Download - Materials Selection for Corrosion Prevention
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CORROSION
CONTROL
MATERIAL SELECTION
ALTERATION OF ENVIRONMENT
PROPER DESIGN
CATHODIC PROTECTION
ANODIC 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, andwood]
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Metals and Alloys
No Environment Proper material
1 Nitric acid Stainless steels
2 Caustic Nickel and nickel
alloys
3 Hydrofluoric acid Monel (Ni-Cu)
4 Hot hydrochloric acid Hastelloys (Ni-Cr-Mo)(Chlorimets)
5 Dilute sulfuric acid Lead
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No Environment Proper material
6 Nonstaining atmospheric
exposure
Aluminium
7 Distilled water Tin
8 Hot strong oxidizing
solution
Titanium
9 Ultimate resistance Tantalum
10 Concentrated sulfuric acid Steel
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NORSOK
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NORSOK
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NORSOK
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NORSOK
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NORSOK
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Table 5 - Materials selection for sub-sea production and flowline systems
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Table 5 - Materials selection for sub-sea production and flowline systems
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Table 5 - Materials selection for sub-sea production and flowline systems
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E.g : Stainless Steels
Stainless steels are
iron base alloys that
contain a minimum
of approximately11% Cr, the amount
needed to prevent
the formation of rust
in unpollutedatmosphere.wt.% Cr
Dissolutionrat
e,cm/sec
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Alloying elements of stainless steel :
Other than Ni, Cr and C, the following alloying elements mayalso present in stainless steel: Mo, N, Si, Mn, Cu, Ti, Nb, Taand/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.
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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.
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Ternary diagram of Fe-Cr-Ni at 6500and 10000C
AISI : American Iron and Steel Institute
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Anodic polarization curves of Cr, Ni and Fe in 1 N
H2SO4solution
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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 (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 (lowcarbon stainless steels to avoid sensitization during
welding).
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Minor alloying elements :
Manganese
Austenitic forming element. When necessary can be used tosubstitute Ni. Concentration of Mn in stainless steel is usually 2-
3%.
Molybdenum
Ferritic forming element. Added to increase pitting corrosionresistance 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 increasedup to 10-14%).
Improves mechanical properties of stainless steel at high
temperature. Increase aqueous corrosion resistance of material
exposed in reducing acid.
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Tungsten
Is added to increase the strength and toughness of martensiticstainless 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.
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Copper
Is 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.
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Influence of alloying elements on pitting
corrosion resistance of stainless steels
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Influence of alloying elements on crevice
corrosion resistance of stainless steels
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Influence of alloying elements on SCC
resistance of stainless steels
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Five basic types of stainless steels :
Austenitic- Susceptible to SCC. Can be hardened by only bycold working. Good toughness and formability, easily to bewelded and high corrosion resistance. Nonmagnetic except afterexcess cold working due to martensitic formation.
Martensitic- Application: when high mechanical strength and
wear resistance combined with some degree of corrosionresistance are required. Typical application include steamturbine blades, valves body and seats, bolts and screws, springs,knives, surgical instruments, and chemical engineeringequipment.
Ferritic- Higher resistance to SCC than austenitic SS. Tend tobe notch sensitive and are susceptible to embrittlement duringwelding. Not recommended for service above 3000C becausethey will loss their room temperature ductility.
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Duplex (austenitic + ferritic)has enhanced resistance toSCC with corrosion resistance performance similar to AISI316 SS. Has higher tensile strengths than the austenitic type,are slightly less easy to form and have weld ability similar tothe austenitic stainless steel. Can be considered as combining
many of the best features of both the austenitic and ferritictypes. Suffer a loss impact strength if held for extended
periods at high temperatures above 3000C.
Precipitation hardening- Have the highest strength butrequire proper heat-treatment to develop the correctcombination of strength and corrosion resistance. To be usedfor specialized application where high strength together withgood corrosion resistance is required.
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Stress Corrosion Cracking of Stainless Steel
Stress corrosion cracking (SCC) is defined as crack nucleationand propagation in stainless steel caused by synergistic actionof tensile stress, either constant or slightly changing with time,together with crack tip chemical reactions or otherenvironment-induced crack tip effect.
SCC failure is a brittle failure at relatively low constant tensilestress of an alloy exposed in a specific corrosive environment.
However the final fracture because of overload of remainingload-bearing section is no longer SCC.
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Three conditions must be present simultaneously
to produce SCC:
- a critical environment
- a susceptible alloy
- some component of tensile stress
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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 during SCC 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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Electrochemical effect
pitting
passive
active
cracking
zones
Usual region for
TGSCC, mostly is
initiated by pitting
corrosion
(transgranular cracking
propagation needshigher 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-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) canmodify the anodic polarization behavior to permit SCC.
Susceptibility to SCC cannot be predicted solely from the anodic
polarization curve.
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Models of stress corrosion cracking
Slip step dissolution model
Discontinuous intergranular crack growth
Crack nucleation by rows of corrosion micro-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 speciesfrom the environment
Add inhibitor
Use more resistant alloys Cathodic protection
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Alteration of Environment
Typical changes in medium are :
Lowering temperaturebut there are cases whereincreasing T decreases attack. E.g hot, fresh or salt water is
raised to boiling T and result in decreasing O2solubility
with T.
Decreasing velocityexception ; metals & alloys thatpassivate (e.g stainless steel) generally have better
resistance to flowing mediums than stagnant. Avoid very
high velocity because of erosion-corrosion effects.
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Removing oxygen or oxidizerse.g boiler feedwaterwas deaerated by passing it thru a large mass of scrap steel.
Modern practicevacuum treatment, inert gas sparging, or
thru the use of oxygen scavengers. However, not
recommended for active-passive metals or alloys. Thesematerials require oxidizers to form protective oxide films.
Changing concentrationhigher concentration of acidhas higher amount of active species (H ions). However, for
materials that exhibit passivity, effect is normally negligible.
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Environment factors affecting
corrosion design :
Dust particles and man-made pollutionCO, NO,
methane, etc.
Temperaturehigh T & high humidity accelerates
corrosion.
Rainfallexcess washes corrosive materials and
debris but scarce may leave water droplets.
Proximity to sea Air pollutionNaCl, SO2, sulfurous acid, etc.
Humiditycause condensation.
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Design Dos & Donts
Wall thicknessallowance 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 involvederosion corrosion.
Avoid crevicese.g weld rather than rivet tanks and other
containers, proper trimming of gasket, etc.
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Avoid sharp cornerspaint 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 andrapid attack occurs.
Avoid hot spots during heat transfer operationslocalized
heating and high corrosion rates. Hot spots also tend to
produce stressesSCC failures.
Design to exclude airexcept for active-passive metals and
alloys coz they require O2for protective films.
Most general rule : AVOID HETEROGENEITY!!!
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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 creates small
anode-large cathode leading to rapid attack at the
damaged areas.
Sacrificial
Zn, Al, Cd on steel. Exposed substrate willbe cathodic & will be protected.
Application hot dipping, flame spraying, cladding,
electroplating, vapor deposition, etc.
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