corrosion protection of ships

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RECOMMENDED PRACTICE

CORROSION PROTECTION OF SHIPS

2000

DETNORSKEERITASVeritasveien 1 , N-1322 Hgvik, Norway Tel.:4 7 7 579900Fax: 4 7 7 57 99 1



FOREWORD

DET NORSKE VERITA S (DNV) is an autonomous and independent foundation with the object of safeguarding life, propertyand the environment at sea and ash ore.

DET NORSKE VERITAS AS (DNV AS), a fully owned subsidiary society of the foundation, undertakes classification andcertification and ensures the q uality of sh ips, mobile offshore units, fixed offshore structures, facilities and system s, and carriesout research in conn ection with these functions. The society operates a world-wide network of su rvey stations and is au thorised

by more than 120national administrations to carry out surveys and, in most cases, issue certificates on their behalf.

Recommended Practices

Recommended Practices (RP) are issued as a supplement to DNV Rules for Classification or DNV Offshore Standards andother recognised codes used by the industry. RPs form a part of the technical basis for DNV classification and verificationservices as well as offering D W s nterpretation of good engineering practice for general use by the industry.

An updated list of Recommended Practices is available on request. The list is also given in the latest edition of theIntroduction-book lets to the "Rules for C lassification of Sh ips", the "R ules for Classification of Mo bile O ffshore Units" andthe "R ules for Classification of High Sp eed, Light Craft and Naval Surfac e Craft".

In "R ules for Classification of Fixed Offshore Installations", only those Recom mended Practices that are relev ant for this type

of structure have been listed.

0 et Norske Veritas 2000

Data processed and typese t by D et Norske Veritas

Printed in Norway by D et Norske Veritas

00-10-24 1 3 ~ 0 0

10.00.2000

Ifany personsuffers loss or damage which is proved to have been causedbyany negli ent act or omission of Det Norske Veritas, then Det Norske Veritasshall pay compensation to such person for hisproved direct loss or damage. However, te compensation shall not exceed an amount equal to en times thefeecharged for the service inquestion, provided that the maximum compensation shall never exceed USD2million.

Inthis provision 'Det Norske Veritas" shall mean the Foundation Det Norske Veritas as well as all its subsidiaries, directors, officers, employees, agents andany other acting on behalf of Det Norske Veritas.



CONTENTS

1.1.11.21.32.1.11.21.31.4

1.5

1.6

1.71.81.91.102.

2.11.21.3

3.

Introduction............................................................. 4General ...................................................................... 4

Limitations................................................................. 4Definitions and description of terms.......................... 4Newbuildings- Corrosion Protection of Ships......Planning..................................................................... 6

Steel surface preparation ........................................... 6

Coating systems for ba llast tanks

Coating systems for oil cargo and slop tanks(Tables 2.1 - 2.3) ..................................................... 13Coating specification for holds in bulk ca rriersor OBOs (Tables 2.1 - 2.3) ...................................... 13Coating for miscellaneous areas .............................. 13Coating for external hull.......................................... 14Cathodic protection of ballast tanks ........................ 15Cathodic p rotection of external hull ........................ 17Ships in Operation - Maintenanceof

Corrosion Protection Systems.............................. 17Ballast tanks and h ull internals................................ 17Externa l hull ............................................................ 20Assessm ent of coating co ndition in ballasttanks (Good, Fair, Poor) .......................................... 20

References .............................................................. 21

Coatings - general...................................................... 9

(Tables 2.1 - 2.3) ....................................................... 9

4.

4.14.25.

5.1

5.2

5.3

6.

7.

7.17.27.3

7.41.2

2.

Appendix 1: Brief Review of the DNV Rules forShips (2000)and IMO Guidelines regarding

Classification requirements...................................... 23IMOGuidelines. orrosion protection ...................23Appendix 2: Properties and Test S tandardsfor Coatings............................................................ 25

Quality control tests for steel surface treatment

Coating m aterials' testing .hermal. mechanical

Docum entation of coating performance properties.

Appendix 3: Coating inspector's duties. hecklist............................................................... 27Appendix 4: Corrosivity and corrosionresistant materials. brief review ....................... 28

The marine environment. orrosivity...................... 28Corrosion mechanisms on steel surfaces in ships ....28

Factors.influen cing steel corrosion rates in ships ..... 8

Meta llic materials other than steel ........................... 29Cargo and ballast handling. esignagainst corrosion ...................................................... 32

Appendix 5: Surfaces - Preparation, Coatingand Corrosion......................................................... 34

corrosion protection of ships................................. 23

and coating ap plication ............................................ 25

and physical properties ............................................ 26

Type app roval of coatings........................................ 26

DETNORSKEERITAS



4 Recommended P ractice

2000

1. Introduction Cathode: The non-corroding or protected part of anelectrochemical cell.

1.1 General

The aim of these guidelines is to indicate different,recognised methods for corrosion protection of ships, withemphasis on tanks and holds. General specifications ofcoating systems I, I1 and I11 of different target useful life ordurability5 , 10and 15 years, respectively, are indicated. Theuseful life is dependent mainly on steel surface preparationand cleanliness.

Use of cathodic protection is recommended in combinationwith coating on surfaces submerged in seawater, e.g.sacrificial anodes in ballast tanks.

The guidelines are primarily not intended for classificationuse but for assisting in the selection of effective corrosionprotection systems by yards and owners, especially fornewbuilding. However, the additional voluntary class

notationsCOAT-I and COAT-2, refer to these guidelines. Asummary of the classification requirements related tocorrosion protection is included.

Definitions and terms, properties and standards for coatings,a check list for coating inspectors, materials and corrosionresistance, surface preparation of steel, and coating conditionevaluation on existing ships are briefly reviewed.

1.2 Limitations

The guidelines cover protection of common carbon steel hullstructures against seawater and the marine environment.

Ballast tanks, oil cargo tanks and holds of bulk carriers orOBOs are known to be most susceptible to corrosion and arethus of prime concern. Other materials and subjects arebriefly reviewed for the sake of completeness.

The guidelines do not address protection against corrosivecargoes or chemicals.

The guidelines are aiming at being easy to use. No subject is

treated in great depth or detail.

1.3 Definitions and description of terms

Alkyd: Alkyds are synthetic resins of polyester type used asbinders in paints or coatings. The name "alkyd" is derivedfrom the parent chemicals alcohol+acid+ster. Alkydpaints cure by air-drying and oxidation.

Anode: The corroding part of an electrochemical corrosioncell (sacrificial anode or impressed current anode).

Anti-foulingpaint: Paint for use on under water areas onhulls to prevent growth of living organisms, usuallycontaining toxic agents (e.g. tin or other biocides).

Binder: The component in paint or coating binding its

constituents together and fixed to the surface. Commonbinders are epoxy, chlorinated rubber, vinyl, and alkyd.

Cathodic protection: Protecting a metal surface fromcorrosion by making it a cathode in an electrochemicalcorrosion cell.

Cathodic protection of a steel surface is obtained byinstalling sacrificial anodes or impressed current anodes in

metallic, electrical contact with the steel within the sameelectrolyte (sea water) system. Protective current passes fromthe anode through the electrolyte to the steel surface.

The term "anodic protection" is not to be used for commonanode systems on ships, either based on sacrificial anodes or

impressed current. These systems belong to the concept"cathodic protection systems". "Anodic protection" isanother, special technique used in chemical industry.

Chlorinated rubber: Binder in paints or coatings based on

dissolved or emulsified (un-vulcanised) rubber polymerssaturated with chlorine. Chlorinated rubber paints are of one-component, air drying type.

Coating: There is no generally valid definition of coating.

Coating is often synonymous with painting, i.e. a protectivefilm of thickness usually about 0.2 - 0.5 mm. Coatings orpaints are usually sprayed on the metal surface.

Conductivity: The inverse of the resistivity (ohm cm). Inthese guidelines: Conductivity, i.e. specific electricalconductance, of an electrolyte, usually seawater.

Corrosion: Chemical degradation of solid material byinfluence from its environment.

Corrosion rate: The rate, usually in mdyear , at which thecorrosion process proceeds. The corrosion rate is always tobe calculated from metal loss on one surface, even whenoccurring on both sides of a steel plate, etc. (Corrosion rate isnot to be confused with "steel thickness reduction rate".)

Dewpoint: The temperature at which air is saturated withmoisture.

Electrochemical cell: See electrolytic corrosion.

Electrolytic corrosion: Corrosion occurring in an electrolyte,i.e. an electrically conductive liquid such as seawater.Anodes and cathodes formed on the steel surface, togetherwith the electrolyte, constitute electrochemical cells.

Epoxy: Common binder type in paints or coatings for marineuse. Epoxies are normally of two component type, epoxyresin (A component) chemically cured with a hardener (Bcomponent, e.g. amine), resulting in a relatively hard film.

DETNORSKEERITAS



Recommended Practice 5

2000

Epoxy coal tar (coal tar ep o v) : Epoxy mixed with coal tar,constituting a part of the binder in paints or coatings for

marine use. Chemical curing is accomplished by means of ahardener, as for pure epoxy. The tar component acts aspigment and influences the flexibility and water resistance ofthe cured coating film.

Film thickness: The thickness of a coating layer or a multi-layer coating system. Dry film thickness DFC is measuredfor cured coatings, in shipbuilding some times specified asaverage thickness. Minimum and maximum thickness canalso be specified. Wet film thickness is usually controlledonly during application by the coating applicator.

General corrosion: Relatively evenly distributed corrosionattacks on a steel surface.

Guidelines (DNV definition): Guidelines are publicationswhich give information and advice on technical and formalmatters related to the design, building, operating,

maintenance and repair of vessels and other objects as wellas the services rendered by the Society in this connection.Aspects concerning classification may be included in thepublication.

Hard coating: Chemically cured coating normally used fornew construction,or non-convertible air drying coating suchas used for maintenance purposes, organic or inorganic(according to IMO (12)). The hard coating concept coverstypical marine coatings such as those based on epoxy, coaltar epoxy, polyurethane, chlorinated rubber, vinyl, zincepoxy, zinc silicate.

Inhibitor: Chemical having an inhibiting effect on corrosion,usually added to a closed liquid or gaseous system.

Lining: Linings are commonly of higher thickness thancoatings, most often above 1 mm, and are usually appliedinternally in tanks, pipes or vessels. Linings may be appliedin sheets or built up with reinforcements.

Localised corrosion: A concept comprising various kinds ofmore or less concentrated or spot-wise corrosion attacks:Typically pitting, corrosion in way of welds, crevicecorrosion, stress corrosion cracking, etc. Localised corrosioncan proceed rapidly and can be dangerous, e.g. in case of lossof weld metal or penetration of a pressure vessel by pitting.

Mechanical cleaning: Power tool cleaning, by means ofgrinding disc, wire brush, or similar.

Marine environment: In this context used in its widest sense,comprising basically sea water and marine atmosphere,including contaminants from cargoes, industry, harbours,wave and weather actions, and operational factors specificfor each ship.

Pigments: Powders added to the coating in liquid conditionto obtain colour. Pigments also influence the coating’sviscosity, application and protective properties.

Primer coating: First layer of a coating system applied in theshipyard (also called touch up primer, to differentiate fromshop-primer).

Recommended Practice (DNV definition): RecommendedPractices (RP) are issued as a supplement to DNV Rules for

Classification or DNV Offshore Standards and otherrecognised codes used by the industry. RPs form a part of thetechnical basis for DN V classification and verificationservices as well as offering DNV’s interpretation of good

engineering practice for general use by the industry.

Resin: Material used as a binder constituent forming a non-crystalline film when dried or cured.

Resistivity: Specific electrical resistance (ohm cm).

Sa I : Light blast cleaning. Loose mill scale, rust and foreignmatter shall be removed. The appearance shall correspond tothe standard photos designated Sa 1. (This originally

Swedish standard SIS 055900-1967is adopted as I S 0standard 8501-1. It is a pictorial surface preparation standardfor painting steel surfaces. The pictures showing the surfaceappearance are not reproduced in this guideline. Grades Sa 1

- Sa 3 describe blast-cleaned surfaces.)

Sa 2: Thorough blast cleaning. Almost all mill scale, rust andforeign matter shall be removed. Finally, the surface iscleaned with a vacuum cleaner, clean, dry compressed air ora clean brush. It shall then be greyish in colour andcorrespond in appearance to standard photos designated Sa 2.(See parenthesis, Sa 1 )

Sa 2,5 (Sa 2 U2): Very thorough blast cleaning. Mill scale,rust and foreign matter shall be removed to the extent that theonly traces remaining are slight stains in the form of spots orstripes. Finally, the surface is cleaned with a vacuum cleaner,clean, dry compressed air or a clean brush. It shall thencorrespond to standard photos designated Sa 2,5. (Seeparenthesis, Sa 1. It should be noted that Sa 2,5 is closer toSa 3 than to Sa 2. Sa 2,5 corresponds to NACE grade No. 2(near white) and SSPC grade SP 10(near white).)

Su 3: Blast cleaning to pure metal. Mill scale, rust andforeign matter shall be removed completely. Finally, thesurface is cleaned with a vacuum cleaner, clean, drycompressed air or a clean brush. It shall then have a uniformmetallic colour and correspond in appearance to standardphotos designated Sa 3. (See parenthesis, Sa 1. Sa 3

corresponds to NACE grade No. 1 (white metal) and SSPCgrade SP 5 (white).)

Shop-primer: Thin (approximately 15 - 25 microns) primercoating applied for temporary protection of steel plates inautomatic plants.

Soft coat: Coating that remains soft so that it wears off whentouched; often based on oils or sheep wool grease.

Semi hard coating: Coating which dries in such a way that it

stays soft and flexible although hard enough to touch andwalk upon.

DETNORSKEERITAS



6 Recommended Practice

2000

St 2: Thoro ugh scraping and w ire brushing - machinebrushing - grinding - etc. The treatment shall remove loosemill scale, rust and foreign matter. Finally, the surfa ce iscleaned with a vacuum cleane r, clean, dry comp ressed air ora clean brush. It should have a faint metallic sheen. Theappearance shall correspond to standard photos designated St

2. (This originally Swedish standard SIS 055900-1967 isadopted as I S 0 standard 8501-1. It is a pictorial surfacepreparation standard for painting steel surfaces. The picturesshowing the surfac e appearance are not reproduced in thisguideline. Grades St 2 - St 3 describe mechanically cleanedsurfaces.)

