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8/12/2019 M172 Section Corrosionfsadf

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Materials (2)

Section 4

Corrosion of Materials Used in Shipbuilding

Section 4 : Corrosion of Materials Used in Shipbuilding 1 of 17Version 1.0 Date: October 200


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Materials (2)

Corrosion!ll "aterials including "etals deteriorate. So"e deteriorate faster than others#

$hile so"e deteriorate "ore than others. %his gradual process of deteriorationor degradation is called corrosion and it affects the "etals used in shipconstruction.

!ll "etals $ill corrode on contact $ith $ater and air. &ffecti'e coating and"ini"isation strategies are essential to the shipbuilding industr( and theeffecti'e life)span of the co"ponents the( "anufacture.

%he reduction or "ini"isation of corrosion is of 'itall( significant indeter"ining the potential life span and suitabilit( of "aterials to be used forshipbuilding# outfitting re*uire"ents and structural applications.

Corrosion process%he corrosion process is an electroche"ical process $hich begins $hen the"etal reacts to its en'iron"ent and its ato"s are e+posed to the "oisture filledat"osphere# gi'ing up electrons and releasing positi'el( charged ions into the

"oisture filled solution $hich beco"e electricall( conducti'e# and is called anelectrol(te. %he point of corrosion is called an !node.

O,% M172-0 - 01 !node &lectrode


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Materials (2)

Anodic site!n anode site can be triggered b( 'er( "inor surface irregularities such ascrac/s# pit "ar/s# ca'ities# pin holes# or bare "etal e+posed to "oisture# or#i""ersed in sea$ater $hich is a corrosi'e electrol(te. &'en salt deposits onthe surface beco"e h(groscopic and act as an effecti'e electrol(te. %he

electrol(te acts as the corrosi'e "ediu"# $here the "etal anode passingthrough it# releases positi'el( charged ions into the solution# at the sa"e ti"ereleasing electrons to another electrode site called a cathode. %he cathode can

be another "etal surface in contact $ith the anode and electrol(te or part ofthe sa"e "etal. %he cathode accepts the release of electrons and sets up acathodic reaction of electrons# flo$ing through the "etal tr(ing to inhibitcorrosion for"ing. %he anode and cathode for" a partnership /no$n as a

couple . %he "etal anode co"ponent of the couple corrodes faster than it$ould nor"all( in an electrol(te# and the other co"ponent# being the cathode#corrodes slo$er than nor"al.

O,% M172-0 - 02 !nodic and Cathodic Cell

%he three co"ponents for" a cell# co"prising an anode# cathode andelectrol(te. %hese are /no$n as anodic and cathodic cells #or potential sites for

bi"etallic# electrol(tic or gal'anic corrosion of the "etals.

Metallic corrosion occurs $hen the follo$ing essential re*uire"ents are "et:! potential difference bet$een t$o dissi"ilar "etals%he presence of an effecti'e electrol(te or electricall( conductingsolution!n electrical connection bet$een the t$o "etals.

3hen these conditions are "et# the "etal in contact $ith the li*uid solution#$ill discharge positi'el( charged particle ions into the solution $here the(

re"ain in negati'e charge. %his is called a "etal s negati'e potential# or#electrode potential# and is an indication of the "etal s corrosion resistance.

Section 4 : Corrosion of Materials Used in Shipbuilding of 17Version 1.0 Date: October 200

Electro-chemical corrosion involving wet corrosion

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Materials (2)

Electrode potential! "etal s electrode potential is graded# and sho$n in an electro)che"icalseries table. %he higher the electrode potential grade# the higher the positionto$ards the anodic end of the table# because of its greater potential tendenc(to corrode.

Electro Chemical Series%he corrosion resistance of a "etal is sho$n b( its position in the electro)che"ical series $hen co"pared to other co""onl( used "etals. 4ts position isarranged according to its relati'e electrode potential as sho$n belo$.

O,% M172-0 - 0 &lectro 5 Che"ical Series

Galvanic corrosion6al'anic corrosion is a co""on proble" $hen dissi"ilar "etals arei""ersed in sea$ater in a "arine en'iron"ent# and an electrical connection is"ade. !n( "etal that is abo'e the other# in the gal'anic series# $ill corrode ata faster rate than the other. One $ill corrode $hile the other is protected. %hefarther the t$o "etals are apart on the gal'anic series# the greater the potentialdifference bet$een the". %he least noble "etal is anodic. 4t $ill be "orereacti'e and corrode at the e+pense of the "ost noble "etal# $hich is cathodicand less reacti'e and protected.

