prevention corrosion

7/29/2019 Prevention Corrosion

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Part I1Pipeline Design

Chapter 15 Corrosion Prevention15.1 IntroductionInfrastructures such as steel pipelines are susceptible to corrosion. This Chapter deals withcoatings and external corrosion protection such as cathodic protection (CP). The preferredtechnique for mitigating marine corrosion is use of coatings combined with CP. Coatings canprovide a barrier against moisture reaching the steel surface therefore defense against externalcorrosion. However, in the event of the failure of coatings, a secondary CP system is required.

Corrosion is the degradation of a metal by its electro-chemical reaction with the environment.A primary cause of corrosion is due to an effect known as galvanic corrosion. All metals havedifferent natural electrical potentials. When two metals with different potentials areelectrically connected to each other in an electrolyte (e.g. sea water), current will flow fromthe more active metal to the other causing corrosion to occur. The less active metal is calledthe cathode, and the more active, the anode. In Figure 15.1, the more active metal Zn is anodeand the less active metal steel is cathode. When the anode supplies current, it will graduallydissolve into ions in the electrolyte, and at the same time produce electrons, which the cathodewill receive through the metallic connection with the anode. The result isthat the cathode willbe negatively polarized, and hence protected against corrosion.t

@- @- @- @-Z n ( W 2

I F 1 I \\

I I

Figure15.1Galvanic corrosion.


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230 Part11Pipeline Design15.2 Fundamentalsof Cathodic ProtectionCarbon steel structures exposed to natural waters generally corrode at an unacceptably highrate unless preventative measures are taken. Corrosion can be reduced or prevented byproviding a direct current through the electrolyte to the structure. This method is calledcathodic protection (CP) as showed in Figure 15.2.

Figure15.2Cathodic protection of pipeline.The basic concept of cathodic protection is that the electrical potential of the subject metal isreduced below its corrosion potential, and that it will then be incapable of going intocorroding. Cathodic protection results fiom cathodic polarizationof a corroding metal surfaceto reduce the corrosion rate. The anodic and cathodic reactions for iron corroding in an aeratednear neutral electrolyte are,

Fe+Fe2++2e- (15.1)

(15.2)respectively.As a consequence of reaction (15.2), pH of the seawater immediate to a metalsurface increases. This is beneficial because of the precipitation of solid compounds(calcareous deposits) by the reactions:

Ca2++HC0i+0H-+H20+CaC03 (15.3)and

Mg2++20H+Mg(OH)2. (15.4)These deposits decrease the oxygen flux to the steel and hence the current necessary forcathodic protection. As a result, the service life of the entire cathodic protection system isextended.


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Chapter 15 Corrosion Prevention 231Offshore pipelines can be protected as a cathode by achieving a potential of -0.80 V&JAgCI ormore negative, which is accepted as the protective potential (E: for carbon steel and lowalloy steel in aerated water. Normally, it is the best if the potentials negative to -1.05 VAg/AgCIare avoided because these can cause a second cathodic reaction (Jones, 1992):

H@+e--+H+OH- (15.5)which results in 1) wasted resources, 2) possible damage to any coatings, and 3) the possibilityof hydrogen embrittlement. .Cathodic protection systems are of two types: impressed current and galvanic anode. Thelatter has been widely used in the oil and gas industry for offshore platforms and marinepipeline in the last 40 years because of its reliability and relatively low cost of installation andoperation. The effectiveness of cathodic protection systems allows carbon steel, which haslittle natural corrosion resistance, to be used in such corrosive environments as seawater, acidsoils, and salt-laden concrete.15.3 Pipeline Coatings15.3.1 Internal CoatingsThe primary reason of applying internal coatings is to reduce the friction and thereforeenhance flow efficiency. Besides, the application of internal coatings can improve corrosionprotection, pre-commissioning operations and pigging operations. Increased efficiency isachieved through lowering the internal surface roughness since the pipe friction factordecreases with a decrease in surface roughness. In actual pipeline operation the improved flowefficiency will be observed as a reduction in pressure drop across the pipeline.The presence of free water in the system isone of the reasons to cause the corrosion of innerpipeline.An effective coating system will provide an effective barrier against corrosion attack.The required frequency of pigging is significantly reduced with a coated pipeline. The wear onpig discs is substantially reduced due to the smoother pipes surface.The choice of a coating is dictated by both environmental conditions and the servicerequirements of the line. The major generic types of coatings used for internal linings includeepoxies, urethanes and phenolics. Epoxy based materials are commonly used internal coatingsbecause of their broad range of desirable properties which include sufficient hardness, waterresistance, flexibility, chemical resistance and excellent adhesion.15.3.2 External CoatingsOil and gas pipelines are protected by the combined use of coatings and cathodic protection.The coating systems are the primary barrier against the corrosion therefore highly efficient atreducing the current demand for cathodic protection. However, they are not feasible to supplysufficient electrical current to protect a bare pipeline. Cathodic protection prevents corrosionat areas of coating breakdown by supplying electrons.