St 3: Very thorough scraping and wire brushing - machinebrushing - grinding - etc. Surface preparation as for St 2, butmuch more thoroughly. After removal of dust, the su rfaceshall have a pronounced metallic sheen and correspond tostandard photos designated S t 3. (See parenthesis, St 2.)

Steel: In these guidelines, if not a m ore precise definition isgiven, "steel" means carbon steel including hull stru cturalsteel.

Stripe coating: Application, normally by brush, of one ormore coating layer on edges, welds or sim ilar to build upadequate total dry film c oating thickness at the actu allocations.

Vinyl:Binder in paints or coatings based on dissolve d oremulsified vinyl chloride or vinyl acetate polymers (Vinylchloride: CH2=CHC l). Vinyl paints are of one-com ponent,air drying type.

Zinc rich paint or coating o r prime r: Products containingusually>85 % of metallic zinc powder in the dry film. Thehigh amoun t of zinc prov ides a sacrificial anode effect. Thebinder usually is on epoxy o r (inorgan ic) silicate basis.

2. Newbuildings - Corrosion Protection ofShips

This chapter aims at indicating coating system s of varyingquality and target useful life or durability levels.

System s of three target useful life or dura bility levels are

indicated for ballast tanks, oil cargo tanks and h olds in bulkcarriers or OB Os:

System I: Target useful life 5 years (k 3 years)

System 11: Target use ful li fe 10 years (k3 years)

System 111:Target useful life 15 years (k 3 years).

The useful life of a coating is consid ered to be until 3 %breakthrough of rust on the coated surface areas: Weld s' andedges' areas, respectively plain and larg e surface areas,considered separately. (I S0 Standard 4628/3 rust scale=Ri

3-4, or Rust Grade 5, ASTM D 610, Appendix 5).

Reasons: See 2.4 and 3.1.1,

The above coating quality or durability levels should be readas target useful lives, estimated based on literature data andexperience , with great uncertain ties and variations. Theuseful life will be prolonged if m aintenance coatingaccording to principles outlined in these guidelines is carriedout.

Som e users may find the indicated durability possibly toooptimistic, others the opposite. As fu rther elaborated below,the useful life of corrosion protection systems is dependenton many fac tors and circumsta nces, including those relatedto the ship type, trade and operation.

2.1 Planning

To ob tain long life protection, the ap plication or installationof corrosion protection system s (coatings and anodes) shouldbe a well planned activity, integrated in the ship yardsconstruction plans. Care should be taken to avoid conflictswith other yard operations, notably pipin g installations andwelding.

Corrosion protection works include

- staging- steel surface preparation , e.g. g rinding of edges and

welds, blast cleaning, mechanical cleaningpartitioning of suitable areas fo r blast cleaning andcoating, e.g. block building

installation and ma sking off of anodes

shielding off already painted areas from blastingoperations in other areas

control of humidity and tempera ture of the air in relationto the steel temperaturescontrol of coating curing conditionsavoidan ce of dama ge to finished coating and installedanodes.

-

- coating application--

- cleaning operations-

--

2.2 Steel surface preparation

The preparation of steel su rfaces is of grea test importance forthe durability of a coating. It may be more impo rtant than theselection of coating type. Us e of a high quality andtechnically sophisticated coating is useless if the steel surface

preparation is neglected. The potentially best coatings maybe the most depen dent on adequate surface preparation. Earlycoating failures are often caused by inadeq uate surfacepreparation.

2.2.1 Shop-primed steel

Steel ships are normally bu ilt of shop-p rimed plates. Theplates are autom atically cleaned by s hot blasting or similarand the shop-primer applied imm ediately afterwards in thesame plant. Th e cleanliness standard for application of shop-primer should be near white m etal or equivalent, i.e. S a 2,5

according to I S 0 850 1 1.

DETNORSKEEFUTAS




2000

The best shop-primers from a corrosion protection point ofview are those containing zinc Zn, preferably inorganicsilicate based. Notably on surfaces not continuallysubm erged, e.g. ballast tank top and internal splash zoneareas, zinc containing shop -primers should be used.

Sweep b lasting of the whole or parts of shop-primed surfacesmay be relevant, dependent on their condition, type of shop-primer and coating system to be applied.

Ultra high pressure water jetting (>=1700bar) withequipment free from back thrust and com bined with chloridecontrol (max imum conductivity corresponding to =20-50mg/m2NaCl, depending on various conditions) may be apossible, alternative method of cleaning shop-primedsurfaces, for specially designed coating s, subject to coa tingmanufacturer’s recom mendation.

Contamination as mentioned below on shop-primed or blastcleaned surfaces may result in early blistering andconsiderably reduced coating lifetime. There are indicationsthat the neg ative effect of surface contam ination with sma llamo unts of salts may be larger than traces of grease.

When relevant, cleaning should be carried out beforeapplication of the first coating layer (primer coat applied inthe yard) to shop-primed surfaces.

Any sa lt contam ination, oil, grease, dust, weld smo ke,meta llic or other particles should be rem oved, e.g. withsolvent cleaning, or washing with fresh w ater containingdetergen t followe d by rinsing with fresh water and drying.Such degreasing , washing and drying of shop-primed

surfaces shall, if required, be carried out before final blastcleaning op erations. The bla sting abrasive should not containcontaminants.

2.2.2 Treatment of sharp edges, welds and burns:

All sharp edges on cu t or burnt steel plates should berounde d or broken before blast cleaning op erations. Aminimum of rounded edges is obtainable by means of asingle pass of a grind ing tool over the ste el edge, braking upon e 90 degrees or sharper edge into two, each approximately90 +45 =135 degrees, as illustrated in figure 2.1. Afterfinished blast cleaning , the edge may beco me sufficientlyrounded for application of coating.

Rounding of sharp edges can also be specified moreaccurate ly, e.g. to a minim um radius, e.g. r =2 mm.

All edges, inc luding cut-outs, rat holes, etc. should beincluded in the above treatment.

Figure 2.1

Upper case: Sharp. Middle case: Broken, after one pass ofgrinding tool. Lower c ase: Rounded after blast cleaning.

Edges from gas and/or plasma cutting are often sharp, thoughnot necessarily alway s so. Edges of rolled sections may moreoften than not be well rounded, though not alway s,

depen ding on types of section s, profiles, and rollers. Adefinition of sharp edges may be: Edg es with radius r <2mm (in som e paint specifications, rounde d edges are definedas with r >2 mm).

Edges on steel plates, enlarged cross section.

Welds should be ground where necessary, so that weldbeads, weld spa tter or othe r surface irregularities that canreduce the coating’s useful life are removed. See A ppendix 5 .

If the abov e treatment of edges and welds is not carried out,coatings will be:

- applied too thin, and-

easily damaged by traffic and flowin g liquids.The con sequen ce will be tha t the coating’s effective lifetimeis reduced to a fraction of what it could have b een.

It is difficult to me asure coating film thick ness on edge s bynon-destructive methods, even when rounded to r =2 nun.

Stripe coating is a comm on means of obtaining the sam e filmthickness on edges as on plain surfaces.

The above surface treatment of edges and welds (2.2.2) isnecessary for System I1 and 111and is generallyrecommended.

For System 111,cleaning as described under 2.2.1 isnecessary and must inc lude welds, burns and edges’ areas.Such cleaning is generally recommended and may benecessary to obtain the desired lifetime also for System II.

2.2.3 Blast cleaning and inspection

In the norm al shipbuilding situation, i.e. for inte rnals andexternals of the hull (excep t cargo tanks for chem icals, etc.)the coating system is built up on intact, clean shop-prim edsurfaces.

Zinc rich shop -primer should be used for System I1 and 111

and is generally recommen ded. Compatibility with Zn rich

primer and coating system must be ensured by the coatingmanufacturer.

DETNORSKEERITAS




2000

After finishing operations 2.2.1 to 2.2.2, blast cleaningshould be carried out on all welds, burns and where the shop-primer otherwise is dama ged.

When blast cleaning is fin ished, broken edges should appearsufficiently rounded fo r coating.

Inspection for ac ceptance of rounded edges, weld surfacesand other possible surface irregularities should be carried outat this stage. After any required re-grindin g or breaking ofedges the inspector will accep t the surfaces for final blastcleaning and coating.

Final blast cleaning should be carried out only when the airand steel temperatures and air hum idity is under control, i.e.

--

the air humidity shall not be abo ve 85 %

the steel temperature shall be 3 OC or more above thedew point.

the above, dry conditions shall be maintained, so that notrace of mo isture condensation on the steel occurs beforethe primer yard coat is applied.

-

Dry conditions are obtainable in tanks and closedcom partme nts by m eans of heating and ventilation. Onsections or blocks in ship n ewbu ilding, dry conditions areobtainable in heated and ventilated building s or tents.

The steel temperature may vary considerably in the sametank, e.g. cond ensation of m oisture more easily occurs oncool steel surfaces deep down in the tanks than in upper,warmer areas.

The cleanline ss of blast cleaned surfac es should be Sa 2,5according to I S 0 8501-1, or better if required by the coatingmanufacturer. Close to Sa 3 is recommended to obtain 15 f 3years useful life, System 111. Other, eq uivalent cleanlinessstandards are stated in Appendix 2 . Bla sting abrasives anddust shall be complete ly removed after finished blastingoperations, e.g. by m eans of vacuum cleanin g, compressedair and brushes.

The surface roughness profile, see e.g. I S 0 8503, should beaccording to the coating manufacturer's recommendations.(The surface roughness profile after blast cleanin g is lessimportant than the surface cleanliness.)

Inspection and accep tance of the surfaces after primercoating should be carried out.

Acceptance criteria, authority, scope of work and reportinglines for the coating inspecto r should be clearly defined.Inspection should be carried out according to specificationsmutually agreed between builder and owner and coatingmanufacturer.

Indications of a coating ins pector's duties are given in 7.

Blast cleaning as described above may not be the normalshipyard practice.

The minimum surface treatment for System I is shop-primerof unspecified type and St 3 on w elds and burns. System I,with target useful life 5 years, is not recommended for ballasttanks.

The minimum surface treatment for System I1 is Zn

containing , silicate based sho p-prime r, broken edge s, Sa 2,5on w elds and burns, mechan ical cleaning to St 3 on blockjoints and damages in coating only, and dry conditions.

The minimum surface treatmen t for System I11 is roundededges, Zn contain ing, silicate based shop-p rimer, blastcleaning to Sa 2,5 or better on all surfaces (including plates,welds, burns and edges, except block joints and a minim umof spots of damaged coated su rface), clean conditions(surfaces not contam inated with salts, dust, hand marks,grease, particles, etc., see 2.2.1 ) and dry cond itions (see2.3.4) during blast cleaning and coating app lication. Th echloride content on surfaces to be coated sha ll be <50

mg/m2.

Blast cleaning to Sa 2 is considered a better s urface treatmen tfor coating application than m echanical cleaning to St 3. St 3may be equivalen t to blast cleaning to somew here betweenSa 1 and Sa 2 regarding coating performance. This is due tothat by brushing, dust and loose rust particles are worked intoand fill up surface pores and irregularities. These bec omefuture spots of osmos is activity and early coating blisters.

Ultra high pressure water jetting ( ~ 1 7 0 0ar) withequipment free from back thrust and comb ined with chloridecontrol (e.g. m aximum conductivity corresponding to=20 ,30 or 50 mg/m2 NaCl) may be a possib le, alternative methodof cleaning, for specially designed coatings, subjec t tocoating manufacturer's recommendation. Water jetting isrelevant primarily for m aintenance coating on ship s inoperation. When m ore experience is gained , water jetting,hydroblastin g, wet sand blasting, etc. also on newbuilding smay possibly be realistic, subjected to specific conditions.

2.2.4 Mechanically cleaned steel

If blast cleaning of w elds and other areas of non-inta ct shop-primer is not obtainable, efforts should be made to m ake thesurfaces as clean and dry a s possible before applica tion oftouch up primer. Mechanical cleaning by means of e.g. wire

brushes is commonly used.

Rotating wire brushes may polish welds and other steelsurfaces, resulting in reduced coating adhesion comparedwith that obtained on blast cleaned surfaces.

The minimum cleanliness standard for any coatingapplication according to System I and Sy stem I1 should be St3 ( IS 0 8501-1) or equivalent standard (for comparison withblast cleaning, see 2.2.3).

Mechanically cleaned steel (wire brushing and sim ilar) isreally not adequate for System I11 but may have to be

accepted in shipbuilding on block joints and on a minim umof spots of damaged coating , for practical reasons.

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2000

2.3 Coatings - general

2.3.1 Coating selection

Coating types adequate for the intended service should beselected in co-operation with the chosen coatingmanufacturer, which should have products of documented

good performance records. The coating manufacturer or hisrepresentative should be capable of rendering adequateadvisory and inspection services, if such are not covered by

other parties.

Due concern should be given to the obtainable steel surfacepreparation in the shipyard, see 2.2.2. to 2.2.4. If blastcleaning, dry conditions, clean conditions, etc. are notspecified, coating systems made to be tolerant of theexpected surface condition should be selected.

Light coloured coatings should be used where relevant tofacilitate inspections, e.g. in ballast tanks.

Coatings for ballast tanks should be chosen with due regardto that the water quality may be bad, e.g. acidic or otherwisecontaminated, notably in harbours.

The below examples of simplified coating specifications arebased on general experience and published literature, see e.g.reference (8).

2.3.2 Coating specification

A coating specification should be mutually agreed betweenbuilder and owner and coating manufacturer. Inspectors'duties and reporting line should be defined. The specification

should describe:

- which coating systems (types of coating, thicknesses and

number of coats) to be applied where

coating manufacturers accepted for delivery

equipment for control of air humidity, temperatures,

ventilation

coating applicator's duties and application equipment

-- yard's coating facilities-

-- steel surface treatment

- coating application and curing

- repair procedures for damages-

2.3.3 Hot surfaces - High strength steels

Coatings for application underneath sun heated decks or onwarm bulkheads adjacent to e.g. the engine room should beable to withstand constant or repeated heating withoutbecoming brittle. Documentation of the coating's long termhigh temperaturestability should be requested from thecoating manufacturer. Brittleness (ageing) may result fromlight components in the coating evaporating with time.

