2.5. CORROSION CONTROL

Materials Selection for Corrosion Prevention

1(1) MATERIAL SELECTION (selection of proper material for a particular corrosive service)Metallic [metal and alloy]Nonmetallic [rubbers (natural and synthetic), plastics, ceramics, carbon and graphite, and wood]2Metals and AlloysNoEnvironmentProper material1Nitric acid Stainless steels2CausticNickel and nickel alloys3Hydrofluoric acidMonel (Ni-Cu)4Hot hydrochloric acidHastelloys (Ni-Cr-Mo) (Chlorimets)5Dilute sulfuric acidLead3NoEnvironmentProper material6Nonstaining atmospheric exposureAluminium7Distilled waterTin8Hot strong oxidizing solutionTitanium9Ultimate resistanceTantalum10Concentrated sulfuric acidSteel4NORSOK

5NORSOK

6NORSOK

7NORSOK

8NORSOK

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10Table 5 - Materials selection for sub-sea production and flowline systems

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Table 5 - Materials selection for sub-sea production and flowline systems12

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14E.g : Stainless Steels

Stainless steels are iron base alloys that contain a minimum of approximately 11% Cr, the amount needed to prevent the formation of rust in unpolluted atmosphere.wt.% CrDissolution rate, cm/sec 15Alloying elements of stainless steel :Other than Ni, Cr and C, the following alloying elements may also present in stainless steel: Mo, N, Si, Mn, Cu, Ti, Nb, Ta and/or W.

Main alloying elements (Cr, Ni and C):

1. Chromium Minimum concentration of Cr in a stainless steel is 12-14wt.% Structure : BCC (ferrite forming element)

* Note that the affinity of Cr to form Cr-carbides is very high. Chromium carbide formation along grain boundaries may induce intergranular corrosion. 16Binary diagram of Fe-Cr

Sigma phase formation which is initially formed at grain boundaries has to be avoided because it will increase hardness, decrease ductility and notch toughness as well as reduce corrosion resistance. 17 2. Nickel

Structure: FCC (austenite forming element/stabilize austenitic structure) Added to produce austenitic or duplex stainless steels. These materials possess excellent ductility, formability and toughness as well as weld-ability. Nickel improves mechanical properties of stainless steels servicing at high temperatures. Nickel increases aqueous corrosion resistance of materials. 18Ternary diagram of Fe-Cr-Ni at 6500 and 10000C

AISI : American Iron and Steel Institute19Anodic polarization curves of Cr, Ni and Fe in 1 N H2SO4 solution

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Influence of Cr on corrosion resistance of iron base alloy2121Material SelectionIJK Solution

Influence of Ni on corrosion resistance of iron base alloy22

Influence of Cr on iron base alloy containing 8.3-9.8wt.%Ni23 3. Carbon

Very strong austenite forming element (30x more effective than Ni). I.e. if austenitic stainless steel 18Cr-8Ni contains 0.007%C, its structure will convert to ferritic structure. However the concentration of carbon is usually limited to 0.08%C (normal stainless steels) and 0.03%C (low carbon stainless steels to avoid sensitization during welding). 24Minor alloying elements :Manganese Austenitic forming element. When necessary can be used to substitute Ni. Concentration of Mn in stainless steel is usually 2-3%.

Molybdenum Ferritic forming element. Added to increase pitting corrosion resistance of stainless steel (2-4%). Molybdenum addition has to be followed by decreasing chromium concentration (i.e. in 18-8SS has to be decreased down to 16-18%) and increasing nickel concentration (i.e. has to be increased up to 10-14%). Improves mechanical properties of stainless steel at high temperature. Increase aqueous corrosion resistance of material exposed in reducing acid. 25Tungsten Is added to increase the strength and toughness of martensitic stainless steel.

Nitrogen (up to 0.25%)Stabilize austenitic structure. Increases strength and corrosion resistance. Increases weld ability of duplex SS.

Titanium, Niobium and TantalumTo stabilize stainless steel by reducing susceptibility of the material to intergranular corrosion. Ti addition > 5x%C. Ta+Nb addition > 10x%C.26CopperIs added to increase corrosion resistance of stainless steel exposed in environment containing sulfuric acid.

Silicon Reduce susceptibility of SS to pitting and crevice corrosion as well as SCC.