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232 Part11Pipeline DesignThick coatings are often applied to offshore pipelines to minimize the holidays and defectsand to resist damage by handling during transport and installation. High electrical resistivityretained over long periods isa special requirement, because cathodic protection is universallyused in conjunction with coatings for corrosion control. Coatings must have good adhesion tothe pipe surface to resist disbondment and degradation by biological organisms, which aboundin seawater. Pipe coating should be inspected both visually and by a holiday detector set at theproper voltage before the pipe is lowered into the water. Periodic inspection of the pipelinecathodic protection potential is used to identify the coating breakdown areas.Coatings are selected based on the design temperature and cost. The principal coatings, inrough order of cost are:

0 Tape wrap0 Asphalt0 Coal tar enamel0 Fusion bonded epoxy (FBE)0 Cigarette wrap polyethylene (PE)0 Extruded thermoplastic PE and polypropylene (PP)

The most commonly used external coating for offshore pipeline is Fusion Bonded Epoxy(FBE) coatings. They are thin film coatings, 0.5-0.6 mm thick. They consist of thermosettingpowders which are applied to a white metal blast cleaned surface by electrostatic spray. Thepowder will melt on the pre-heated pipe (around 230 "C), flow and subsequently cure to formthicknesses of between 250 and 650microns.15.4 CP Design ParametersThis section specifies parameters to be applied in the design of cathodic protection systembased on sacrificial anodes.15.4.1 Design L ifeThe design life tr of the pipeline cathodic protection system is to be specified by the operatorand shall cover the period from installation to the end of pipeline operation. It is normalpractice to apply the same anode design life as for the offshore structures and submarinepipelines to be protected since maintenance and repair of CP system are very costly.15.4.2 Current DensityCurrent density refers to the cathodic protection current per unit of bare metal surface area ofthe pipeline. The initial and final current densities, i, (initial) and i, (final), give a measure ofthe anticipated cathodic current density demands to achieve cathodic protection of bare metalsurface. They are used to calculate the initial and final current demands that determine thenumber and sizing of anode.


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Chapter 15 Corrosion Prevention 233

WaterTemp. (C)ocationGulf of Mexico 22U.S. West Coast 15N. North Sea 0-12S. SouthSea 0-12

The initial design current density is necessarily higher than the average final current densitysince the calcareous deposits developed during the initial phase reduces the current demand. Inthe final phase, the developed marine growth and calcareous layers on the metal surface willreduce the current demand, However, the final design current density shall take into accountthe additional current demand to re-polarize the structure if such layers are damaged. The finaldesign current density is lower than the initial.

Design Current Density (mA/m2)Initial M ean Final110 55 75150 90 100180 90 120150 90 100

The average (ormaintenance) design current density is a measure of the anticipated cathodiccurrent density, once the cathodic protection system has attained its steady-state protectionpotential. This will simply imply a lower driving voltage and the average design currentdensity is therefore lower than both the initial and final design value. Tables 15.1 gives therecommended design current density used for the cathodic protection system of non-buriedoffshore pipelines under the various seawater conditions in different standards. For bare steelsurfaces fully buried in sediments a design current density of 20 mN m2 is recommendedirrespective of geographical location or depth.