Coating to be applied on high strength steels should haveadequate relative elongation to cope with increased strainsand movements in ships built from such steel. As above, the

long term ability of the coating to stay flexible is important.Documentation of relative elongation should be requestedfrom coating manufacturers.

test methods, equipment and acceptance criteria.

A relevant minimum figure for the relative elongation of acoating film may be about 4 per cent. For test method, seeAppendix. Testing should be carried out on adequately agedcoating, not only on new, fresh coating.

2.3.4 Coating application and curing

Coatings, including primers and intermediate coating layers,should be applied on dry, clean surfaces (i.e. blast cleaned,shop-primed, mechanically cleaned, washed and dried if

necessary) and according to the coating manufacturer'srecommendations. The degree of cleanliness and dryness willbe varying according to the alternative quality levels SystemI, I1or 111. Stripe coating is recommended for edges andwelds.

Clean conditions: See 2.2.1 to 2.2.2.

Dry conditions means:

--the air humidity shall not be above 85 %.the steel temperature shall be 3 "C or more above thedew point.

the above dry conditions shall be maintained so that nocondensation of moisture occurs on any surface when anew coating layer is applied. They are also valid forshop-primer application.

-

For System 111, clean and dry conditions are required.

For System 11, dry conditions are required. Clean conditionsare necessary in the sense that intact shop-primer, welds andedges should be cleaned if contaminated with salts, weldsmoke, etc. impairing coating adhesion.

For System I, dry, clean conditions are not required (butrecommended).

In general, for application of coatings the following is valid:

- coatings should be applied by spraying, except "stripecoats" applied to build up thickness on edges notproperly rounded and in areas difficult to access

the dry film thickness should be as recommended by thecoating system manufacturer

each coating layer should be adequately cured beforeapplication of the next coat. The time required for curing

will generally be longer at lower temperatures. Coatingmust not be applied below the manufacturer'srecommended minimum temperature.

intermediate coats should not be contaminated with dirt,grease, dust or salts

for curing of zinc silicate based primers, the air humiditymust be brought above a certain minimum percentage(contrary to other coatings - though not until theapplication process is finished).

-

-

-

-

2.4 Coating systems for ballast tanks (Tables 2.1 -2.3)

In Tables 2.1 - 2. 3 are indicated three alternative coating

specifications for ballast tanks, System I, I1 and 111.

DETNORSKEERITAS




The indicated surface preparation in the tables is theminimum surface quality needed to obtain the stated targetuseful coating life. It is a shorthand summ ary of the detaileddescription under 2.2.1 - 2.2.4.

For clarification: Touch -up priming of areas with no n-intact

shop-primer is not intended in these guideline s, but may b eadvantageou s, depending on the coating m anufacturer’srecommendation.

The estimated target useful life span ranges (durabilityranges) are meant to b e rough indications of durability, basedon collected experience and inform ation. It is to be taken intoconsideration that the durab ility of coatings and thesusceptibility to corrosion of different surfaces w ithin thesame tank may vary w idely. Maintenance of the coating willincrease the useful life. Notably the area underneath sun-heated decks, at w arm bulkheads and internal splash zonesare representing severe exposure conditions for coa tings. See

2.3 and 8.

There are indications that the useful life of coating may bereduced in deep w ater ballast tanks e.g. in very large crudeoil carriers, due to increased tendency to blistering withincreasing hydrostatic pressure. The explanation may be thatrapid decompression during deballasting causes small orpremature blisters to grow and large blisters to break. Thisprocess may proceed at an increasing rate with increasingdepth in deep w ater ballast tanks.

Useful life of a coating is considered to be until 3%

breakthrough of rust o n th e coated s urface areas: We lds’ andedges’ areas, respectively plain and large surface areas,considered separately. ( IS0 Standard 4628/3, rust scale=Ri

3-4, or Rust Grade 5, A S TM D 610,9) . See also 3.1.1.

Maintena nce coating should be carried ou t beforebreakthrough of rust reaches 1 % of the surface area: Welds’and edges’ areas, respectively plain and large surface areas,considered Separately. (Rust Grade 6, A STh4 D 6 10, or =ustscale Ri 3, I S 0 Standard 4628/3).

The abov e definitions are chosen as practical compromisesamong several possible alternative definitions.

The coating system s indicated in these guidelines may be

considered as general coating types, commonly used. Theyare not meant to exc lude other recognised or newlydeveloped systems.

Light coloured coatings should be used wh ere possible tofacilitate inspections.

DFT m eans dry film thickness in microns (=0,001 mm).

The total nominal dry film thickness NDTF is stated in theseguidelines. However, the av erage DFT is also used inshipbuilding.

In case the average thickness is used, it should be sp ecifiedsufficiently high to ob tain a target minimum DFT . E.g.minimum 80 % of all thick ness measurem ents should be 1the specified DFT. Of the rem aining 20 % of themeasure ments, none should be below 80%of specified DFT(80/20 practice). E.g. to obtain a minimum D FT of 200

microns, the average or nominal DFT should be about 250microns.

For som e coating types, it may be,important that a maximumthickness is not exceede d. Coating manufacturer’srecommendations should be followed.

In doub le bottom ballast tanks in bulk carriers, the coating onthe tank top may be dam aged by grab impact. It is importan tthat the app lied coating stays flexible. Relative elongationfigures for the coating should be requested. See 2.3.3.

- System I for ballast tanks (not recommended)

Estimated useful life span range: 5 +3 years.

System I is not recommen ded for ballast tanks due to theshort life expectancy. (It is, however, still in use, and may besuitable for some areas in oil cargo tanks and carg o holds inbulk carriers. See below.)

Surface preparation: Steel plates shop-p rimed on blastcleaned or equivalent surface to S a 2 - 2,5. Welds and bumsmechan ically cleaned to m inimum St 3. To obtain a coating

durability 25 years the steel surfac e preparation for shop-priming should be Sa 2,5.

Coating ap plication (Table 2.1): One stripe coat should beapplied if necessary to obtain the stated thickness on edges,etc. Increasing of the average thickness to obtain 200microns minimum D m s recommended for a coating

durability 2 5 years.

- System I1 for ballast tanks

Estima ted useful life span range: 10 3 years:

Surface preparation: Steel plates w ith Zn containing, silicatebased shop-prim er applied on surface blast cleaned to Sa 2,5or better. Sharp edges broken or rounded. Dam aged shop-

primer blast cleaned to S a 2,5, including welds and burns.

Dry conditions: Air humidity 5 85 % and steel temperature 2

3 “C bove the dew point during surface preparation andcoating operations.

Coating application : Stripe coating with brush should beapplied as an additional coat where required to obtain thethickness specified.

- System I11 for ballast tanks

Estimated useful life span range: 15*3 years.

DETNORSKEERITAS



Recommended Practice

Target durabilityCoating system I )

Coats and thickness

11

5 yearsEpoxy based

Other recognised hard coating

1coat 3).

Total nominal dry film thickness (NDFD 200microns 4,

2000

~

Secondary surface preparation

Clean conditions

Surface preparation: Steel plates with Zn containing, silicate

based shop-primer applied on surface blast cleaned to Sa 2,5or better, or, preferably, all shop-primer blasted off untilfresh Sa 2,5 surface is obtained on all areas, including welds,burns and edges. Sharp edges rounded or broken.

~ ~

Welds and bums mechanically cleaned to minimum St. 3

Any visible salt contamination, oil, grease, dust, weld smoke or dirt on shop primed or other surface tobe coated, to be removed by cleaning

Clean conditions: Any salt contamination, weld smoke, dust,particles, grease, oil, hand marks, etc. on shop-primed orother surface to be coated, to be removed by cleaning before

final blasting operations (assuming that the blasting abrasiveis clean).

Dry conditions: Air humidity 585 % and steel temperature 2

3 "C above the dew point during blast cleaning and coating

operations.

Coating application: 2 stripe coats with brush should beapplied where required to obtain the stated thickness. In way

of suctions, erosion of the paint film should be taken intoaccount. Heavy duty coating ,e.g. glass flake reinforcedepoxy 500 microns DF T and/or doubling plates may beapplied.

Comment regarding the estimated 15 5 3 years target usefullife of System 111:

Increased tendency to blistering of coatings with increasing

depth of water ballast tanks is probably caused by increasedosmotic pressure gradient and rapid de-pressurising when de-ballasting. Cleanliness of the surface underneath the coating

is a necessary condition for good adhesion. Sufficientstrength of the coating film is also necessary to obtain thiscoating life.

Primarv surface DreDaration I Steel dates shoo Drimed on blast cleaned surface to Sa 2 - Sa2.5 (Sa 2.5 recommended)

The thermal and hygrometric conditions related to air humidity and steel temperature shall be within theI limits set by the coating manufacturerThermal and hygrometricconditions

Comments to system I:

1)

2)

3)

4)

Light coloured coatings are recommended. Tar containing coatings are dark.The selection of a recognised coating may depend on the type of compartment and it's function.

One stripe coat to be applied on edges, welds and in areas where spraying may not be fully effective.

Nominal dry film thickness shall follow the "80/20rule" and is for system I defined as follows:

The averageDFTbased on measurements shall always be equal to or larger than the NDFT. Up to 20% of the area (measuredpoints) may have a thickness between 100% and 80%of the NDm, but the measured dry film thickness shall always be largerthan 80% of the NDFT.

The measured DFT shall not exceed the maximum dry film thickness defined by the paint manufacturer.

DETNORSKEVERITAS




2000

Table 2.2 Coating system No. I1 - Target useful life 10 years

Target durability

Coating system I )

Coats and thickness

Primary surface preparation


Clean conditions

Thermal and hygrometricconditions

10years

Epoxy based

Other recognised hard coating ')

2 coats 3).

Total nominal dry film thickness (NDFT) 3 00 microns 4 ) 6 )

Zinc containing, silicate based pre-fabrication primer on surface blast cleaned to minimum Sa 2,5

Sharp edges to be removed

Damaged shop primer blast cleaned to Sa 2,5, including welds and bums. Intact shop primer canremain. Mechanical cleaning to St 3 acceptable on block joints and damages ') to the applied coatingsystem

Any visible salt contamination, oil, grease, dust, weld smoke or dirt on sho p primed or other surface tobe coatedThe chloride content on surfaces to be coated shall be within the limit set by the coating manufacturer,if any

Air humidity 5 85 YO nd steel temperature L 3 "C above the dew point during blast cleaning andcoating application operations

Comm ents to system 11:1)

Light coloured coatings are recommended. Tar containing coatings are dark. If coal tar epoxy is used the epoxy to tar ratio shallnormally not be less than 60 to 40. If paint with a less epoxy to tar ratio is used (less epoxy, m ore tar), the thickness of thecoating may have to be increased

The selection of a recognised coating may depend on the type of compartment and it's function

On e stripe coat to be applied prior to each full coat on edges, welds and in areas where spraying may not be fully effective

Nominal dr y film thickness shall follow the "80/20 ule" and is for system I1 defined as follows:The average D FT based on measurements shall always be equal to or larger than the NDFT . Up to 20%of the area (measuredpoints) may have a thickness between 100% and 80% of the NDFT, but the measured dry film thickness shall always be largerthan 80%of the NDFT

The measured DFT shall not exceed the maximum dry film thickness defined by the paint manufacturer

2 )

3)

4)

5 )

6 )

Only applicable for a reasonable amount of damages. Otherwise the basic surface preparation to be re-applied

The N DFT depends on the type of coating. Refers to epoxy based coatings only

Table 2.3 Coating system No . 111- Target useful life 15 years

Target durability

Coating S ystem I )

Coats and Thickness

Primary surface preparation


Clean conditions

Thermal and hygrometricconditions

15 years

Epoxy based

Other recognised hard coating 2,

2 to 3 coats 3,

Total nominal dry film thickness (NDFT) 300-400 m icrons 4,

Zinc containing, silicate based pre-fabrication primer on surface blast cleaned to minimum Sa 2,5

Sharp edges to be removed

Damaged shop primer blast cleaned to Sa 2,5, including welds and bums. Intact sho primer to be

coating svstem

sweep blasted. Mechanical cleaning to St 3 acceptable on block joints and dam agesR o the applied

Any visible salt contamination, oil, grease, dust, weld smoke or dirt on shop primed or other surface to

be coated, to be removed by cleaningThe chloride content on surfaces to be coated shall b e less than 50 mg/m2 (as NaCl) acco rding to Bresleconductimetric method, or an equally recognised method

Air humidity I 8 5 % and steel temperature ? 3 "C above the dew point during blast cleaning andcoating application operations

Com ments to system 1111)

P

3)

4)

Light coloured coatings are recommended. Tar containing coatings are dark. If coal tar epoxy is used the epoxy to tar ratio shallnot be less than 60 to 40

The selection of a recognised coating may depend on the type of compartment and it's function

On e stripe coat to be applied prior to each full coat on edges, welds and in areas where spraying may not be fully effective

Nominal dry film thickness shall follow the "90/5 rule" and is for system 111 defined as follows:The average DF T based on measurements shall always be equal to or larger than the NDFT. Up to 5% of the area (measuredpoints) may have a thickness between 100 % and 90%of the NDFT, but the measured dry film thickness shall always be larger

than 90% of the NDFTThe measured DFT shall not exceed the m aximum dry film thickness defined by the paint manufacturer

Only applicable for a reasonable amount of damages. Otherwise the basic surface preparation to be re-applied)

DETNORSKEERITAS



Recomm ended Practice 13

2000

2.5 Coating systems for oil cargo and slop tanks(Tables 2.1 - 2.3)

Three alternative coating specifications for oil cargo tanks,

Systems I, I1 and 111, are indicated in Tables 2.1 - 2.3.