27Influence of alloying elements on pitting corrosion resistance of stainless steels

28Influence of alloying elements on crevice corrosion resistance of stainless steels

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Influence of alloying elements on SCC resistance of stainless steels30Five basic types of stainless steels :Austenitic - Susceptible to SCC. Can be hardened by only by cold working. Good toughness and formability, easily to be welded and high corrosion resistance. Nonmagnetic except after excess cold working due to martensitic formation.Martensitic - Application: when high mechanical strength and wear resistance combined with some degree of corrosion resistance are required. Typical application include steam turbine blades, valves body and seats, bolts and screws, springs, knives, surgical instruments, and chemical engineering equipment.Ferritic - Higher resistance to SCC than austenitic SS. Tend to be notch sensitive and are susceptible to embrittlement during welding. Not recommended for service above 3000C because they will loss their room temperature ductility. 31Duplex (austenitic + ferritic) has enhanced resistance to SCC with corrosion resistance performance similar to AISI 316 SS. Has higher tensile strengths than the austenitic type, are slightly less easy to form and have weld ability similar to the austenitic stainless steel. Can be considered as combining many of the best features of both the austenitic and ferritic types. Suffer a loss impact strength if held for extended periods at high temperatures above 3000C.

Precipitation hardening - Have the highest strength but require proper heat-treatment to develop the correct combination of strength and corrosion resistance. To be used for specialized application where high strength together with good corrosion resistance is required.32

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40Stress Corrosion Cracking of Stainless SteelStress corrosion cracking (SCC) is defined as crack nucleation and propagation in stainless steel caused by synergistic action of tensile stress, either constant or slightly changing with time, together with crack tip chemical reactions or other environment-induced crack tip effect.

SCC failure is a brittle failure at relatively low constant tensile stress of an alloy exposed in a specific corrosive environment.

However the final fracture because of overload of remaining load-bearing section is no longer SCC. 41Three conditions must be present simultaneously to produce SCC: - a critical environment - a susceptible alloy - some component of tensile stress

42Tensile stressCorrosive environmentSusceptible materialStress corrosion crackingTensile stress is below yield pointCorrosive environment is often specific to the alloy systemPure metals are more resistance to SCC but not immune and susceptibility increases with strength43Typical micro cracks formed during SCC of sensitized AISI 304 SS

Surface morphology44

Example of crack propagation during transgranular stress corrosion cracking (TGSCC) brass45

Example of crack propagation during intergranular stress corrosion cracking (IGSCC) ASTM A245 carbon steel46

Fracture surface of transgranular SCC on austenitic stainless steel in hot chloride solutionFracture surface of intergranular SCC on carbon steel in hot nitric solution47

Fracture surface due to intergranular SCCFracture surface due to local stress has reached its tensile strength value on the remaining section 48Electrochemical effect

pittingpassiveactivecracking zonesUsual region for TGSCC, mostly is initiated by pitting corrosion (transgranular cracking propagation needs higher energy)Usual region for IGSCC, SCC usually occurs where the passive film is relatively weakZone 1Zone 249Note that non-susceptible alloy-environment combinations, will not crack the alloy even if held in one of the potential zones. Temperature and solution composition (including pH, dissolved oxidizers, aggressive ions and inhibitors or passivators) can modify the anodic polarization behavior to permit SCC.Susceptibility to SCC cannot be predicted solely from the anodic polarization curve.50Models of stress corrosion crackingSlip step dissolution modelDiscontinuous intergranular crack growthCrack nucleation by rows of corrosion micro-tunnels Absorption induced cleavageSurface mobility (atoms migrate out of the crack tips)Hydrogen embrittlementHIC51Control/prevention :