ArabianGul fCook Inlet

Table15.1 Summaryof recommended design current densitiesfor baresteel.

30 130 65 902 430 380 380

TropicalSub-TropicalTemperateArctic

>20 1501130 70160 9018012-20 1701150 80170 1101907-12 20011 80 100180 130111020 70160 90180

12-20 80170 110f907-12


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234 Part I I Pipeline Design

Depth(m

0-30>30

15.4.3 Coating Breakdown FactorThe coating breakdown factor describes the extent of current density reduction due to theapplication of coating. ,=O means the coating is 100% electrically insulating.f,=l implies thatthe coating can not provide any protection.

Coating CategoryI I1 111 I V

kl=O.1 kl=0.05 k1=0.02 kl=O.02k2 k2 k2 k20.1 0.03 0.015 0.0120.05 0.02 0.012 0.012

The coating breakdown factor is a function of coating properties, operational parameters andtime. The coating breakdown factor fc can be described asf , =k,+k, at (15.6)

where t is the coating life time, k, and k2 are constants that are dependent on the coatingproperties.There are four paint coating categories defined for practical use based on the coatingproperties in DNV (1993):

0

0

Category : One layer of primer coat, about 50 pm nominal DFT (Dry Film Thickness)Category 11: One layer of primer coat, plus minimum one layer of intermediate topcoat, 150 to 250 pm nominal DFTCategory 111: One layer of primer coat, plus minimum two layers of intermediatehopcoats, minimum 300 pm nominal DFTCategory I V: One layer of primer coat, plus minimum three layers of intermediate topcoats, minimum 450 pm nominal DFT0

The constantskl and k2 used for calculating the coating break-down factors are given in Table15.2.

For cathodic protection design purpose the average and final coating breakdown factors are tobe calculated by introducing the design life trf, average)=k,+k, t, I 2f , J inal)=k, +k, t,

(15.7)(15 . 8 )


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Chapter 15 Corrosion Prevention 235

Anode M aterial TypeAI-baseZn-base

15.4.4 Anode M aterial PerformanceThe performance of a sacrificial anode material is dependent on its actual chemicalcomposition. The most commonly used anode materials are A1 and Zn. Table 15.3 gives theelectrochemical efficiency E of anode materials applied in the determination of required anodemass.

Electrochemical Efficiency (Ahlkg)2000 (max 25 "C)700(max 50 " C)

Anode M aterial Type

The closed circuit anode potential used to calculate the anode current output shall not exceedthe values listed in the Table 15.4.

Closed Circuit Anode Potential(V rei. AgIAgCI seawater)nvironment

Table 15.4 Design closed circuit anode potentials for AI and Zn based sacrificial anode materials@NV,1993)

AI-baseI seawater -1.05sediments -0.95seawatersedimentsZn-base

-1.00-0.95

15.4.5 ResistivityThe salinity and temperature of seawater have influence on its resistivity. In the open sea, thesalinity doesn't vary significantly. The temperature becomes the main factor. The resistivitiesof 0.3 and 1.5 ohmam are recommended to use to calculate the anode resistance in seawaterand marine sediments respectively when the temperature of surface water is between 7to 12C(DNV, 1993).15.4.6 Anode Utilization FactorThe anode utilization factor indicates the fraction of anode material that is assumed to providecathodic protection current. Performance becomes unpredictable when the anode is consumedbeyond a mass indicated by the utilization factor. The utilization factor of an anode isdependent on the detailed anode design, in particular dimensions and location of anode cores.Table 15.3 gives the anode utilization factor for different types of anodes (DNV, 1993).


