Corrosion Prevention 316

8/8/2019 Corrosion Prevention 316

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CORROSIONCONTROL

MATERIAL SELECTIONALTERATION OF ENVIRONMENT

PROPER DESIGNCATHODIC PROTECTIONANODIC PROTECTION

COATINGS & WRAPPING


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(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]


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Metals and Alloys

No Environment Proper material

1 Nitric acid Stainless steels

2 Caustic Nickel and nickelalloys

3 Hydrofluoric acid Monel (Ni-Cu)

4 Hot hydrochloricacid

Hastelloys (Ni-Cr-Mo) (Chlorimets)

5 Dilute sulfuric acid Lead


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No Environment Proper material6 Nonstaining

atmospheric exposureAluminium

7 Distilled water Tin8 Hot strong oxidizing

solutionTitanium

9 Ultimate resistance Tantalum

10 Concentrated sulfuricacid

Steel


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E.g : StainlessSteels

Stainless steels are

iron base alloys that

contain a minimum of

approximately 11%Cr, the amount

needed to prevent the

formation of rust in

unpollutedatmosphere.wt.% Cr

Dissolu

tionrate,

cm/se

c


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Binary diagram of Fe-Cr

Sigma phaseformation whichis initially formed

at grainboundaries has tobe avoidedbecause it willincrease

hardness,decrease ductilityand notchtoughness as well

as reducecorrosion


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2. Nickel

Structure: FCC (austenite formingelement/stabilize austenitic structure)

Added to produce austenitic or duplex stainlesssteels. These materials possess excellent ductility,formability and toughness as well as weld-ability.

Nickel improves mechanical properties ofstainless steels servicing at high temperatures.

Nickel increases aqueous corrosion resistance ofmaterials.


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Ternary diagram of Fe-Cr-Ni at 6500 and 10000C

AISI : American Iron and Steel Institute


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Anodic 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 alloy


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Influence of Ni on corrosion resistance of iron base alloy


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Influence of Cr on

iron base alloy

containing 8.3-

9.8wt.%Ni


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3. Carbon

Very strong austenite forming element (30xmore 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 isusually limited to 0.08%C (normal

stainless steels) and 0.03%C (low carbonstainless steels to avoid sensitization duringwelding).

nor a oy ng


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nor a oy ngelements :

Manganese

Austenitic forming element. When necessary can beused to substitute Ni. Concentration of Mn in stainlesssteel is usually 2-3%.

Molybdenum

Ferritic forming element. Added to increase pittingcorrosion resistance of stainless steel (2-4%).

Molybdenum addition has to be followed by decreasingchromium concentration (i.e. in 18-8SS has to bedecreased down to 16-18%) and increasing nickelconcentration (i.e. has to be increased up to 10-14%).

Improves mechanical properties of stainless steel athigh temperature. Increase aqueous corrosionresistance of material exposed in reducing acid.


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Tungsten

Is added to increase the strength and toughnessof martensitic stainless steel.

Nitrogen (up to 0.25%)Stabilize austenitic structure. Increases strength

and corrosion resistance. Increases weld ability ofduplex SS.

Titanium, Niobium and Tantalum

To stabilize stainless steel by reducing susceptibilityof the material to intergranular corrosion. Tiaddition > 5x%C. Ta+Nb addition > 10x%C.


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Copper

Is added to increase corrosion resistance of

stainless steel exposed in environment containingsulfuric acid.

Silicon

Reduce susceptibility of SS to pitting and crevicecorrosion as well as SCC.


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Influence of alloying elements onpitting corrosion resistance ofstainless steels


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Influence of alloying elements oncrevice corrosion resistance ofstainless steels


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Influence of alloying elementson SCC resistance of stainlesssteels


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Five basic types of stainlesssteels :

Austenitic - Susceptible to SCC. Can be hardenedby only by cold working. Good toughness andformability, easily to be welded and high corrosionresistance. Nonmagnetic except after excess coldworking due to martensitic formation.

Martensitic - Application: when high mechanicalstrength and wear resistance combined with somedegree of corrosion resistance are required. Typicalapplication include steam turbine blades, valvesbody 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 toembrittlement during welding. Not recommended forservice above 3000C because they will loss their

room temperature ductility.


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Duplex (austenitic + ferritic) has enhancedresistance to SCC with corrosion resistance performancesimilar to AISI 316 SS. Has higher tensile strengths thanthe austenitic type, are slightly less easy to form and haveweld ability similar to the austenitic stainless steel. Can beconsidered as combining many of the best features of

both the austenitic and ferritic types. Suffer a loss impactstrength if held for extended periods at high temperaturesabove 3000C.

