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4.0 CORROSION PREVENTION

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Title: 4.0 CORROSION PREVENTION


1
4.0 CORROSION PREVENTION
  • MATERIAL SELECTION
  • ALTERATION OF ENVIRONMENT
  • PROPER DESIGN
  • CATHODIC PROTECTION
  • ANODIC PROTECTION
  • COATINGS WRAPPING

2
  • (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

3
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
4
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
5
E.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.
Dissolution rate, cm/sec
wt. Cr
6
Alloying 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.

7
Binary 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.
8
  • 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.

9
Ternary diagram of Fe-Cr-Ni at 6500 and 10000C
AISI American Iron and Steel Institute
10
Anodic polarization curves of Cr, Ni and Fe in 1
N H2SO4 solution
11
Influence of Cr on corrosion resistance of iron
base alloy
12
Influence of Ni on corrosion resistance of iron
base alloy
13
Influence of Cr on iron base alloy containing
8.3-9.8wt.Ni
14
  • 3. Carbon
  • Very strong austenite forming element (30x
    more effective than Ni). I.e. if austenitic
    stainless steel 18Cr-8Ni contains 0.007C, its
    structure will convert to ferritic structure.
    However the concentration of carbon is usually
    limited to 0.08C (normal stainless steels) and
    0.03C (low carbon stainless steels to avoid
    sensitization during welding).

15
Minor 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.

16
  • Tungsten
  • 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 Tantalum
  • To stabilize stainless steel by reducing
    susceptibility of the material to intergranular
    corrosion. Ti addition gt 5xC. TaNb addition gt
    10xC.

17
  • 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.

18
Influence of alloying elements on pitting
corrosion resistance of stainless steels
19
Influence of alloying elements on crevice
corrosion resistance of stainless steels
20
Influence of alloying elements on SCC resistance
of stainless steels
21
Five 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.

22
  • Duplex (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.

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Stress Corrosion Cracking of Stainless Steel
  • Stress 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.

32
  • Three conditions must be present simultaneously
    to produce SCC
  • - a critical environment
  • - a susceptible alloy
  • - some component of tensile stress

33
Pure metals are more resistance to SCC but not
immune and susceptibility increases with strength
Tensile stress is below yield point
Susceptible material
Tensile stress
Stress corrosion cracking
Corrosive environment
Corrosive environment is often specific to the
alloy system
34
Typical micro cracks formed during SCC of
sensitized AISI 304 SS
Surface morphology
35
Example of crack propagation during transgranular
stress corrosion cracking (TGSCC) brass
36
Example of crack propagation during intergranular
stress corrosion cracking (IGSCC) ASTM A245
carbon steel
37
Fracture surface of intergranular SCC on carbon
steel in hot nitric solution
Fracture surface of transgranular SCC on
austenitic stainless steel in hot chloride
solution
38
Fracture surface due to local stress has reached
its tensile strength value on the remaining
section
Fracture surface due to intergranular SCC
39
Electrochemical effect
Usual region for TGSCC, mostly is initiated by
pitting corrosion (transgranular cracking
propagation needs higher energy)
pitting
Zone 1
passive
cracking zones
Zone 2
Usual region for IGSCC, SCC usually occurs where
the passive film is relatively weak
active
40
  • 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) can modify the anodic
    polarization behavior to permit SCC.
  • Susceptibility to SCC cannot be predicted solely
    from the anodic polarization curve.

41
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 embrittlement?HIC

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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 inhibitor
  • Use more resistant alloys
  • Cathodic protection

43
2. Alteration of Environment
  • Typical 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.

44
  • Removing 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.

45
Environment 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 sea
  • Air pollution NaCl, SO2, sulfurous acid, etc.
  • Humidity cause condensation.

46
3. Design Dos Donts
  • Wall 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.

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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 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!!!

48
4. 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 will be cathodic will be protected.
  • Application hot dipping, flame spraying,
    cladding, electroplating, vapor deposition, etc.

49
  • Surface 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.

50
5. Cathodic and Anodic Protection5.1 Cathodic
Protection
  • Cathodic Protection (CP) was employed before the
    science of electrochemistry had been developed
  • CP is achieved by supplying electrons to the
    metal structure to be protected. (M Mn
    ne) and (2H 2e H2)
  • Examination of equation indicates the addition of
    electrons to the structure will tend to suppress
    metal dissolution and increase the rate of
    hydrogen dissolution
  • If current is considered to flow from () to (-),
    then a structured is protected if current enters
    it from the electrolyte
  • Conversely, accelerated corrosion occurs if
    current passes from the metal to the electrolyte,
    this current convention has been adopted in
    cathodic protection technology and is used here
    for consistency

51
  • There 2 ways to cathodically protect a structure
  • (i) by an external power supply
  • (ii) by appropriate galvanic coupling

52
  • Figure illustrates CP by impressed current
  • Here, an external dc power supply is connected to
    an underground tank, the negative terminal of the
    power supply is connected to the tank, and the
    positive terminal to an inert anode such as
    graphite or Duriron.
  • The electric leads to the tank and inert the
    electrode are carefully insulated to prevent
    current leakage
  • The anode is usually surrounded by backfill
    consisting of coke breeze, gypsum or bentonite
    which improves electric contact between the anode
    and the surrounding soil.
  • In figure, current passes to the metallic
    structure and corrosion is suppressed

53
  • CP by galvanic coupling to Mg is shown in Fig
    6.2
  • Mg is anodic with respect to steel and corrodes
    preferentially when galvanic coupled The anode
    in this case is called a sacrificial anode since
    it is consumed during the protection of the steel
    structure
  • CP using sacrificial anodes can also be used to
    protect buries pipelines shown Fig 6.3. the
    anodes are spaced along the pipe to ensure
    uniform current distribution

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5.2 Anodic Protection
  • In contrast to CP, anodic protection (AP) is
    relatively new
  • This technique was developed using electrode
    kinetics principles and is somewhat difficult to
    describe without introducing advanced concepts of
    electrochemical theory
  • Simply, AP is based is based on the formation of
    a protective film on metals by externally applied
    anodic currents.
  • This usually except for metals with
    active-passive transitions such as Ni, Fe, Cr, Ti
    and their alloys
  • If carefully controlled anodic currents are
    applied to these materials, they are passivated
    and the rate of metal dissolution is decreased
  • To anodically protect a structure, a device
    called a potentiostat is required
  • A potentiostat is an electronic device that
    maintains a metal at a constant potential with
    respect to a reference electrode.

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  • The potentiostat has three terminals, one
    connected to the tank, another to an auxiliary
    cathode (a platinum or platinum-clad electrode)
    and the third to a reference electrode (e.g.
    calomel cell)
  • In operation, the potentiostat maintains a
    constant potential between the tank and the
    reference electrode
  • The optimum potential for protection is
    determined by electrochemical measurements

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