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236 Part I I Pipeline Design

AnodeTypeLong 1) slender stand-off

A node Utilization Factor0.90 I~ ~I Long 1)flush-mounted I 0.85

Bracelet, half-shell typeBracelet, segmented type

I Short2) flush-mounted I~

0.800.75

0.80 I

15.5 Galvanic Anodes System Design15.5.1 Selection of Anodes TypePipeline anodes are normally of the half-shell bracelet type (see Figure 15.6). The braceletsare clamped or welded to the pipe joints after application of the corrosion coating. Strandedconnector cables are be used for clamped half-shell anodes. For the anodes mounted on thepipeline with concrete, measures shall be taken to avoid the electrical contact between theanode and the concrete reinforcement.Normally, bracelet anodes are distributed at equal spacing along the pipeline. Adequate designcalculations should demonstrate that anodes can provide the necessary current to the pipelineto meet the current density requirement for the entire design life. The potential of pipelineshould be polarized to -0.8V A g / ~g ~~r more negative. Figure 15.4 shows the potential profileof a pipeline protected by galvanic bracelet anodes.

Figure 15.4Potential profileof a pipeline protected by bracelet anodes.


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Chapter 15 Corrosion Prevention 237Since the installation expense is the main part of CP design, larger anode spacing can reducethe overall cost. However, the potential is not evenly distributed along the pipeline. Thepipeline close to the anode has a more negative potential. The potential of middle point on thepipeline between two anodes is more positive and must be polarized to -0. 80VA~/ A~CIr morenegative in order to achieve the cathodic protection for the whole pipeline. Increased anodespacing brings bigger mass per anode therefore cause more uneven potential distribution. Thepotential close to the anode could be polarised to negative than -1.05 VA~/ A~CIwhich shouldbe avoided because of reaction 1.5. Figure 15.5 schematically illustrates the anticipatedpotential attenuation for situations of large anode spacing (Ham et al, 2004).

Figure15.5 Pipeline potential profile for large anode spacing.

15.5.2 CP Design PracticeOffshore pipeline CP design includes the determination of the current demand I,, requiredanode mass M and number and current output per anode I,. The current demand is a functionof cathode surface area, A,, a coating breakdown factor, c, and current density, , ,and can beexpressed as ( DNV, 993):

I , =A, . c , . i , (15.9)where i, depends upon water depth, temperature, sea water versus mud exposure, and whetheror not the mean or final life of the CP system is being evaluated. Current density i, isnormally in the range 60- 170mNm ( DNV, 1993). As the initial polarization periodproceeding steady-state conditions is normally quite short compared to the design life, the


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238 Part11Pipeline Designmean (time-averaged) design current density i , becomes very close to the steady state currentdensity. Therefore, it is used to calculate the minimum mass of anode material necessary tomaintain cathodic protection throughout the design life. Correspondingly,M can be calculatedas:

8,760.i;Tu * c=

where u is a utilizationpotential is assumed tocalculated by:

(15.10)factor, C is anode current capacity and T is design life. The cathodebe spatially constant. Therefore, the current output per anode can be

(15.1 1)where bCand 4, are the closed circuit potential of the pipe and anode, respectively, and R, isthe anode resistance.15.5.3 Anode Spacing DeterminationBethune and Hart (2000) have proposed a newly attenuation equation to modify the existingdesign protocol interrelating the determination of the anode spacing L,. L, can be expressedas:

(15.12)where

4c,,,,, :the free corrosion potential;a : he polarization resistance;yrp :the pipe radius.

:the reciprocal of coating breakdown factor3Assumptions have made in this approach: 1) total circuit resistance equal to anode resistance;2) all current enter the pipe at holidays in the coating (bare areas); and 3) bCand4 beconstant with both time and position. The I S0 standards recommend the distance betweenbracelet anodes should not exceed300m (IS0 14489, 1993).15.5.4 Commonly Used Galvanic AnodesThe major types of galvanic anodes for offshore applications are slender stand-off, elongatedflush mounted and bracelet (Figure 15.6). The type of anode design to be applied is normallyspecified by the operator, and should take into account various factors, such as anodeutilization factor and current output, costs for manufacturing and installation, weight and dragforces exerted by ocean current. The slender stand-off anode has the highest current outputand utilization factor among these commonly used anodes.