Precipitation hardening - Have the highest strength but

require proper heat-treatment to develop the correctcombination of strength and corrosion resistance. To beused for specialized application where high strengthtogether with good corrosion resistance is required.


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Stress Corrosion Cracking of StainlessSteel

Stress corrosion cracking (SCC) is defined ascrack nucleation and propagation in stainlesssteel caused by synergistic action of tensilestress, either constant or slightly changing with

time, together with crack tip chemical reactionsor other environment-induced crack tip effect.

SCC failure is a brittle failure at relatively lowconstant tensile stress of an alloy exposed in a

specific corrosive environment.

However the final fracture because of overload ofremaining load-bearing section is no longer SCC.


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Three conditions must be presentsimultaneously to produce SCC:

- a critical environment

- a susceptible alloy

- some component of tensilestress


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Tensile

stress

Corrosive

environment

Susceptible

material

Stress

corrosion

cracking

Tensile stressis below yield

point

Corrosive

environment is

often specific to

the alloy system

Pure metals are more

resistance to SCC but not

immune and susceptibility

increases with strength


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Typical micro cracks formed duringSCC of sensitized AISI 304 SS

Surface morphology


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Example of crack propagation during

transgranular stress corrosion cracking (TGSCC)brass


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Example of crack

propagation during

intergranular stresscorrosion cracking

(IGSCC) ASTM A245

carbon steel


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Fracture surface oftransgranular SCC on

austenitic stainless steel in

hot chloride solution

Fracture surface of

intergranular SCC oncarbon steel in hot nitric

solution


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Fracture surface due to

intergranular SCC

Fracture surface due to

local stress has reached its

tensile strength value on the

remaining section


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Electrochemical effect

pitting

passive

active

cracking

zones

Usual 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

weak

Zone 1

Zone 2


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Note that non-susceptible alloy-environmentcombinations, will not crack the alloy even if held inone of the potential zones.

Temperature and solution composition (includingpH, dissolved oxidizers, aggressive ions andinhibitors or passivators) can modify the anodicpolarization behavior to permit SCC.

Susceptibility to SCC cannot be predicted solelyfrom the anodic polarization curve.


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Models of stress corrosioncracking

Slip step dissolution model

Discontinuous intergranular crack growth

Crack nucleation by rows of corrosionmicro-tunnels

Absorption induced cleavage

Surface mobility (atoms migrate out of the

crack tips)Hydrogen embrittlementHIC


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Control/prevention :

Reduce applied stress level

Remove residual tensile stress (internal

stress) Lowering oxidizing agent and/or critical

species from the environment

Add inhibitorUse more resistant alloys

Cathodic protection


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Alteration of Environment

Typical changes in medium are :

Lowering temperature but there are caseswhere increasing T decreases attack. E.g hot,fresh or salt water is raised to boiling T andresult in decreasing O

2solubility with T.

Decreasing velocity exception ; metals &alloys that passivate (e.g stainless steel)generally have better resistance to flowingmediums than stagnant. Avoid very highvelocity because of erosion-corrosion effects.


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Removing oxygen or oxidizers e.g boilerfeedwater was deaerated by passing it thru a largemass of scrap steel. Modern practice vacuumtreatment, inert gas sparging, or thru the use of

oxygen scavengers. However, not recommended foractive-passive metals or alloys. These materialsrequire oxidizers to form protective oxide films.

Changing concentration higher concentrationof acid has higher amount of active species (H ions).However, for materials that exhibit passivity, effectis normally negligible.

Environment factors


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Environment factorsaffecting 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 sea

Air pollution NaCl, SO2, sulfurous acid, etc.

Humidity cause condensation.


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Design Dos & Donts

Wall thickness allowance to accommodate for corrosioneffect.

Avoid excessive mechanical stresses and stressconcentrations 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 velocitiesand/or solid in suspension are involved erosion corrosion.

Avoid crevices e.g weld rather than rivet tanks and othercontainers, proper trimming of gasket, etc.


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Avoid 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 againstconcentrated sulfuric acid. But if remaining liquid isexposed 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 toproduce 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!!!

Protective Coatings /


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Protective Coatings /Wrapping

Provide 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 createssmall anode-large cathode leading to rapid attackat the damaged areas.

Sacrificial Zn, Al, Cd on steel. Exposed substratewill be cathodic & will be protected.

Application hot dipping, flame spraying,cladding, electroplating, vapor deposition, etc.


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Surface modification to structure or composition by useof directed energy or particle beams. E.g ion implantationand 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. Coatingliquid generally consists of solvent, resin and pigment.

The resin provides chemical and corrosion resistance, andpigments may also have corrosion inhibition functions.

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Corrosion Prevention 316