Reduce applied stress levelRemove residual tensile stress (internal stress)Lowering oxidizing agent and/or critical species from the environmentAdd inhibitorUse more resistant alloysCathodic protection52Alteration of EnvironmentTypical changes in medium are :Lowering temperature but there are cases where increasing T decreases attack. E.g hot, fresh or salt water is raised to boiling T and result in decreasing O2 solubility with T.Decreasing velocity exception ; metals & alloys that passivate (e.g stainless steel) generally have better resistance to flowing mediums than stagnant. Avoid very high velocity because of erosion-corrosion effects.53Removing oxygen or oxidizers e.g boiler feedwater was deaerated by passing it thru a large mass of scrap steel. Modern practice vacuum treatment, inert gas sparging, or thru the use of oxygen scavengers. However, not recommended for active-passive metals or alloys. These materials require oxidizers to form protective oxide films.Changing concentration higher concentration of acid has higher amount of active species (H ions). However, for materials that exhibit passivity, effect is normally negligible.54Environment factors affecting corrosion design :Dust particles and man-made pollution CO, NO, methane, etc.Temperature high T & high humidity accelerates corrosion.Rainfall excess washes corrosive materials and debris but scarce may leave water droplets.Proximity to seaAir pollution NaCl, SO2, sulfurous acid, etc.Humidity cause condensation.55Design Dos & DontsWall thickness allowance to accommodate for corrosion effect.Avoid excessive mechanical stresses and stress concentrations in components exposed to corrosive mediums. Esp when using materials susceptible to SCC.Avoid galvanic contact / electrical contact between dissimilar metals to prevent galvanic corrosion.Avoid sharp bends in piping systems when high velocities and/or solid in suspension are involved erosion corrosion.Avoid crevices e.g weld rather than rivet tanks and other containers, proper trimming of gasket, etc.56Avoid sharp corners paint tends to be thinner at sharp corners and often starts to fail.Provide for easy drainage (esp tanks) avoid remaining liquids collect at bottom. E.g steel is resistant against concentrated sulfuric acid. But if remaining liquid is exposed to air, acid tend to absorb moisture, resulting in dilution and rapid attack occurs.Avoid hot spots during heat transfer operations localized heating and high corrosion rates. Hot spots also tend to produce stresses SCC failures.Design to exclude air except for active-passive metals and alloys coz they require O2 for protective films.Most general rule : AVOID HETEROGENEITY!!! 57Protective Coatings / WrappingProvide barrier between metal and environment. Coatings may act as sacrificial anode or release substance that inhibit corrosive attack on substrate.Metal coatings : Noble silver, copper, nickel, Cr, Sn, Pb on steel. Should be free of pores/discontinuity coz creates small anode-large cathode leading to rapid attack at the damaged areas.Sacrificial Zn, Al, Cd on steel. Exposed substrate will be cathodic & will be protected.Application hot dipping, flame spraying, cladding, electroplating, vapor deposition, etc.58Surface modification to structure or composition by use of directed energy or particle beams. E.g ion implantation and laser processing.Inorganic coating : cement coatings, glass coatings, ceramic coatings, chemical conversion coatings. Chemical conversion anodizing, phosphatizing, oxide coating, chromate.Organic coating : paints, lacquers, varnishes. Coating liquid generally consists of solvent, resin and pigment. The resin provides chemical and corrosion resistance, and pigments may also have corrosion inhibition functions.59VALVE BODIES (ALLOY STEEL)ADEQUATE FOR BENIGN SERVICE LOW CO2WELDABLECRITICAL SEALING AREAS NEED TO HAVE ALLOY 625 OVERLAYGOOD TOUGHNESSSOME LOW ALLOY MATERIALS HAVE LOW HARDENABILITY6060Material SelectionIJK SolutionNACE REQUIREMENTS HARDNESS OF LOW ALLOY MATERIALS LIMITED TO HRC 22 MAXIMUMNICKEL CONTENT IS LIMITED TO 1.0 PERCENT MAX FOR LOW ALLOY MATERIALSNACE MR0175 (ISO 15156) COVERS METALLIC MATERIALS REQUIREMENT FOR RESISTANCE TO SULFIDE STRESS CRACKING (SCC)STANDARD IS NOT INTENDED AS DESIGN SPECIFICATIONEACH TYPE OF MATERIAL HAS SPECIFIC HEAT TREATMENT REQUIREMENTS SUCH AS ONE OF THE FOLLOWING:HOT ROLLEDANNEALEDNORMALIZEDNORMALIZED, AUSTENITIZED, QUENCHED AND TEMPEREDAUSTENITIZED, QUENCHED AND TEMPERED6161Material SelectionIJK SolutionAPI MATERIAL CLASS REQUIREMENTS