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Chapter I 5 Corrosion Prevention 239

15.5.5 Pipeline CP System RetrofitCathodic protections systems retrofits become necessary as the pipeline systems age. Animportant aspect of such retrofitting is determination of when such action should take place.Assessment of cathodic protection systems upon pipelines isnormally performed based uponpotential measurements. As galvanic anodes waste, their size decreases; and this causes aresistance increase and a corresponding decrease in polarization. Models have beenconstructed for potential change that occurs for a pipeline protected by galvanic braceletanodes as these deplete were also developed. Anodes depletion is time dependent in themodel.

STAND OF FLrII-INGLEBRACELET

DUAL

FLUSH MOUNTED

Figure15.6 Commonlyused anodes.

Bracelet anodes have been used for cathodic protection of marine pipelines, especially duringthe early period (roughly 1964-1976) when many oil companies had construction activitiesin the Gulf of Mexico. According to recent survey data, many of these early anode systemshave depleted or are now depleting. Retrofitting of old anode systems on pipelines installed in1960s and 1970s and even newer ones is required since these are still being used for oiltransportation. A nodes can be designed as multiples or grouped together to form an anodearray (anode sled) (See Figure 15.3). Anode arrays typically afford a good spread of protectionon a marine structure. They are a good solution for retrofitting old cathodic protectionsystems.15.5.6 Internal Corrosion InhibitorsCorrosion inhibitors are chemicals that can effectively reduce the corrosion rate of the metalexposed to the corrosive environment when added in small concentration. They normally workby adsorbing themselves to form a film on the metallic surface (www.corrosion-doctors.org).Inhibitors are normally distributed from a solution or dispersion. They reduce the corrosionprocess by either:

0 Increasing the anodic or cathodic polarization behavior;


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240 Part I .Pipeline Design0 Reducing the movement or diffusion of ions to the metallic surface;0 Increasing the electrical resistance of the metallic surface.

Inhibitors can be generally classified as follows (www.corrosion-doctors.org):0 Passivating inhibitors;0 Cathodic inhibitors;0 Precipitation inhibitors;0 Organic inhibitors;0 Volatile corrosion inhibitors.

The key to select an inhibitor is to know the system and anticipate the potential problems inthe system. The system conditions include water composition (such as salinity, ions and pH),fluid composition (percentage water versus hydrocarbon), flowrates, temperature and pressure.Application of the inhibitors can be accomplished by batch treatments, formation squeezes,continuous injections or a slug between two pigs.Inhibitor efficiency can be defined asInhibitor efficiency (%)=I oo* (CRuninhibitedCRinhibited)/ CRuninhibited

Where CRuninhibited :the corrosion rate of the uninhibited system;Typically the inhibitor efficiency increases with an increase in inhibitor concentration

CRinhibited : he corrosion rate of the inhibited system.

15.6 References1. Bethune, K. and Hartt, W.H. (2000), A Novel Approach to Cathodic Protection Designfor Marine Pipelines: Part 11-Applicability of the Slope Parameter Method, presented atCorrosion, paper no.00674.2. DNV Recommended Practice RP B401 (1993), Cathodic Protection Design, Det NorkeVeritas Industry AS, Hovik.3. Hartt, W.H., Zhang, X. And Chu, W. (2004), Issues Associated with Expiration ofGalvanic Anodes on Marine Structures. Presented at Corrosion, paper no. 04093.4. Jones, D.A. (1 992), Principles and Prevention of Corrosion, First Edition, Macmillan Inc.,New Y ork, pg. 437-445.5 . NACE StandardRP 0176 (1994), Corrosion Control of Steel-Fixed Offshore PlatformsAssociated with Petroleum Production, Houston, TX.6. ISO/TC 67/SC 2 NP 14489, Pipeline Cathodic Protection-Part 2: Cathodic Protection ofOffshore Pipelines (1993), International Organization for Standardization, Washington,DC.7. Sunde, E.D. (1 968), Earth Conduction Effects in Transaction Systems, Dover PublishingInc., New Y ork, NY , pg. 70-73.8. www corrosi on-doctors. org/ l nhi bi tors/ l essonI .htm.

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