Section 4 : Corrosion of Materials Used in Shipbuilding of 17Version 1.0 Date: October 200

ELECTRO-C E!"CAL SER"ES ELECTRO#E $OTE%T"AL

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Materials (2)

O,% M172-0 - 0 6al'anic Series

Corrosion $rotection Strategies

&' Cathodic $rotectionCathodic protection is used to protect steel ship hulls fro" such reacti'it( andcorrosion. or e+a"ple a co""on "anganese bron8e propeller is usuall( 'er(close to the steel hull# and is an e+a"ple of bi)"etallic coupling of differing"etals# i""ersed in corrosi'e electrol(te# sea$ater. Under this condition thesteel hull $ould corrode 'er( fast and its ser'ice life $ould be s"aller than ifit had cathodic protection.

%o offer cathodic protection to the "anganese bron8e propeller and the steelhull it is co""on practice for the bi)"etallic couplings to be connected

electricall( b( 8inc cathodes in the for" of thic/ 8inc plates attached to thehull along its length.

%he 8inc plates are strategicall( placed close to the propeller and i""ersed inthe electrol(tic sea$ater. %his pro'ides cathodic protection of the steel hull asthe 8inc assu"es an anodic role offering the steel and the propeller cathodic

protection. %he 8inc slabs corrode at a faster rate than the steel hull or propeller that the( protect. %he 8inc plates can be replaced on a regular basise+tending the ser'ice life of both the propeller and the steel hull.


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Materials (2)

4n the e+a"ple abo'e# 8inc plates are used to induce sacrificial corrosion ofthe (elow the waterline ; co"ponents such as hull# stern and rudder areas.%he techni*ue is also suitable for other co"ponents sub of 17Version 1.0 Date: October 200


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Materials (2)

O,% M172-0 - 09 4"pressed Current S(ste"s

3hen corrosion does occur# it occurs in a gal'anic cell# unifor"l( and predictabl(# enabling pre)e"pti'e action to be ta/en.%he i"pressed current protection s(ste" has the follo$ing benefits:

4"pressed current s(ste"s force a protecti'e current to flo$.4t enables the ser'ice life of the product to be esti"ated.%he 8inc plates used as sacrificial anodes can be replaced on a regular

basis.?re'entati'e "aintenance schedules to be planned and progra""ed$ith a degree of certaint( and reliabilit(.4t allo$s for correcti'e "easures to be i"ple"ented# before se'ereconditions# or co"plete failure e'entuates.

Galvanic cell

Corrosion ta/es place in a gal'anic cell co"prising of dissi"ilar "etals incontact $ith a corrosi'e electrol(te at the anode @AB end# $hile a reduction incorrosion occurs at the cathode @)B end. %he anode end $ill be reacti'e and corrode# $hile the cathode end $ille+perience a reduction reaction# re"aining protected and relati'el( intact b(the application of cathodic current.


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Materials (2)

O,% M172-0 -0> 6al'anic cell

ecause of the high 8inc content of brass# na'el bron8e# and "anganese bron8e# fasteners or fittings "ade fro" these "aterials sho*ld not, be usedfor e+posed to $ater applications. or used for co"plete sub"ersion insea$ater# as the( $ill corrode at a rapid rate. 4nstead# alu"iniu" bron8e#silicon bron8e# or phosphor bron8e fasteners# and fittings# should be used.,o$e'er# precautions "ust be ta/en to ensure that the( are not# in contact$ith dissi"ilar "aterials# unless ade*uate corrosion protection strategies arein place such as# 8inc cathodic protection# i"pressed current s(ste"s# or

pre'entati'e coatings such as epo+( paint ha'e been used.

!ustenitic stainless steel fasteners should be used to secure alu"iniu"co"ponents such as "asts# out)dri'es# fittings e+posed to cli"atic conditionsand sea$ater as long as the( are onl( in contact $ith alu"iniu"# stainlesssteel# gal'anised steel# or "onel "etal. Monel fasteners should be used tosecure under$ater stainless fittings.