The indicated surface preparation is the minimum levelneeded to obtain the stated potential useful coating life. It is abrief summary of the detailed description under items 2.2.1 -2.2.4.

General about estimated life span ranges, definitions andexplanations: See 2.4.

Coating is recommended in cargo oil tanks bottom platingand structures and underneath deck and deck-structures.System I1 is preferable for cargo tank inner bottom anddeckhead, while System I may be sufficient on upper part ofstringers.

On the bottom plating the coating is to prevent bottom pittingcorrosion that can occur in sour, foul water precipitated fromoil cargoes, from condensation of moisture in the air, etc.Bacterial corrosion attacks superposed on commonelectrolytic corrosion may give very high corrosion rates onbottom plates unless properly protected by coating,especially in double hull vessels due to elevated cargotemperatures being maintained for a prolonged time period(thermal isolation or "thermos bottle effect" of the doublehull).

The top side of horizontal stringer decks should be protectedwith coating due to their susceptibility to pitting corrosion

due to deposits settling out from the cargo, as for the bottomplating.

Plating and associated structures underneath deck are oftensusceptible to more or less uniform corrosion attacks due tocyclic sun heating and cooling, condensation water, abundant

oxygen supply, washing and sloshing of cargo. Properapplication of a suitable coating is recommended. Sacrificialanodes will not be effective due to that here is no bulkelectrolyte for distribution of protective current.

The coating types suitable for cargo oil tanks must be oilresistant and will usually be epoxy based. The coating typeshould be selected in co-operation with coating

manufacturers. Coal tar epoxies will usually not be oilresistant and may easily become brittle due to that light tar

components may be washed away andlor evaporated due tocyclic sun heating.

Slop tanks should be coated on all surfaces with System 11

(or better).

2.6 Coating specification for ho lds in bulk carriersor OBOs (Tables 2.1 - 2.3)

The three alternative coating specifications Systems I, I1 andI11 as indicated in Tables 2.1 - 2.3 are applicable also for

holds in bulk carriers and OBOs.

Surface preparation, estimated life span ranges, definitions

and explanations: As for 2.4.

Coating systems I, I1 and 111are generally applicable but maybe modified as regards coating thickness and number ofcoats, if relevant. The coating is to be epoxy or equivalent,

rendering adequate corrosion protection to the surfaces inquestion, considering the cargo type and mode of operationof the ship.

All internals of cargo holds except flat tank top areas, hoppertanks sloping plating and transverse bulkheads bottom stool

approximately 300 mm below shell frame and brackets are tobe coated. Internal and external surfaces of hatch coamingsand hatch covers are also to be coated, referring to thecurrent Rules for Classification of Ships (9).

Coating of the flat tank top areas and other surfacesexempted from the rule requirement (see above), is alsorecommended, provided a coating or floor covering can be

found that is sufficiently tough to withstand the impacts,abrasion and generally rough treatment by grabs, bulldozers,etc. during loading and unloading.

Due concern should be given that coating used for grain orother edible cargoes is properly certified for the purpose bythe responsible authorities. The inner bottom should becoated, covered by a wooden deck, or similar.

Floodable cargo holds for harbour filling should be coatedwith due regard to that harbour ballast water may be pollutedand corrosive.

For some dry cargoes, light coating with 1-

2 coats of holdpaints may be useful. A variety of hold paints (epoxy andnon-epoxy based) and surface preparation methods areavailable. Especially ultra high pressure water jetting may beuseful in cargo holds.

2.7 Coating for miscellaneous areas

For the sake of completeness, a few examples of coatingsystems for accommodation areas, engine rooms and freshwater and petroleum product tanks are listed below. The

suggested systems for use underneath thermal isolation in

reefers and for voids i n LN G tankers are practically the sameas System 11, Table 2.2. For other allocations, special

systems are suggested that are more or less different from theSystems I - I11 presented in Tables 2.1 - 2.3.

The durability level or estimated useful life of the coating is.dependent on wear and tear as well ason maintenancecoating. The corrosion protection of these areas is, however,usually not critical for the operation or safety of the ship.

No coating systems are indicated for corrosive cargoes,chemicals or gases.

DETNORSKEERITAS




2000

For coating or lining of tanks for chemic al cargoes, shop-primer should be removed before applica tion of the primeryard coat. The cleanliness of blasted surfaces should be S a2,5 - Sa 3. Som e of the coating s may be very sensitive tooverthickness. Th e coating manufacturer’s recom mend ationsshould generally be followed as regards surface preparation,

coating application and curing.I Table 2.4 Coatings for miscellaneous areas

Allocation

Freshwatertanks

Producttanks

Accom-modationandEnginerooms

Reefers,underneaththermalinsulationon tank topor innerbottomplate

Voids inconnectionwithsphericaltanks,LNGcarriers

Voidspaces(exceptdry,sealed-offcompart-ments)

Surfacepreparation

Sa 2,s

Sa 3

Sa 2 - St 3

Sa 2.5

Sa 2,s

Sa 2,s - St 3

Coating

PPe

EPOXY

EPOXY

EPOXY

Phenolic

Zinc

silicate

Alkyd,etc.

EPOXY

NominalDFTmicrons

200

300-350

300-350

75-100

100-150

300

300

200- 300

Number ofcoatsminimum

2

2 - 3

2 - 3

1

2

2

2

1 - 2

2.8 Coating for external hull

A coating specificatio n for the ship will inc lude a descriptionof the external hull coating. The g eneral contents of a coatingspecifications is described under 2.3.

For the hull a coating specification with two alterna tive steel

surface preparations is indicated below:

Surface preparation alternative I :

Shop-primed steel, shop-primer applied on plate surface blastcleaned to Sa 2,5 and preferably of Zn contahing type. Allwelds and other areas of damaged shop -primer blast cleanedto minimum Sa 2,5. The coating should be applied under dryand clean conditions and as indic ated under 2.2.1,2.2.3 nd2.3.4.This surface preparation will give the most durablecoating.

Surface prepa ration alternative 2:

Shop-primed steel, preferably Zn containing shop-primer,and mechanical cleaning, e.g. wire brushing, of welds andother areas of damaged sho p-prime r to St 3.

The dura bility of a coating on areas with surface preparationalternative 2 will be considerably reduced com pared with thesame coating applied on a surface according to alternative 1.

The coating manufacturer’s recommendations shouldgenerally be followed.

Anti-fouling coating should likewise be applied inaccordan ce with the m anufacturer’s recom mend ation, also

considering the target lifetime.

Indications of hull coating system s are given in Table 2.5.

DETNORSKEERITAS




Table 2.5 Bull coating systems and allocation

Allocation Coating type

External hull, under water including boot-top* area

* between loaded and ballasted water line

As above Vinyl tar

Epoxy or Epoxy coal tar

+

Anti-fouling paint

+

Anti-fouling paint

As above Chlorinated rubber

Anti-fouling paint

+

External hull, in ice

External hull, above water, and deck, deckhouse,

Epoxy, hot applied, solvent free

Chlorinated rubbersuperstructure

As above EPOXY

As above Vinyl

As above Epoxy Mastic+

Topcoat

2000

Total average NumberDFT microns of coats

300-350 2-3

250-350 2-3

300-350 2-3

250-350 2-3

300-350 3

250-350 2-3

600-1500 1-2

250-300 3

250-300 2-3

250-300 3

300-350 2

Note:

Stripe coating of insufficiently rounded edges, etc. should be canied out to build up coating thickness.

2.9 Cathodic protection of ballast tanks

Cathodic protection is recommended in ballast tanks incombination with coating, as no coating will be free of poresand defects. Notably in the bottom areas of tanks which areseldom com pletely dried, sacrificial anodes installed as close

as possible to the bottom p lates can prevent o r reduce pittingcorrosion starting from local coating defects.

To o btain full cathodic pro tection in large, uncoated tanksmay be difficult in practice due to the often complexstructure of tanks and great number of anodes required tocover all surfaces.

Cathodic protection system s are without effect when thetanks are empty, and it will take so me time (1/2 day or more)to obtain full effect (polarisation) of subm erged steelsurfaces after filling with sea water. In the ullage spa ce orunder deck area on top of tanks sacrificial anodes will not beeffective unless the tank is com pletely filled.

Sacrificial anode systems designed according to the belowprinciples will prevent corrosion on surfac es duringsubm ersion in ballast water (minu s the time needed for re-polarisation after refilling).

2.9.1 Cathodic protection system design basis - ballasttanks

2.9.1.

The tanks’ ize, shape and areas to be pr otected should bepresented accura tely and in detail to the cathodic protectionsystem supplier.

2.9. .2

Areas coated, respectively uncoated should be specified.

2.9.1.3

Th e ballasting routines, including th e percentage of the total

time the tanks will likely be filled with ballast wa ter, theprobable duration of ballasted periods, and quality of ballastwater should be indicated, if possible.

2.9.1.4

The criterion of cathodic protection is that the potential ofthe protected surfaces shall be - 0,80 V or higher negativevalues measured with a silver or silver chloride Ag/AgClreference electrode, or equivalent potential with otherreference cell. For ballast tanks, the goal of the cathodicprotection designer will be to obtain this condition as quicklyas possible after each ballasting and during ballasted periods

for a defined lifetime of the anodes.

Cathodic protection may induce hydrogen stress cracking inextra high strength steels with specified minimu m yieldstrength >550 MPa, which are not used in commonshipbuilding. Potentials more positive than - 1,05 V Ag/AgCIare generally recommended.

2.9.1.5

The average current density demanded to obtain full cathodicprotection according to 2.9.1.4may be as given in Table 2.6.

~ ~ ~

DETNORSKEERITAS




Table 2.6 Average current density

Area CurrentdensiymA/m

loo lo

120

100 - 110

80 - 90

Clean ballast tanks (bare steel)Upper wing tanks (bare steel)

Fore and aft peak tanks (bare steel)

Lower wing tanks and double bottom tanks (bare steel)

Cargo or dirty ballast tanks (bare steel)

Paint coated surfaces (in intact condition)

Soft coats (see 3.1.2. in intact condition)

.____

;m -- 1 0

20 - 40

2000

A1 or Zn based anode alloys sho uld be of recogn ised type,known to be efficient in ships. The current capacities andconsumption rates, respectively should be as given in Table2.7.

Table 2.7 Current capacities and consumption rate

Anode and environment Current Consumptioncapacity rate kg/AA w 3 Year

A1 based anodes in sea water

A1 based anodes in marine

2300 - 2650

1300 - 2300

3,3 - 3,8

3,8 - 6,7sediments

Zn based anodes in marinesediments

I I 1760-780 I 11,2-113 I' I Zn based anodes in sea water

750- 780 11,2- 11,7 II

The current density demand to horizontal upwards facingsurfaces in combined oil cargo or ballast tanks can be higher,e.g. up to 200 mA/m2(2) and more.

For short ballast voyages the current densities should beincreased by installing more an odes, e.g. for voyag es shorterthan 5 days, the above current densities should be increasedby 25 % (if the future trade ro utes of the ship are known thecathodic protection system may to some degree be designedfor specific ballasting routines).

2.9.1.6

For cathodic protection current output calculations aresistivity of 25 ohm cm can be used as an average figure forsea water ballast. For brackish or river wa ter, the resistivitycan be 100- 200 o hm cm (allowance for increased resistivitycan be made if the trading routes are known).

2.9.2 Sacrificial anodes - ballast tanks

2.9.2.1

Anode alloy m aterials based on aluminium or zinc areacceptable. Magnesium based alloys are not acceptable.

Ballast tanks adjacent to tanks for liquid cargo with flashpoint c 60 "C are considered as gas dangerous areasaccording to the D NV Rules for Ships. Aluminium alloyanodes are to be so located that a kinetic energy of I 2 7 5 J is

developed in case of their loo sening and falling down, i.e.

H 28/W,

where H (m) is the height above tank bottom, deck orstringer, W (kg) is the anode gross mass.

It is a presumption for the abov e calculation of H that thelargest diameter of holes or sca llops in deck or stringer is lessthan 1/2of the ano de lengtb. From corrosion and safety pointof view, welded anodes are preferable. Bolted aluminiumanoy anodes w ith at least 2 through-bolts per anode anddouble locking nuts (for easy renewal) are, however,acceptable.

WI = -

El

L =effective life of the anodes (years)

W =net mass of one anode (kg)

E =consumption rate (kg/A year)

I =mean current demand per anode (A) during the lifetime,based on mean current density.

Sacrificial ballast tank anode systems should be designed tolast for minimum the planned docking interval and can bedesigned for 3,4,5 r more years effective life, according to

need.

2.9.2.4

The current output capacity i of the anode should becalculated from Ohm's law

i = U / R

U =the driving voltage difference betw een anode and

cathodically protected steel

R =the anodic resistance, to be calculated based on theanode geometry by a recognised formula (5 ) .

2.9.2.5

Anode size and distribution should be based on the abov einformation and calculations. Th e anode distribution, type,weights and dim ensions should be show n on drawings.Sacrificial anode systems should be delivered by arecognised company with good references.

See also 5 .

DETNORSKE ERITAS




2.9.2.6

Anode renewals should be carried out well in time before theold anodes are fully consumed. T he design of renewalsystems should be based on the abov e principles modifiedaccording to experience gained with the actual ship.

2.9.2.7

Inspection of sa crificial anode installations can be carried outby the coating inspector, see 2.3 and 7.

2.9.3 Impresse d current systems

2.9.3.1

Impressed current system s should not be used in ballast tanksdue to developm ent of chlorine and hydrogen, which canresult in an explosion hazard.

2.10 Cathodic protection of external hull

2.10.1 Cathodic protection system design basis - externalhull

2.10.1.1

The following should be specified:

--

size, shape and area of the extern al hullspeed and probab le trade or trading routes, includingtime in harbourspropeller(s)’ yp e, size, speed, m aterials and location(s)

propeller shaft material, and if grounded to the hull

sea wa ter intakes, sea chests, and any other hullaccessories.

--- rudder and rudder accessories-

All external hull items in electrical contact with the hull shallbe included in the cathodic protection system. The propellerwill be insulated from the hull when ro tating. Recognisedslip-rings and brushes shou ld be installed to include thepropeller in the cathodic protection system.

