4.0 CORROSION PREVENTION

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4.0 CORROSION PREVENTION

• 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, and wood

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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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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
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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.

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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.
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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 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 (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).

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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.

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• 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.

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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 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.

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• 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.

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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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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
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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
stress corrosion cracking (IGSCC) ASTM A245
carbon steel
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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
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Fracture surface due to local stress has reached
its tensile strength value on the remaining
section
Fracture surface due to intergranular SCC
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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
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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) can modify 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 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

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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.

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• 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.

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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.

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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!!!

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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.

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• 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.

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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

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

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• 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

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• 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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