Copper# copper allo(s# brass and bron8e "ust not be


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Materials (2)

O,% M172-0 -07 Cathodic protection fro" corrosion

Corrosion $revention Strategies

' $rotective Coatings o. marine str*ct*res - !etallic coatings

Metallic coatings are applied to deposit thin la(ers of "etal such as#chro"iu"# nic/el# copper# cad"iu" and 8inc# to the base "etal of the "arinestructure to be protect fro" corrosion such as steel# ferrous allo( often caused

b( the sea$ater and the pre'ailing cli"atic conditions.%he coating pro'ides a durable# corrosion resistant la(er protecting the part atthe e+pense of the coating "ediu".So"e of the "ethods used are the application of 8inc rich paints#electroplating# hot)dip gal'anising and fla"e spra( coating.4n electroplating an electrical current is passed through a dissol'ed "etal ionsolution containing the part to be plated# $hich beco"es the cathode in theelectroche"ical cell that for"s. %he part deri'es its ions fro" the solution itco"es into contact $ith plating the surface $ith a thin barrier coating.

! 'ariet( of "etals can be used for electroplating such as alu"iniu"# brass# bron8e# cad"iu"# copper# chro"iu"# iron# lead# nic/el# tin# 8inc# gold# platinu" and sil'er. %hese are


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Materials (2)

Electroless $lating

!n &lectroless nic/el @& B plating process uses a che"ical catal(tic reduction process and a "etal plating "ediu" such as nic/el in solution. %he nic/el is

placed in an a*ueous solution containing a reduction agent that causes thenic/el ions to flo$ through the a*ueous solution.

%he solution of nic/el ions is agitated thoroughl( to ensure unifor"it( of aco"bined "i+ture of dissol'ed nic/el ions and solution. 3hen the ferrous"etal allo( is placed into this "i+ture the disposition adheres onto the part

being coated in the for" of a consistent plating barrier coat. %he thic/ness ofthe plating is unifor" to all internal and e+ternal features# faces and parts. 4t isa "easure of its corrosion resistant *ualities.

or gal'anic protection Copper allo(s are so"eti"es coated $ith @& B nic/el because of its higher position on the gal'anic series.& plated ob


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Materials (2)

gal'anic corrosion# rust $ill e'entuall( begin to for". !nd# if left untreatedfor an e+tended period# the ferrous "etal $ill be consu"ed in the corrosion

process.

,ot 5dip gal'anising is used for "an( shipbuilding applications $here ferrous

"etals are used such as fabricated co"ponents# cargo holds# sheet "etal parts# piping# cladding# e+posed tubes# rails# balustrading# dec/ing and "an( otherapplications that re*uire a durable corrosion and rust resistant barrier coating.%he gal'anising process consists of the follo$ing steps.

Galvanising $rocess

O,% M172-0 -0E 6al'anising process 5 Step 1

O,% M172-0 -0F 6al'anising process 5 Step 2


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The cleaning process is ver+ important in preparing the part .or acid (athing

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Acid (athing o. the parts ens*res scale and r*st removal (e.ore rinsingand dr+ing'


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O,% M172-0 -10 6al'anising process 5 Step

O,% M172-0 -11 6al'anising process 5 Step


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/or chemical cleaning the parts are immersed in a hot .l*0 sol*tion o.1inc ammoni*m chloride

$arts are galvanised (+ immersion in molten 1inc hot-dip galvanising(ath'


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Materials (2)

O,% M172-0 -12 6al'anising process 5 Step 9

%he gal'anising protects the ferrous "etal fro" the corrosi'e en'iron"ent b(gi'ing Cathodic and sacrificial protection. !nd irregular# co"plicated shapes#including internal surfaces such as piping and tubes are e*uall( suitable forhot)dipped gal'anising.

%he gal'anised products ha'e a high resistance to "echanical da"age as $ellas anti)corrosi'e properties in a thic/ 8inc coating seal allo(ing $ith the base"etal# eli"inating "aintenance re*uire"ents for long periods.

!fter careful surface preparation and special adhesion treat"ents such as etch pri"ers# secondar( coatings can be applied. &po+( based paints and othersuch protecti'e coatings can be applied o'er the gal'anising for e+tradurabilit( or decorati'e re*uire"ents.