CRA (Corrosion Resistant Alloy) = Any one or Sum of Co, Cr, Mo, Ni, and Ti exceeds 50 percentWellhead equipment to satisfy the above API corrosive requirements can be group as follows:Low alloyStainless SteelCombination of the aboveDuplex stainless steelsCRAsEach of these alloys has successfully been used for subsea wellhead application but all have some type of environmental limitations. 6262Material SelectionIJK SolutionAPI 6A AND 17DSPECIFY STRENGTH LEVELS FOR VARIOUS PRESSURE LEVELSFOR PRESSURE LEVELS UP TO AND INCLUDING 10.000 PSI, YIELD STRENGTH OF 60,000 PSI MINIMUMFOR PRESSURE LEVELS EXCEEDING 10,000 PSI, YIELD STRENGTH OF 75,000 PSI MINIMUMSPECIFY PSL (PRODUCT SPECIFICATION LEVELS ) PSL 1-4PSL LEVELS ARE QUALITY RELATEDSPECIFIES IMPACT REQUIREMENTS FOR SUBSEA EQUIPMENT TEST TEMPERATURE IS 0 DEGREES FTEST COUPON SIZE ( 4-INCH X 4-INCH CROSS-SECTION)6363Material SelectionIJK SolutionMATERIALS FOR SEAWATER INJECTIONFACTORS:TEMPERATURECHLORINE LEVELPITTING RESISTANCE EQUIVALENT (PRE) OF MATERIALSOXYGEN CONTENT

CANDIDATE MATERIALS:PRE>40 FOR MATERIALS FOR USE IN AERATED SEAWATERLOWER PRE WITH LOWER OXYGEN6464Material SelectionIJK SolutionMATERIALS SELECTION CONSIDERATIONS INCLUDECOMPOSITION OF PRODUCED FLUIDS IN CONTACT WITH VALVE BODY AND INTERNAL PARTSSERVICE TEMPERATUREPRESSURE RANGESGALVANIC EFFECTS DUE TO CONTACT BETWEEN DIFFERENT MAERIALSCREVICE CORROSION RESISTANCE AT SEAL AND FLANGE FACESWEAR AND GALLING RESISTANCE FOR MOVING PARTSTEMPERATURE AND CHEMICAL RESISTANCE FOR NON-METALLIC MATERIALSCP ON METALSEFFECTIVENESS OF COATINGS USED ON MATERIALSWELDABILITY FOR WELD OVERLAYMATERIAL AVAILABILITY AND COSTCOMPATIBILITY OF MATERIALS WITH INJECTED FLUIDS6565Material SelectionIJK SolutionSTAINLESS STEEL 410

GOOD CORROSION RESISTANCE FOR MANY APPLICATIONSLOW TOUGHNESS IN WROUGHT CONDITIONLOW TOUGHNESS IN WELDED CONDTIONSCRITICAL SEALING AREAS NEED ALLOY 625 OVERLAY

STAINLESS F6NM

SOMEWHAT BETTER CORROSION RESISTANCE THAN 410 SSGOOD WELDABILITYGOOD TOUGHNESS IN LOW TEMPERATURESCRITICAL SEALING AREAS NEED ALLOY 625 OVERLAYGOOD HARDENABILITY

6666Material SelectionIJK SolutionNICKEL ALLOYS EXCELLENT CORROSION RESISTANCE IN SUBSEA AND PRODUCED FLUID ENVIRONMENTSDIFFICULT IN MACHININGEXPENSIVEDIFFICULTIES FOR WELD REPAIR IN AGED CONDITIONCRITICAL SEALING SURFACES DO NOT REQUIRE OVERLAYLARGE COMPONENTS REQUIRE LONG LEAD TIME PROCUREMENTNO API 6A OR 17D ENVIRONMENTAL LIMITS6767Material SelectionIJK SolutionDUPLEX STAINLESS STEELS GOOD CORROSION RESISTANCE IN MOST PRODUCED FLUIDSMORE RESISTANT TO STRESS CORROSION CRACKING IN CHLORIDE ENVIRONMENTS THAN AUSTENITIC SSGOOD WELDABILITY FOR STANDARD DUPLEX (2205)WELDING ISSUES WITH SUPER DUPLEX (2507)GOOD WELDABILITY WITH HEAT INPUT CONTROLGOOD TOUGHNESSEXHIBIT PROBLEMS WHEN HEAT TREATED IN LARGE SECTIONSMAY EXHIBIT HYDROGEN EMBRITTLEMENT WITH CPLIMITED IN H2S SERVICE6868Material SelectionIJK SolutionMATERIAL LIMITS FOR PRODUCTION EQUIPMENT AlloyH2SCO2ChloridesLow TempAlloy SteelUnlimitedLowLimitedUnlimited 410 SSTracesUnlimitedLimitedLimitedF6NMLimitsUnlimitedLimitedUnlimitedDuplexDepending of Cl & H2SYesUnlimitedNo limitsNickel AlloysNo LimitsNo LimitsNo LimitsNo limitsAlloy + Selective CladYesLimitsYesFew limits6969Material SelectionIJK SolutionMISCELLANEOUS MATERIALS STAINLESS (3XX)COPPER BASE6 MO13 CRNITRONIC (50 OR 60)PRECIPITATION HARDENED (A286, 925, 718, 625+)ULTRAHIGH STRENGTH (MP35N)7070Material SelectionIJK SolutionPITTING RESISTANCE EQUIVALENT (PRE)VSCRITICAL CREVICE/PITTING TEMPERATUREPitting Resistance Equivalent (PRE) = %Cr + 3.3(%Mo) + 16(%N)