2.10.1.2

The criterion of catho dic protection is as per 2.9.1.4.

2.10.1.3

Average current densities needed to obtain full cathodicprotection of well coated ships’ hulls w ill usually be ab out 1 0mA/m2or more. In special cases, e.g. on ice break ers, up toabout 60 mA/mZmay be needed. The current density demandwill vary depen ding on factors mentioned under 2.10 .1.1,and w ill be different at different locations of the hull and itsaccesso ries such as propeller, rudder, and sea chests.

2.10.1.4

Wa ter resistivities: See 8.

2.10.2 Sacrificial anodes - external hull

2.10.2.1

Aluminium or zinc anode m aterials should be used.

2.10.2.2

Typical current capacity or consumption rate figures, lifetimeand current output capacity calcula tions are describe d under2.9.2.

2.10.2.3

Ano de size and distribution should be based on theinformation provided from 2.10.1.1 and calc ulations asindicated in 2.9.2.3 - 2.9.2.4. The anode distribution, type,weights and dime nsions should be shown on drawings.

2.10.2.4

Ano de renewals should be carried out well in time before theold anodes are fully consumed, preferably during docking.

2.10.3 Impresse d current systems

2.10.3.1

The size, number of and distribution of impressed currentanod es should be based on information as per 2.10.1.

2.10.3.2

The impressed current system design and installation,including anode alloy type, design, location and distribution ,reference electrode s, rectifiers, cabling , hull penetrations,cofferdams, monitoring units, anode shields, etc. should bedelivered by a rec ognised specialist compan y with goodreferences.

2.10.3.3

Impressed current systems should be checked and adjustedby the supp lier regularly for prop er functioning , e.g. every2nd year.

3. Ships in Operation -Maintenance ofCorrosion Protection Systems

Ships of some age can have corrosion prevention systems invarying condition. Systems suitable for new buildings maynot be so for ships in service. In the follow ing some methodsfor prevention or limitation of corrosion attacks aresuggested for ships in service.

3.1 Ballast tanks and hull internals

3.1.1 Maintenan ce coating - general

Maintenance coating should be carried out beforebreakthou gh of rust reaches 1% of the surface area: Welds’and edge s’ areas, respectively plain and large surface area s,considered separately. This corresponds to Rust G rade 6,

ASTM D 610 (9) or =rust scale Ri 3, I S 0 Standard 4628/3.The ASTM D 610 has a very sim ple and useful figure forestimation of area percentag es and is reproduce d as figure9.1 in these guidelines. More standards on evaluation of thecondition of worn or damag ed coatings may be foun d in

Appendix 2.

DETNORSKEERITAS




Allocation

Ballast tanks andhull internals

As above

2000

Coating type Total average DFT micron Number of coatsminimum

Epoxy based, "surface tolerant", "high tech", 300 - 350 1 - 2

Other recognised system 300, or as recommended by 1 - 2

"mastic", etc., preferably light coloured

manufacturer

When rust is breaking through on as much as 1 % of thecoated area, much of the remaining coated surface is soondue for the sam e. When possible, maintenance coatingshould be started earlier.

Often those areas which are most in need of protection will

suffer the hardest from coating breakdown: W elds, burns andedges of cut-outs, etc. The reason is the com monly occurringsubstanda rd surface prepa ration of welds, burns and edg es,see 2.2.

Maintenance coating may be carried out with the sam ecoating system as originally used on the new building. Thesurface treatment and hum idity or temperature conditionsrequired for a satisfactory result are, however, not alwayspractically obtainable.

The shipowner has to options for maintenance coating:

- dry-dock repairs, or

- repairs at sea.For ballast tanks, according to IMO 12), maintenance of thecorrosion prevention system shall be included in the ship'soverall maintenance scheme.

Mud, sludge and foul water in the bottom of tanks should beremoved on a planned and continuos basis in order to preventpitting corrosion, bacterial growth and development ofbacterial corrosion. If bacterial corrosion is superposed onthe common electrolytic corrosion, the corrosion rate can bevery high. This phenomenon is especially relevant in d oublehull tankers' cargo tanks, due to prolonged period of ele vatedcargo temperature (isolating effect of empty ballast tanks).

Special coating systems designed to be more tolerant oflower quality surface treatment and humidity or temperatureconditions may be m ore effective. In the Table 3.1 are listedexamples of paint coating systems designed for maintenancecoating. The list is not m eant to exclude other recognised ornew developed systems. Documentation of obtained results

should be requested before selection of coating type.

The steel surface cleanliness should be m inimum St 3

according to I S 0 8501-1, or degreased, clean, sound coatingor shop-primer, and as dry conditions as practically possible.

Salt contamination is a comm on problem in m aintenancecoating. Salts on surfaces underneath applied coating willpromote early blistering due to osm osis. Salt should beremoved a s far as possible by mea ns of fresh water washing.The salt content on surfaces to be coated should at least bebelow 60 and preferably below 30 mg/m2 (as NaCl). Arecognised test method for salt is IS 0 8502-9.

The significance of condensation of m oisture on steelsurfaces and its relation to air humidity and temperatureconditions must be understood by the personnel involved, so

that adequate ventilation, and dehumidification if necessary,is provided.

It is important that repaired, welded or heat affected zoneareas are given adequate surface preparation and are properlycoated, otherwise accelerated galvanic corrosion attack mayoccur.

The ship's safety and tank entry procedures must be strictlyadhered to during work in connection with maintenance

coating, Personnel must be adequately trained in safe usageof all equipment.

Table 3.1 Maintenance coating

I 1 - 2

As above Semi hard or similar coating, approved According to manufacturer's

recommendationNotes to Table 3.1:- Coatings with aluminium A1 content above 10% in the dry film shall not be used in gas hazardous areas.

- Maintenance coating systems should be specially developed for use on non-blast cleaned surfaces.

- Cargo holds for clean cargo should be maintenance coated with the same system as used originally, and the coating must becompatible with the original coating and the cargo.

General guidelines for inspection m a y be found in 2.2.4 and 7.

Regarding selection of coatings sufficiently flexible on sun heated or hot surfaces and coatings for high strength steels, see 2.3.

3.1.2 Surface preparation - hydro-jetting (grit injection available)

Several methods of surface preparation are relevant, and in1999 the preferred of these seem to be:

- ultra-high pressure hydro-jetting (grit injection

available)

- slurry blasting- power tool cleaning (rotary grinders, wire brushes, - grit blasting

needle gun, etc.) - magnesium d e-scaling

DETNORSKEERITAS




2000

- hydrochloric acid descaling

- spong e-jet blasting.

Power tool cleaning is suitable for sm all repairs. A re alisticuseful life span range obtainable with a good coating may beabout 2 - 5 years.

Hydro-jetting with w ater pressure above about 100 MPa w illgive varying results depending on nozzle design, speed ofopera tion, etc. Loose rust, scale and coating will generallycome off, while mill-scale and hard, black rust (magnetite-scale) will not be removed. Expected useful life for good ,surface tolerant coatings can be 5 years or more. (So called"mo isture tolerant" coating s may not ye t be sufficientlydeveloped to be successful for such use.)

Ultra-highpressure hydro-jetting with pressures above about200 MP a gives faster and usually be tter results than theabove method. D epending on the cleanliness achieved, theexpected useful life of good coatings applied on surface s

prepared by mea ns of this method may be v arying but insome cases up to 10 years.

Sl ur v blasting is similar to dry grit blasting but w ater is usedas propellant instead of air. The advantages are reduced dustand salt levels, the disadva ntage is that the surface is wetted,which implies re-rusting. The expected useful life of a goodcoating may be about 5 years but strongly dependent on thedegree of re-rusting.

Grit blasting is technically the best method for obtaining adurable coating. How ever, grit-blasting may often not befeasible for m aintenance coating. Special grit blasting unitssuitable for maintenance work are, how ever, available; e.g.vacuum, back-pack, mini-pot, etc. equipment. Expecteduseful life of a good co ating applied on grit blasted surface ina m aintenance-situation may b e 10years or more.

Magnesium descaling may give a surface suitable for asurface tolerant coating if white calciudm agn esiumcarbonate powder formed during descaling is quicklyremoved by fresh water wash ing. Large am ounts of hydrogengas formed during descaling can represent a safety hazard ifnot properly ventilated, and mag nesium me tal itself is a veryreactive metal, e.g. thin flakes or chips may catch fire.Expected useful life of a good coating may be 2 - 5 years.

Hydrochloric acid descaling (diluted HCl) may be used fordescaling. Hydrochloric acid represents also a potentialsafety hazard. All traces of acid m ust be removed by freshwater washing befo re coating. Expe cted useful coating lifemay be about as for magnesium descaling, 2 - 5 years,depending on e.g. re-rusting and dryness of surfaces duringcoating application.

Sponge-jet blasting is a newly developed m ethod that maypossibly be used for preparing block joint surfaces andsimilar. No records giving indications on expected usefulcoating life are available.

3.1.3 Soft coats, semi hard coatings, inhibitors

This category of products is of different chemical origin andhave different properties. Terms such as for example "softcoats", "semi hard" coating and "inhibitors" may occur. Softand semi hard coatings may be based on for examplepetroleum oils, vegetable oils or wool g rease (lanolin).

The basic idea of soft and semi hard coats or coatings isusually that they shall be able to pene trate rust and adhere tonon-blast cleaned steel, due to their content of su rface activechem icals. They should stay soft or sem i hard, i.e. flexibleand non-brittle. The corrosion protection mechanism may bedue to chem ical constituents acting as corrosion inhibitorsand/or in combin ation with a barrier effect. They are usuallyintended primarily for maintenance coating.

- "Soft coat" may be defined as: Co ating that remains softso that it wears off when touche d; often based on o ils orsheep wool grease.

Semi hard coating" may be defined as: Coating whichdries in suc h a way that it stays soft and flex iblealthough hard enough to touch and walk upon."Inhibitors" for corrosion protection are gene rallychemicals having an inhibiting effect on c orrosion andcan be of film forming (i.e. coating), anodic or cathodic

type.

-

-

Soft and semi hard coating s will norm ally be of shorterdurability than com mo n paint coatings and w ill normallyhave to be renewed annually or every 2nd year.Manufacturers of such coatings should normally have readymade procedu res for re-coating.

A draw back with some of these coatings, notably woolgrease based soft coat$ is that hot work or w elding on theoutside or inside of coated plates may ca use fire orexplosions due to gas development from the coating whenheated. Careful removal of the coating is thus necessarybefore any hot work is carried out.

Another drawback, also most significant with wool g reasebased soft coats, is that the coated surfaces stay soft andslippery and make inspection wo rk e.g. in ballast tanksdifficult and dirty. For this reason IACS (13) has issued arecomm endation that practically will rule out the use of softcoats in ballast tanks.

Some soft coat products are applied in relatively highthickness, e.g. 1 mm and above. These will impose thegreatest difficulties with respect to clean ing, hot w ork, accessand possibilities for inspection .

The heat resistance of the co ating products should becarefully checked before application in ballast tanks,considering that sun heating, adjacent cargo tanks and engineroom may cause elevated temperature,

Semi hard coatings approved by DNV will be accepted foravoida nce of annua l survey of ballast tanks. How ever, softcoats will not qualify for relaxation of requirem ents for

annual examination of w ater ballast tanks. t is also decided(1995)that DNV approval will no more be issued for softcoat products, due to the drawbacks described above.

DETNORSKEERITAS




Epoxy based, "surface tolerant","high tech", "mastic",etc.,

+

2000

300 350

Classification survey with steel thickness measurements if

necessary should be carried out after cleaning but prior toapplication of any maintenance coating, in order to verify

that the hull structure is in sound condition.

Antifouling paint

including coal tar epoxy, vinyltar, chlorinated rubber tar, etc.

Antifouling paint

Other recognised system,

+

Coated tanks will normally have to be properly cleaned

before tank surveys, e.g. the coating is to be removed incritical areas. The extent of cleaning will be to the discretionof the surveyor. It should be considered that some coatingsmight be more difficult to clean off steel surfaces than others.

Varying Varying *

manufacturer300, or as recommended by 1 - 2

Varying Varying *

3.1.4 Other coatings

Several coatings have been marketed for application to non-

blast cleaned surfaces but with little documented success.

Relatively new products within the groups "surface tolerantepoxies" and "rust converter +" may seem promising.

Documentation of obtained results should be requestedbefore taken into use.

3.1.5 Cathodic protection

Sacrificial anodes of zinc or aluminium may effectivelyreduce corrosion in the under water region of ballast tanksfor the ballasted periods. They can prevent pitting corrosion

in tank bottom areas mostly flooded with water. Sacrificialanodes will not have any effect on areas not submerged in

water. They should preferably be used in combination withcoating, to reduce the protective current demand and increaseanode life (care is to be taken such that anodes are not

covered by coating).

Sacrificial anode systems installed in ships in service should

be designed in accordance with the principles of these

guidelines as described in 2.9. (Too few and/or too smallanodes have often been installed in ships tanks.)

Impressed current systems should not be used in ballast

tanks, see 2.9.

Generally, anodes should be renewed before the sacrificial

material has disappeared. If designed for a defined usefullife, anodes should be renewed in accordance with the

predetermined schedule.

3.2 External hull

3.2.1 Maintenance coating

External hull coatings should be renewed when necessary.When the necessary control with blast cleaning, humidityand temperature conditions during coating application can

not be obtained, special coating systems designed formaintenance may be used. Examples of such systems areindicated in Table 3.2.

3.2.2 Cathodic protection - maintenance - renewal

Sacrificial anode systems should be renewed in accordancewith the principles outlined in 2.9 and 2.10.

Impressed current cathodic protection systems need regularspecialist survey by the supplier for maintenance of proper

functioning.

Table 3.2 Maintenance paint coating systems - external hull

Allocation I Coatingwpe I Nominal DFT micron I Number of coats minimum

External hull

External hull

Notes to Table 3.2:

I 1 - 2

- On submerged areas anti-fouling paint should be applied

- Tar containing coatings have shown good performance as corrosion protection in under water applications but are now prohibited inseveral countries due to their carcinogenic properties.

