corrosion prevention 316
TRANSCRIPT
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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.