7171Material SelectionIJK SolutionCREVICE CORROSION Narrow spaces between metal-to-metal componentsAttack occurs in small volumes of stagnant solution Occurs under bolt heads and gasketsCrevice can occur in absence of pitting for marginally pitting resistant alloysCrevice sequence includes depletion of oxygen in crevice, followed by chloride ion concentration and increase in acidity within crevice7272Material SelectionIJK SolutionFASTENERS Common low alloy steel bolting materials include either ASTM A193 Grade B7 or A320 Grade L7These materials are not corrosion resistant and require CP if not coatedIf coating is used, electrical continuity is required between bolt and assembly for CP performanceEach bolt must be connected to CP system Low alloy bolting may be used on CRA type flanges if connected to CPCRA type bolting is preferred if flanges are CRA type materialLow alloy bolting can result in hydrogen embrittlement from CP system. Hardness should be limited to HRC 34Typical CRA type bolting include A453, 718, 925, 625 and Alloy K5007373Material SelectionIJK SolutionPITTING CORROSIONExtreme localized form of corrosion that results in holes in the metalCharacterized by attacks at small discrete areasSmall on the surface but may have larger cross sectional area deeper inside the metalPits are considered self-forming crevicesPits usually grow in the direction of gravityPits develop and grow downward from horizontal surfacesAssociated with stagnant conditions such as liquid in a tank or liquid trapped low in the componentOrdinary steels are more resistant to pitting than most stainless steelAttack occurs in acidic solutions containing chlorides7474Material SelectionIJK SolutionCHEMICAL INJECTION FLUIDSPARAFFIN INHIBITORSASPHALTENE DISPERSANTASPHALTENE SOLVENT TOLUENESCALE INHIBITORSCORROSION INHIBITORSBIOCIDESALL CHEMICAL INJECTION FLUIDS HAVE PROPRIETARY ADDITIVES THAT MAY AFFECT CERTAIN MATERIALSCOMPATIBILITY DATA IS REQUIRED FOR METALLIC AND NON-METALLIC MATERIALS7575Material SelectionIJK SolutionFLUID COMPATIBILITYPART: TUBING HEAD BODYMATERIAL: (ALLOY BODY)EXPOSEDCOMPATIABLE PARAFFINNON/AINHIBITORNOOKASPHALTENENONEN/A DISPERSANTNONEASPHALTENE SOLVENTNONEN/AHYDRATE INHIBITORNONEN/ACONTROL FLUIDSNONEN/APRODUCTION FLUIDSNONEN/A 7676Material SelectionIJK SolutionSPECIAL CONSIDERATION FOR SUBSEAAPPLICATIONS (CP & HE)MATERIAL CP NEEDEDSUSCEPTIBLE TO HE

316 SSYESNO17-4 PHYESYES2205 DUPLEXYESYESA286 ASTM A453YESNOX-750YESYESMONEL K-500YESYES2507 DUPLEXTEMP RELATEDALLOY 625NORARELYALLOY 718NORARELY7777Material SelectionIJK SolutionPRICE

78Terima Kasih

7979Material SelectionIJK Solution