*Development are in progress and will continue due to restrictions on the use of antifouling containing tin.

If the coating is found in Poor condition, or where coatingwas not applied (relevant for some ships built before thegeneral reauirement of coating ballast tanks was introduced).

3.3 Assessment of coating cond ition in ballast tanks(Good, Fair, Poor)

Y u ,,

Ballast tank coatings will be assessed by the classification

surveyor as being either in Good, Fair or Poor condition.

If the coating is found in Good condition, the extent of close-

up examination and steel thickness measurements requiredby the classification society may be reduced.

retention of class will be subject to the tank in question beingexamined at annual intervals.

DETNORSKEEREAS




Item con sidered or comparablestandard

2000

~ ~ ~

Coating condition (Limit o r rating)

Good Fair Poor

To avoid the requirement of annual exam ination, the ballasttanks with coating in Poor condition will have to be re-coated. Due to high costs of carrying out proper surfacepreparation for app lication of an epoxy based, hard coating,semi-hard or similar coating m aterials have been especiallydeveloped for the m aintenance coating market of ageing

ships' ballast tanks. In ships classed with DN V, such coatings(other than hard coats) are required to be Typ e Approved.

Type A pproval is offered as a service to coatingmanufacturers. The criterion for Typ e A pproval is, briefly,satisfactory performa nce in either actual field exposure(ballast tank) for a minimum of 2 years, or laboratory testingby approved methods. Coating materials that are Typ eApprove d by DN V are thus products of a high and definedquality (7, 14).

Table 3.3 Coating conditions

ASTM D 610

(all coating breakdown and rustadded, see fig. 9.1)

I S 0 462813

(area rusted)

The three main coating conditions are commonly defined bythe classification societies as follows:

1 % 20% >20%

Rust Scal e Rust Scale Rust Scale

Ri 3 ( 1 %) Ri 4 (8%) >Ri 4 (8%)

--

Good: Condition with only minor spot rusting.

Fair: Cond ition with local breakdow n at edges ofstiffeners and weld conn ections and/or light rusting over

20 % or mo re of areas under conside ration, but less thanas defined for Poor condition.

Poor: Con dition with general breakdown of coating over20 % or more of areas or hard scale at 10 % or more ofareas under con sideration.

-

The definitions of Good, Fair and Poor coating conditionscan be interpreted as given in Table 3.3.

I

I spot rust I minor ( 1 %) I I I

I Light rust (surface rust) I minor(1 120% I > 2 0 % IEdges and welds, coatingbreakdown

minor ( 1 %) >20%I 2 0 % II Hard Scale ' I minor(1 %) I 10 % I > l o % II General coating breakdown I minor (1 %) I 2 0 % I >20% I

A sim plified interpretation for practical purposes may be thatcond ition Good can be given if all observed coating damagesand rust spots added together make less than 1 % of thesurface area considered when compared with the A STM D610 figure, reproduced as figure 9.1 in these guid elines,Appendix 5 .

Condition Poor should be given if all observe d areas ofcoating damage and rust added together make more than 20% of the area under consid eration when compared with thesame Figure 9.1 in these guidelines.

Accordingly, condition Fair can be given for correspondingarea percentages 1 - 20 %.

4. References

(1) L. L. Shreir, Corrosion, Newnes-Butterworths, 1979.

(2) NTNF-prosjekt Extended Lifetime for Ships, DNVReport No. 89-0205 dated 5.5.91, NTNF MV.24918.

(3 ) R. Sund by, Syn punk ter pB k orrosjonsskydd i

havsvattenkylda varm evaksler, Stal-Lava1Turbin AB, 197 3.(In Swedish)

(4) NACE Corrosion Engineers Reference Book, NACE1979, after F.L. LaQue, M arine Corrosion Causes andPrevention.

( 5 ) Recom mended Practice RP B40 1 Cathodic ProtectionDesign, DNV , 1986 and 1993.

(6) Rules for Classification of Ship s, DN V, last edition.

(7) Type Approval Programm e No. 1- 602.1 for Protective

Coating Systems, DNV , 19 99.

(8) G.H. Brevoort and A.H. Roebuck , Selecting Cost-Effective Protective Coa ting Systems, M aterialsPerformance, February 1 991.

(9) ASTM D 610 - 85,Evaluating Degree of Rusting onPainted Steel Surfaces. ASTM Annual B ook of S tandards,Vol. 06.01.

(10)Camrex Limited, Manual of Tank Coating - Procedures& Standards. Camrex Limited, Washington, UK.

(1 1) Y. Akita, Ships damages and the counterplans to protectships. Nippon Kaiji Kyokai. Paper presented in London , June1980.

DETNORSKEERITAS



22 RecommendedPractice

2000

(12) International Maritime Organization,IMO. esolution

A.798(19) adopted 23 November 1995. Guidelines for the

Selection, Application and Maintenance of Corrosion

Protection Systems of dedicated Sea Water Ballast Tanks.

(13) International Association of Classification Societies

IACS, RecommendationNo. 44, urvey guidelines for tanks

in which soft coatings have been applied, 1996.

(14) E. Askheim, Ballast Tanks&Cargo Holds, Protective

Coatings Europe, June 1997.

DETNORSKEENTAS



Recommended Practice

2000

Guidelines, Chapter and subject

2.1,2.2.3 Planning and access forinspection

23

Item, briefly

Permanent and temporary staging andpassages through hull structures to be

5. Appendix 1: Brief Review of the DNV Rules for Ships (2000) and IM O Guidelinesregarding corrosion protection of ships

5.1 Classification requirements

Th e below is a br ief review of DNV class requirements forcorrosion protection, presented in the same order of

occurren ce as the related su bjects are treated in these

guidelines. The detailed requirements are stated in th e DN VRules for Classification of Sh im (See re fe rence ( 6 ) ) .

2.9 Gas produced by anodes

3.1 Coating, Ships in Service

Vent pipes are to be installed both fore andaft in tanks w here sacrificial anodes areinstalled.

Corrosion protection system definition .

Definition of coating co nditions Good, Fair,

and Poor.Annual survey, coating.

Intermediate survey, coating.

Renewal survey, coating.

I provided.

2.2.1 Shop-primer To be approved with respect to that it willhave no detrimental effect on welds.

2.3 to 2.7 Coatin g, newbuildings All steel surface s are to be co ated, except intanks other than ballast tanks.

Tanks for ballast water are to be protectedwith effective coating.

2.3 to-2.7 Coating specifica tion Items to be described in a coatingspecification are outlined.

2.6 Co ating, holds in bulk carriers

2.3 to 2.7 Coating containing aluminium

Corrosion protection to be speciallyconsidered.

Details regarding coating of cargo holds.

Use in gas hazardous areas limited to A1content maximum 10 % by weight in the dryfilm, due to sparking hazard.

2.9 Sacrificial anodes’efficiency in ballasttanks

If anodes are to installed, calculation detailsand distribution drawings are to besubmitted for, respectively, information andapproval

2.9 Sacrificial anodes’ astening andinstallation

To be approved with respect to fastening ingas hazardous areas, e.g. tanks adjacent tooil cargo tanks. Maximum kinetic energy27 5 J developed in case of anodes fallingdown.

DN V Rules for Ships, reference (2000)

Pt.5 Ch.3 Sec.3 D300

Pt.7 Ch.2 Sec.1 E200

Pt.3 Ch.1 Sec.18 B201

Pt.3 Ch.1 Sec.18 BlOlPt.3 Ch.1 Sec.2 D201

Pt.3 Ch.1 Sec.18 A200

Pt.3 C h.2 Sec.14 A200

Pt.3 Ch.1 Sec.18 BlOl

Pt.5 Ch.2 Sec.5 2500

Pt.3 Ch. 1 Sec. 18 B202

Pt.3 Ch.1 S ec.18 A200

Pt.3 Ch.1 Sec.18. B301 to B304

Pt.4 Ch.1 Sec.4 KiO l

Pt.7 Ch.2 Sec .2 A108

Pt.7 Ch.2 Sec.2 A109

Pt.7 Ch.2 Sec .2 B601Pt.7 C h.2 Sec.2 C102, C303 and C400

Pt.7 Ch.2 Sec.2 Dl0 0

organisational matters concerning shipowners, shipyards,

f lag state adm inistration, c lassif ication societies and others.

me c l as s if ic a tion soc ie ty ’s i nvolvem e n t i n f o ~ ~ o w - ~ pf

ships’corrosion protection systems is currently beingdiscussed.

5.2 IMO Guidelines - Corrosion protection

Selected i tems of the lM0 guidelines concerning corrosion

protection in such forms as they appe ar in January 1996, re

presented in the below table. The guidelines deal with

technical aspe cts of corrosion protection as well as

DETNORSKEERITAS




2000

Table 5.2 IMO G uidelines - Corrosion p rotection

IMO reference

Res. A.798 ( 19)adopted 23Nov. 1995

Guidelines - Heading and selected items

GUIDELINES FOR THE SELECTION, APPLICATION AND MAINTENANCE O F CORROSION PROTECTIONSYSTEMS OF DEDICATED SEA WATER BALLAST TANKS

invites governments to apply the guidelines a.s.a.p. to new bulk carriers and oil tankers

shipyard and / o r its subcontractors should provide clear evidence of their experience in coating application

coating stand ard, jo b specification, inspection, ma intenance and repa ir criteria should be agreed by the shipyardan do r its subcontractors, owner and manufacturer, in consultation with the Administration or an organizationrecognised by the Administration

multi-coat treatments with coating layers of different colours are recommended

use of a hard coating is the most common practice

the effectiveness of a hard coating can b e achieved only if the manufacturer's technical pro duct data sheet and jo bspecification is carefully followed

the last layer of each co at shou ld preferably be of a light colo ur in order to facilitate in-service insp ections

due regard should be given to the possible poor edge covering of hard coatings with a high solids content

the surface preparation should be in accordance with the coating manufacturer's specifications and recommend ations

the conditions under which blast cleaning is performed should preclude condensation

it is not recommended to carry out blast cleaning w henthe relative humidity is above 85 %, o r

the steel surface temperatu re is less than 3 "C above the dew point, or

there are traces of moisture, or condensation occurs before the primer coat is applied

inspection relevant to surface preparation and coating application should be agreed upon between shipowner andshipyard under the manufacturer's advice

activities that shou ld be overseen , inter alia, are

working conditions, e.g. illumination, access, staging, etc.

environmental conditions, e.g. temperature and moisture

removing of sharp edges

blast cleaning or mechanical cleaning

cleaning up after blast cleaning

shielding of painted surfaces from blasting operations

coating application equipment

curing times for intermediate coats in relation to temperature and humidity

cleaning of coated surfaces before application of next coat

handling or storing or transport of coated objects

coating repairs, when damaged

precautions are to be taken to reduce health, fire, explosion an d other safety risks which should be in accord ancewith the regulations of the Administration

cathodic protection by means of sacrificial anodes may be used in combination with coating to prevent or reducepitting corrosion starting from local defects in the coating

anodes should be designed in terms of size, weight, and distribution to give an adequate life commensurate with theservice period

anode design documents should be available for maintenance purposesalternative corrosion protection methods may be used, provided they give the same level of corrosion protectionaccomplished by means of hard coatings

maintenance of the corrosion protection system should be included in the overall ship's maintenance schemes

the effectiveness of the corrosion protection system should be verified during the ship's life by the Administration oran organization recognised by the Administration

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2000

6. Appendix 2: Properties and Test

Standards for Coatings

Some recognised standard methods for optional use inquality control, testing of properties and evaluation of the

condition of coatings are listed below. Other test methodsmay be used as a well.

Abbreviations:ASTM: American Society for Testing and MaterialsBS :British StandardDIN :German StandardIS0 :International Standardization OrganizationNACE :National Association of Corrosion Engineers (USA)SIS :Swedish StandardSSPC :Steel Structures Painting Council (USA)

6.1 Quality control tests for steel surface treatment

and coating applicationCoating adhesionProper coating adhesion (bonding) to the steel surface andbetween individual coats is most important for the qualityand durability of the coating. Inadequate adhesion results in amechanically weakened coating layer which may soon belifted, blistered or peeled off by moving water, weatheringactions, impacts or traffic.

Proper adhesion is obtained by following the aboveguidelines for surface preparation and coating application.

Criteria for minimum acceptable adhesion may be specified,

referring to either a cross cut test or pull-off test. Adhesiontesting is, however, destructive and is normally used only in

cases of complaint, not as a routine test.

Adhesion (bonding) may be tested by the pull-off method,cross cutting or tape test, manual peel testing, etc.

Standards for cross cut test, e.g.: IS0 2409, ASTM D 3359,BS 3900 part E6.

Standards for pull-off test: IS0 4624, BS 3900 part E10, DIN53232.

Coating dry film thickness DFT

The minimum dry film thickness DFT should be stated foreach coating layer and for the full coating system. If it isconsidered more practical to specify the average DFT, itshould be increased so that it will comply with a statedminimum DFI'. See examples discussed in 2.4 of theseguidelines.

Dry film thickness DFT test methods:Electromagnetic and magnetic type instruments are used forcoatings on steel. Eddy current based instruments may beused on non-magnetic substrates.

Standards concerning dry film thickness measurements:

I S 0 2808, BS 3900PartC5, BS 5494, ASTM D 1005, D1400, D 2691, SIS 184160.

Due consideration to surface profile should be taken whencalibrating DFI' measuring equipment.

Blasted steel surface cleanliness and profileVisual, pictorial standards for surface cleanliness are usuallysufficient, such as

---

IS0 12944 is a relatively new standard, 1stedition 1998,comprising Paints and Varnishes for corrosion protection ingeneral, consisting of 8 parts. The part no. 4 concerns steelsurface preparation. National and EN standards are issuedwith the same number and same content.

The grades Sa 3 and 2,5 approximately correspond to,respectively,

IS0 8501-1, with grades Sa 3 and Sa 2,5

SIS 055900, with grades Sa 3 and Sa 2,5

DIN 55928, with grades Sa 3 and Sa 2 3.