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A.ter galvanising the parts are chilled to .orm a (onded 1inc coating'


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$ac2 Cementation

Thermal Spra+ Coating

%he ther"al spra( coating process is used to appl( a "etallic coating to thesurface of the part and e+tend its ser'ice life.%here are three "ethods of ther"al spra( coating.

la"e spra( using co"bustion torch.,igh)'elocit( o+()fuel @,VO B and detonation torch.&lectric $ire arc and plas"a arc spra(.

%he process in'ol'es the use of a high)energ( spra( s(ste"# such as a high'elocit( o+()fuel @,VO B together $ith po$dered particles of "etal# cera"icor carbide co"pounds that are heated to high te"peratures until the( reach a

plastic state.! strea" of co"pressed air or gas is used to spra( the se"i)"olten "etal

particles onto the prepared substrate to $hich the( adhere and for" a per"anent durable tough coating. On stri/ing the substrate the po$dered particles flatten and for" thin adhered o'erlapping porous la(ers that needsealing before use.

!s $ell as po$dered "aterials# rod and $ire can be used in this process.Materials used for ther"al spra( coating include "etal# cera"ic or carbides.&+a"ples of use include bra/e discs# piston rings# gas turbine blades# steeringshafts# co"bustion c(linders# in


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Materials (2)

Organic coating

Various paints and resins such as al/(ds# epo+ies# esters# 'in(ls# rubbers# coaltar epo+ies# aro"atic pol(urethane# aliphatic pol(urethane# late+# acr(lics#

phenolics# and 8inc) rich products can be used to pro'ide a protecti'e fil" that

increases the corrosion resistance of the part and protects against corrosionand en'iron"ental da"age.Surface preparation including thorough cleansing is 'er( i"portant to ensureade*uate bonding adhesion and longe'it( of coating protection.?rior to use obtain @MSDSB Material Safet( Data Sheets for each of the

products to be used and follo$ safe handling# storage and use instructions.Manufacturers instructions "ust be follo$ed carefull( in all aspects of

product preparation and use.

3arrier coating o. al*mini*m allo+s - Anodising! barrier coating can be applied to alu"iniu" allo(s using an electrol(tic

process of treat"ent called anodising that uses sulphuric# chro"ic# or phosphoric acid solutions# to for" a thin o+ide scale protecti'e fil"# on the"etals surface.%he process is an electroche"ical process in'ol'ing an anode# electric currentand an electrol(te# usuall( sulphuric acid. %he alu"iniu" itself is the anode.%he electric current passing through the electrol(te coats the anodicalu"iniu" surface lea'ing a alu"iniu" o+ide protecti'e fil" that hasincreased abrasion# corrosion resistance# and anodic protection in a corrosi'eseafaring en'iron"ent. !nodising is not considered a stand alone process inshipbuilding applications and is usuall( acco"panied b( a secondar(application of finishing b( painting# po$der coating or other protecti'eorganic coating "ediu" especiall( in highl( corrosi'e# al/aline or acidicsituations.

O,% M172-0 -1 !lu"iniu" anodised outfitting co"ponents


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The anodised .ilm (ecomes part o. the s*(strate and does not r*( o.. and is anideal preparator+ (ase .or painting or secondar+ conversion coating'


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S*r.ace preparation

efore the application of an( barrier coating the surface "ust be thoroughl(cleaned to aid adhesion and "echanical bonding. efore grit blasting thesurface should be degreased. Conta"inants "ust be re"o'ed using onl(

reco""ended cleaners that don t lea'e cleaning co"pounds on the surfacethat "a( inhibit proper coating adhesion and reduced durabilit( protection.Organic sol'ents# high)pressure stea"# caustic solutions# detergents# $ater of 17Version 1.0 Date: October 200

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O,% M172-0 -19 4nhibitors and their application %able 2

apo*r $hase "nhi(itorsVapour phase inhibitors are co"pounds used in closed or confined spaces b('olatilisation to pre'ent corrosion. %he( do this b( neutrali8ing acidic carbondio+ide# or b( "odif(ing the p, 'alues# fro" acid and corrosi'e# to less acidand corrosi'e.On contact $ith the "etal surface# the 'apour inhibitor condenses and ish(drol(sed b( "oisture to release protecti'e ions and acting inhibition fore+tended periods of enduring protection.

O,% M172-0 -1> 4nhibitors and their application %able

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igh volatilit+ compo*nds are *sed .or .ast acting inhi(ition5 while lowvolatilit+ compo*nds are less .ast acting (*t more end*ring over longer periods

m172 section corrosionfsadf

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