-- BS 4232, grades First Quality and Second QualityNACE, grades No. 1 (white metal) and No. 2 (near-

white)- SSPC, grades SP 5 (white) and SP 10 (near-white).

Surface roughness profile after blast cleaning can be checkedwith a surface profile gauge, or a "Rugotest" set or similarfor visual comparison, commonly Rugotest no. 3. It isdescribed in I S 0 8503. Commonly used surface profiles forcorrosion protection paint coatings are 60 - 90 microns Rwhere R is the peak to valley profile depth.

Mechanically cleaned steel surfacesThe common minimum cleanliness standard for mechanical

cleaning is St 2.

The St 2 cleanliness standard is described in the abovementioned standards IS0 8501-1, SIS 055900, DIN 55928,etc.

Wet film thickness WFTWFT is usually measured only by the coating applicator.Rollers and comb type of equipment are in use. The dry filmthickness DFT can be estimated from theWFT:

DFI' =WFT x volume % solids/100.

Relevant standards for measurement of WFT:

ASTM D 1212,BS 3900 Part C5, BS 5493.

Holiday detection or spark testingHoliday detection, or spark testing, or continuity testing, isnot commonly used on paint coatings but can be essential forlinings for chemicals, pipe coating, and other critical coatingor lining applications. The equipment must be calibratedstrictly according to the manufacturer's instructions and duly

considering the coating or lining type and thickness. Lowvoltage equipment only is relevant for paint coatings.

A recognised standard indicating test voltages versus coatingthickness is NACE RP-02-74. Useful advice is also given inBS 6374.

DETNORSKEERITAS



26 Recomm ended Practice

2000

6.2 Coating materials’ testing - thermal,mechanical and physical properties

The below tests for m aterials properties are listed foroptional use and are not meant to exclud e other test methods.

Heat resistanceCoatin gs with sufficient heat resistance should be selectedfor application to surfa ces exposed to elevated temperatures,such as bulkheads against heated cargo and decks expose d tosunshine. For exam ple tar containing coatings may containvolatile comp onents which tend to evapo rate when heated,rendering the remaining coating less fle xible and prone tocracking when aged .

Service temperature limits of coatings may be determined assoftening point, or by means of registering signs ofdegradation or decomposition, e.g. change of colour.

Standards for heat resistance testing:

BS 3900 Part G 7, BS 4164, BS 4692, ASTM D 2485.

Flexibility or relative elongationCoatin gs should have sufficien t flexibility or relativeelongation for the actual application. Loss of flexibility mayresult from prolonged exposure to elevated temperature. Ingeneral, the coating should have h igher relative elongationthan the steel it is applied to, considering even that somelocal deformatio n of the steel may o ccur. Coatings to be usedin ships tanks should have minim um 4 % relative elongationat relevant conditions of ex posure (ageing effects and asafety factor taken into ac count).

Test standards for coating flexibility:

Tensile elongation, free film: AST M D 2370.

Cup ping of coated plates (E richsen test):

IS0 1520,BS 3900 Part E 4, DIN 53156.

Bending over mandrel:

BS 3900 Part E 1,ASTM D 522, ASTM D 1737.

Abrasion resistance

For floor coatings and tra ffic paint abrasion resistance isimportant. Relevant test standards may be:

ASTM D 1395, D 658,968,821.

Drying time of coatingsTest standards:

ASTM D 3732,1640,1953,711, I S 0 1517, SIS 184153.

Binder/Pigment/Volatile/Solids ontentThe percen tages of the respective componen ts of a coatingmaterial may be determ ined by various standa rd tests, e.g.:

ASTM D 2697,2832, 1259,2621, IS0 3233.

Impact resistanceRelevant test standards (to be chosen according to type andthickness of coating):

BS 3900 Part E3 and E7, ASTM G 14, D 2794.

HardnessIf hardnes s testing is relevant, due concern must be given tothe type and thickness of coating. Releva nt test standards:

ASTM D 2134,2240, BS 4164, SIS 162201.

Flash pointThe flash point of a liqu id coating may be determined by e.g.tests:

ASTM D 93,3278,56,1310, DIN 53213,51755,51758.

ViscosityRelevant tests:

ASTM D 1200, DIN 51550,1342,53177.

Gloss

Tests for gloss of architectural paints, etc.:

ASTM E 430, BS 3900 Part D 50,DIN 67530.

6.3 Documentationof coating performan ceproperties - Type approv al of coatings

Independ ent records of s atisfactory perform ance as corrosionprotection in a relevant enviro nmen t should be requested andevaluated before selection of coating type and manufacturer.

DNV offers type approv al of protective coating systems tothe m anufacturers, see Type Approval Programme. The typeapproval is mainly based on indepe ndent docum entation of

the coating’s long time durability in real life exposure. Typeapproved coatings, their areas of use and requ iremen ts tosteel surface treatment are describe d in the DNV publicationsseries Type Approved Products.

Standards which may be used in evaluation of coatingperform ance or describing the condition of a coating:

IS0 4628, in which the part 3 is most useful, and AST M D

610, D 659, D 661, D 714, D 772, D 1654, DIN 53209, NS5400-5408,etc.

DETNORSKEEFUTAS




2000

7. Appendix 3: Coating inspector's duties -Checklist

The status and auth ority of the coating insp ector or team ofinspectors should be stated in the contract betwe en ship-

builder and owner. The following are indications of relevanttasks and equipm ent for the inspector that may be co nsideredin such contract.

The coating inspector should have documentedqualifications, including knowledge about health and firehazards concerning this work. Certification arrangements forcoating insp ectors are established in som e countries.

Sacrificial anod es' installation can be cov ered by the coa tinginspecto r. Installations of imp ressed current systems shouldbe surveyed by a represe ntative from the sup plier.

Th e coating inspector should be involved in the following

items described in the guidelines:

planningsteel surface preparation

coating selection and applicationcoating quality control and testing (of the testsmentioned in Appendix 2 only a few of those mentionedunder 6.1 are normal routine tests for comm on paintcoatings)sacrificial anodes installation, if any.

Deta ils of testing and quality control to be carried out by thecoating inspector should be defined. During coating

operatio ns the inspector's daily duties may be a s follows:Practical inspection workSurvey o f

illumination, access, and co mpartments for conditioningof tempera tures and m oisturebreaking or rounding of edges

blast cleaningcleaning up after blast cleaning

shielding off painted surfaces from blasting operations

conditioning of steel temperatures and air temp eraturecontrol of humidity and dew point calculation

coating application equipment and methodsdry film thicknessescuring times for indiv idual coats in relation totemp erature and hum iditystoring of coating materials and abrasives

specified type of coating to be appliedcleanin g of coated surfac es before ap plication of nextcoathandling or storing or transport of coated ob jectscoating repairs, when dam aged

sacrificial anod es, installation in accord ance withspecification and drawingssurvey that sacrificial anodes are not coated or o therwise

damaged.

Paper work

Reporting, including:

- documentation of the quality of coated surfaces versusthe quality specified

tests carried out (chec k of surface cleanliness, film

thicknesses, air and temperature controls)

docu men tation of installed numbe rs and locations of anyanodes, to be in accordance with specification anddrawings.

-

- deviations from specified quality-

Inspection equipmentThe coating applicator or coating subcontractor shouldnormally have the necessary inspection equipment on site.The most important equipment for the co ating inspector willbe:

- specification or pictorial standard s for steel surface

preparation

- pocket knife- psychrometer (for wet and d ry air tem perature

measurement) or hygrometerdew point calculator, electronic type

dry film thickness measuring equipment.

-- steel surface thermometer-

Useful additional equipment may be:

- inspection m irror- pull-off adhesion test equipment- Rugotest or surface profile gauge

- paint inspection gauge.

DET ORSKEVERITAS




2000

8. Appendix 4: Corrosivity and corrosionresistant materials - a brief review

8.1 The marine environment - corrosivity

The corrosivity of sea water varies relatively little in theoceans. Even in local areas like the Baltic Sea, where the saltcontent is considerably lower than in the large oceans, thereis enough salt to make the water an electrolyte and thuscorrosive. The resistivity of the water is a measure of itselectrolytic properties. Typical resistivities are for example

(see references (1 ) and ( 5 ) )

--

in open sea water, temperature about 25 "C, 20 ohm cm

in open sea water, temperatures below 10"C,30 ohmcm

in sea bottom mud or sediments, 75 - 150ohm cm

in brackish river water, 200 ohm cm

in distilled water, 500000 ohm cm.

---The corrosivity of sea water as regards general corrosion onsteel increases with increasing temperature, oxygen content,water velocity, content of corrosive contaminants, erodingparticles, and conductivity.

Localised corrosion, notably on stainless steels andaluminium alloys, will often be promoted by stagnant waterlow in oxygen.

Bacterial corrosion (also called micro-biological, anaerobic,etc.) may occur e.g. in ships' oil tanks, ballast tanks, cargopiping, etc., due to that local environments and conditions for

bacterial activity are prevailing. These conditions are, briefly

- stagnant (anaerobic) water- hydrocarbons nourishing bacteria, e.g. crude oil,

possibly some coatings or soft coats, etc.

sulphates present in sea water (the most commoncorrosion promoting bacteria utilise sulphate for"breathing" instead of oxygen)

ideal temperatures for bacterial growth (about 20 to 40

"C)sufficient numbers of bacteria to flourish under aboveconditions are often present in water.

-

-

-

The corrosivity of the marine atmosphere is dependent on its

content of chlorides, sulphates or sulphites and other aircontaminants, including soot and dust particles. Thecorrosivity thus generally decreases with increasing heightabove sea level.

The corrosion promoting effect of salts, dust etc. in themarine atmosphere is due to, mainly

--

increasing conductivity of moist film on metal surfaces

prevention of a moist film on metal surfaces from dryingout, and

breaking up passive, oxide films on e.g. stainless steelsand A1 alloys.

-

8.2 Corrosion mechanisms on steel surfaces inships

Common corrosion on steel in ships is of the electrolyticcorrosion type. General-, pitting-, crevice-, galvanic- orbimetallic-, intercrystalline-, erosion-, etc.- corrosion are allvariants of principally the same, electrolytic type. The

smaller the anodic (corroding) surface in relation to thecathodic (non-corroding) surface, the more localised,concentrated and rapidly occurring is the corrosion process,e.g. pitting corrosion rates may be very high. Corrosionfatigue and stress corrosion cracking is also of the

electrolytic type, though combined with mechanical action.

Dry, high-temperature corrosion may occasionally occurunder special conditions, as in engines.

For steel submerged in sea water the accessibility of oxygento the surface is governing the corrosion rate. This implies

that the corrosion rate for different steel grades in

submerged, static condition is approximately the same,independent of minor alloying elements.

High strength steels may more often than common steel besubject to high stress levels or cyclic stresses. This can implyincreased corrosion rates, due to the stresses as such (stresscorrosion cracking or corrosion fatigue), and also due to thata protective layer of rust is prevented from being formed atspots receiving high stress levels or fatigue loads. Fresh,unprotected metal surface is then continuously exposed to acorrosive environment.

Due to high local stresses and strains, the useful life of

coatings may be reduced. If so, the corrosion process mayget an early start.

Bacterial corrosion, if occurring, may proceed locally at highrates. Indications of bacterial activity are

rotten smell of hydrogen sulphide H2S (Caution: H2S in

high concentrations is odourless to humans. It is verypoisonous and also explosive)

corrosion may occur as smooth pitting

corrosion products are initially black coloured by ironsulphides

the black colour disappears rapidly in air due to

oxidation of the iron sulphides.For steel exposed to marine atmosphere the corrosion rate isgoverned by the rate of transfer of metal ions at the anode,which in its turn is dependent on the amount ofcontamination elements and alloying elements in the steel.The atmospheric corrosion rate will thus vary significantlywith the steel type.

8.3 Factors influencing steel corrosion rates inships

The corrosion rate on unprotected surfaces, like otherchemical reaction rates, generally increases with thetemperature.

DETNORSKEERITAS




2000

The corrosion rates in a ship can, however, be constant,increase or decrease with the time, dependent on e.g. thefollowing factors:

- a layer of built up corrosion products (rust) on a steelsurface will have a protective (coating) effect by limiting

the access of oxygen to the steel, thus lowering thecorrosion rate

a layer of corrosion products may render parts of the

surface cathodic in relation to other, anodic, parts of thesurface lacking such layer experiencing increasedcorrosion ratesurfaces exposed to vibrations and/or high stress levels

may have increased corrosion rates with time, due to thatthe thickness reduction of steel plates reinforcesvibrations and stress levels

macro-elements or large aeration cells caused byvariations in the oxygen concentration, e.g. at differentdepth levels in a ballast tank and over or under

sediments, may create anodic parts experiencingaccelerated corrosion and other parts cathodic, non-corroding

areas with locally degraded coating may become anodiccompared with areas with intact coating, resulting in

pitting corrosion.

-

-

-

-

Other, operational factors may also influence the corrosionrates in a ship, e.g.:

percentage of time in ballast or ballasting routine

moisture content of empty tanks

temperature of cargo or fuel in adjacent tanks

cathodic protection, application and design and anode

distributioncoating type and application, including steel surfacepreparation

maintenance of corrosion protection systems

structural design of ship and tanks

frequency and method of tank washing

clean or dirty ballast

cargo type and composition, including contamination

use and type of inert gas

trade, speed and sailing route

etc.

Depending on the above factors, unprotected steel internally

in ships ballast tanks may typically, allowing for greatdeviations, experience average corrosion rates (see reference(2))of the order of 0.2 - 0.4 d y e a r . Local corrosion, e.g.pitting and corrosion in way of welds, may proceed at muchhigher rates.

8.4 Metallic materials other than steel

Significant features concerning the corrosion or protectionaspects of a few important groups of metallic materials arebriefly mentioned below:

8.4.1 Stainless steels

The so called stainless steels owe their corrosion resistanceto a thin surface film of oxides, called a passive film. Theoxide film is cathodic compared with the base metal, andwhen broken, the adjacent base metal exposed to sea waterwill act as a sacrificial anode. Pitting will occur, often at a

high rate.

The passivity of the oxide film is dependent on oxygensupply and on the chemical-metallurgical composition of thebase metal. Notably the content of molybdenum must beabove a certain minimum to withstand depassivation bychlorides. Only a few types of "high molybdenum"- typestainless steels are resistant to sea water, i.e. those with Mo-contents above about 6 %, e.g. 0,02 % C, 20% Cr, 18% Ni,6, l % Mo, 0,2%N, 0,7 %Cu. At temperatures>15 "C,however, the 6% MO stainless steels can suffer from crevicecorrosion in sea water.

The most commonly used austenitic type stainless steels, e.g.0,03 % C, 18,5%Cr, 143% Ni, 3,3 %Mo (AISI 316 andrelated products) are not resistant to sea water. Modernduplex steels may be somewhat more resistant.

Pitting or other localised attacks in stainless steels are thusoften due to:

- lack of oxygen, e.g. in stagnant sea water, underneathdebris or adherent particles

local chemical or metallurgical surface defects, e.g.caused by welding.

It follows that success of stainless steels in sea water are

dependent on a non-intermittent flow of water and of thesurface being kept smooth and clean. Proper weldingprocedures must be strictly adhered to.

Further, stainless steels are more noble, i.e. of significantcathodic character compared with common ship constructionsteel. When sea water or moist marine air is present,unprotected black steel in electrical contact with stainless

will corrode more quickly than if it were alone, due togalvanic corrosion.

Stainless steel types used for chemical cargoes should beselected based on a critical evaluation of informationprovided by the manufacturers of the steel and of thechemicals. Close attention should be given to the weldingprocedures, and to the contaminants of th e cargo, which maybe more corrosive than its main constituent.

-

8.4.2 Aluminium alloys

The sea water resistance of Al-alloys depends, as forstainless steels, of a thin, cathodic film of oxides. If the filmis destroyed, the Al-alloy is likewise prone to pitting,according to the same mechanisms as stainless steels.

Contrary to stainless steels, Al-alloys are anodic, i.e. lessnoble, compared with black steel. In metallic, electrical

contact and exposed to sea water or marine atmosphere, theAl-alloy will corrode, sacrificing itself and protecting thesteel.

DETNORSKEERITAS



30

Alloy

AlMg2.5 - AlMg4

AlMg3Mn

AlMg4.5Mn

Recommended Pra ctice

2000

Si Mg Mn

maximum 0.5 2.2 - 4.6 maximum

maximum 0.5 2.4 - 3.4 0.3 - 1.0

maximum 0.5 4.0 - 4.9 0.3 - 1.0

0.4 - 0.8 *

Typical Al-alloy s used in marine constructio n are given inTable 8.1.

Tube material

Al-brass

Cu-Ni 90/10

Cu/Ni 70/30

8.4.3 Co pper alloys

Typical propeller casting materials are the Ni-Al-bronzes.

Maximumflow veloci& d s

2.1

2.4

3.0

AlMeSil I 0.6- 1.6 1 0.4 - 1.4 I 0.4 - 1.0.

The typical composition of Ni-Al-bronzes is minimum 78 %Cu, 8 - 11 %Al, 3 - 6,5 % Fe, 3 - 6.5% Ni, maximum 3 %

Mn and total other elements maximum 0.5 %.

Ni-A l-bronzes are resistant to high w ater velocities and puresea water. Like all other Cu-alloys, they are susceptible tosulphide polluted waters.

Filling of newly installed cuprous alloy piping systems withfoul harbour water should be avoided.

~ ~ ~

* maximum Mn content varying from 0.4% for Mg 2.5 to 0.8%for Mg 4.

Other elements in Table 8.1, maximum values, %: Cu: 0.10,

Fe: 0.50,Cr: 0.35, Zn: 0.20, Ti: 0.20,Other: Each 0.05, Total0.15.

Al-alloys, including the above , may suffer from localisedcorrosion such as pitting. Cast alloys of AlMgSi type arerelatively prone to p itting corrosion. The general corrosionrate in sea water is, however, norma lly low, i.e. ( 0.005

mt dye ar. Pitting may be initiated on e.g. A1 hulls of vesselswhen idle in harbours with stagnan t sea water, underneathmarine growth or anti-fouling coating.

The AlMg alloys may be subjected to stress corrosioncracking at contents of Mg >=4.5 % in strain hardened andstabilised condition. Ma terials for bolts and rivets shouldhave< 3.5 % Mg.

Galvan ic corrosion is well know n on A1 alloys in thepresence of an electrolyte such as sea w ater or a chloridecontaining film of moisture . Metallic contact between A1 ande.g. copper, nickel, chromiu m, stainless steels and mild steelshould be avoided in the presence of sea water or moist,marine atmosphere. A n n e.g. galvanised steel),AYcadmium and usually A ylead are harmless.

Stainless steel screws or bo lts are often used in A1 alloyconstructions. When subm erged in se a water or when a filmof moisture will often be prese nt at the surface, electricalinsulation is nece ssary between stainle ss steel and Al.Metallic contact Allstainless steel screw or bo lt may be

adequate in marine atmosphere at som e height above thewater level (e.g. som e helicopter deck constructions)provided the surface connectio n is kept dry most.of the timeby abundance of fresh air. AYstainless steel contacts areharmless in a dry , indoor atmosphere.

Copp er content in A1 alloys above the abo ve limit of 0,lO %may initiate intergranular corrosion.

AlSiCu and AlC uTi cast alloys should thus not be used inmarine construction.

AlZnMg alloys m ay be prone to stress corrosion cracking

and should thus be avoided.

Typical tube materials are

Al-brass :76 - 79 % Cu, 0.02 - 0.035 % As, 2 % A1 and Znrest

Cu-Ni 90/10: 10%Ni, 1.0- 1.8 % Fe , 0.5 - 1 % Mn

Cu-Ni 70/30: 30 %Ni, 0.4 - 1.0% Fe , 0.5 - 1.5 % Mn.

Copp er has relatively low chemical reactivity and iscathodic, i.e. of noble or inert character compared with steel.Th e corrosion resistance of Cu-alloys, however, oftendepend s on a thin film of surface oxides. If the protectivesurface oxide film on C u-alloys are broken down by too highwater velocity, erosion or contamin ants in harbour waters,corrosion attacks may occur.

For tube m aterials in heat exchang ers (see reference (3))recognised flow velo cities should not be exceeded to avoid

corrosion attacks as given in Table 8.2.

Joining of tubes by welding or other metho ds may intro ducelocal deviations from th e base material composition of greatconsequence for the corrosion resistance. Proper proceduresfor joining and in stallation should be strictly adhered to.

8.4.4 Galvanic series of metals and alloys

Joining together different metals or alloys may result in rapidgalvanic corrosion on the less noble alloy, which will act as asacrificial anode relative to the nobler, cathodic alloy.Necessary conditions for such corrosion are that the twoalloys stay in electrical contact and that sea water or a film ofmoisture is present. Insu lating flanges and bolts, coating, ordry conditions, will prevent galvanic corrosion fromoccurring.

DETNORSKEERITAS




2000

Below is inserted a table of galvanic series in sea water (seereference (4)) for relevant metals and alloys, with relativepotential differences (from F. L. LaQue, Marine Corrosion -Causes and Pre vention). For pra ctical purposes, galvaniccorrosion will usually not be sig nificant at potentialdifferences less than 50 - 100mV.

Generally, the surface ratio between an odic (less noble) andcathodic (more noble) metals will influenc e the galva nic(bimetallic) corrosion attack on the a nodic surface. Tw ocommon conditions are, however, worth considering: Thefully submerged and the moist surface film condition,respectively:

Fully submerg ed in bulk sea wa ter (electrolyte):

E.g. a stainless steel bolt (cathodic) with small su rface areain contact with a large surface area of aluminium (an odic) isnormally far less harmful than if the opposite situationshould occur, because the corrosion a ttack on the A1 alloysurface will be spread out evenly on a large surface.

Moist surface film (electrolyte):

Due to the low thickness of the m oist film, the surface effectof a large anode versus a small cathode will be can celled (nospreading effect of corrosion current through the bulkelectrolyte). E.g. a stainless steel bo lt in an aluminium alloyconstruction will give local corrosion attack on thealuminium adjacent to the bolt.

DETNORSKEERITAS




2000

-1-60.2 0 4.2 -0.4 4.6 -0.8 -1.0 -1.2 -1.4

L

10.416

toys

,347

Figure 8-1 Galvanic series in sea water

Volts: Saturzted calomel half-cell reference electrode

Note regarding Figure 8.1:

Alloys are listed in the order of the potential they exhibit in

flowing sea water (2,4 - 4 dsec, temperatures 10 - 27 "C).

Certain alloys indicated by black rectangles in low-velocityor poorly aerated water, and at shielded areas, may becomeactive and exhibit a potential near - 0,5 V.

8.5 Cargo and ballast handling - design againstcorrosion

Ballast water and cargoes often promote corrosion, as in thetypical cases of:

Tankers for oil: Acid water containing sulphurouscomponents from the oil may settle out in the bottom ofcargo tanks, ballast tanks and cargo piping, causing corrosionproblems.

DETNORSKEERITAS




2000

Ore carriers: Impacts from grabs may damage the corrosionprotection systems on both sides of exposed plates.

Ore carriers: Corrosion attacks may occur from wateracidified by the ore.

Ballast tanks: Heavily exposed to sloshing sea water, cyclicchanges of temperature and hydrostatic pressure, wetting anddrying, often of intricate construction, with difficult access,inadequate drainage, etc., invite corrosion to unprotectedsurfaces.

In designing and constructing of the ship attention should bepaid to cargo handling and ballasting operations, to facilitatedrainage, cleaning and drying up of empty tanks and holds.

Access must be ensured for application of adequate corrosionprotection systems where most likely needed. Details oftenoverlooked are for instance small cut-outs in corners, whichare made too small for proper surface treatment and fo raccess of painters’ spraying equipment. “Mice holes“ shouldbe enlarged to ”rat holes” with diameter minimum 100 mm.

DETNORSKEERITAS




. .

L

2000

In Figure 9.1 are show n some figures illustrating variousaspects of steel surface preparation and coating related to the. Appendix 5: Surfaces - Preparation,

Coating and Corrosion

9

3

I-

16%

subjects treated in these guidelines.

8 7

.

0,1%0,3%

2 1

33% 50%

The fig ure is referred to in the guidelin es for defining

--

Figure 9-1 Examples of area percentages

The black spots of the figure representing 3 % of the surfaceis denoted as Rust Grade 5 , those representing 1 % is

denoted Rust Grade 6. Figure copied from ASTM (see

reference (9)).

due time for m aintenance coating ( 1 %=Rust Grade 6) ,and

useful life of a coating (3 %=Rust Grade 5 ) .




2000

k-..

SharpEdge

Weld Spatter

Sharp Profile peaks to be smoothed usinggrinder.

Lamination

Undercut

ManualWeldBead

A. Remove by grinder or disc sander.as CutEdge

\ .

B. Rolledsteelsections normally have radiusededges. Thereforeca n be left untreated.

@ J

A . Remove spatter observed before grit-blastingwithgrinderor chipping hammer.

7

B . Fo r spatter not readily removed. Removeusing gn‘ndertdisc.

Remove using grinder.?/I ’/ / / ‘1.m

?,,,,,,7

Undercuts exceeding classification rulingshould be repairedby weldingandgrinding.

Figure 9-2 Preparation of steelThe figure (originally made by Camrex L imited (seereference (9)) illustrates impo rtant surface details to beprepared for coating application. All the above preparationsshould be carried out before blast cleaning.

DETNORSKEERITAS




2000

'8

3

1: TFigure 9-3 Stripe coating locations

(fromCamrex (see reference (9))

1: Inside edge of cut outs. 2: Edge part of stiffeners.3: Welding beads. 4:Whe re spraying is difficult.

DETNORSKE ERITAS




2000

C o a t i n g

coating

Increased plate thicknes5.Coating or wooden floor

for grain cargoes, etc .

Figure 9-4 Recommended extent of coating in cargo holdson bulk carriers

Regarding the extent of coating, see also the DNV Rules,reflecting IACS Unified Requirement. The inner bottomshould have increased steel plate thickness due tosusceptibility to corrosion and physical wear. Holds for grainand other cargoes to be kept clean should have coated inner

bottom.

Figure 9-5 Recommended extent of coating in cargo holdson bulk carriers

Cargo holds:

a) Lower end of hold framesb) Lower end of water tight bulkhead (not illustrated)c) Corners of lower decksd) Bilge wellsDouble bottom ballast tanks:

e) Upper surface of face plates of bottom and tanklongitudinals

f ) Floor plate around filler plates to slotsg) Upper surface of tank top plating(from Y. Akita (see reference (1 1))

Figure 9 -6 Parts liable to corrosion in bulk carriers orOBOs

Wing ballast tanks:

a) Upper part of transverse and longitudinal bulkheads (notillustrated)

b) Upper part of deck transverses

c) Longitudinalsd) Cut edge of slots and drain holes in transversese) Block butts in internal members and in bulkheadsf ) Junction of cross ties to side transverses or vertical webs

Holds and bulkheads:

Bulkhead plate at the level of double bottom tank top (notillustrated)Upper surface of tank top plating incl. hopper tankplatingHold frames, particularly lower part, upper part andframe bracket at toe and HAZAt about40%of height (normally top of cargo with

cargoes not trimmed)

Figure 9-7 Pa rts liable to corrosion in oil tankers

DETNORSKEEIUTAS




Water ballast tanks: f) Upper surface of horizontal girders

a) Uppe r part of transverse and longitudina l bulkheadsb) Uppe r part of deck transversesc) Deck longitudinalsd) Upp er surface of horizontal stiffeners and brackets (not tanks:

illustrated)e) Cut edge of slots and lightening holes in horizontal

girders

g) Upper surface of shell and bulkhead longitudinalsh) Upper surface of face plate of bottom longitudinals,

bottom girders and bottom transverses

i) Structural members in vapour spaces of tanks (notillustrated)

corrosion protection of ships

Documents

foundation det norske

behalf of det norske

omission of det norske

provision det norske

dnv classification

classification of ships

dnv rules

dnv offshore standards