CORROSION

1
CORROSION

• OXIDATION
• CORROSION
• PREVENTION AGAINST CORROSION

Principles and Prevention of Corrosion D.A.
Jones Prentice-Hall, Englewood-Cliffs (1996)
2
Attack of Environment on Materials

• Metals get oxidized
• Polymers react with oxygen and degrade
• Ceramic refractories may dissolved in contact
  with molten materials
• Materials may undergo irradiation damage

3
Oxidation

• Oxide is the more stable than the metal (for
  most metals)
• Oxidation rate becomes significant usually only
  at high temperatures
• The nature of the oxide determines the rate of
  oxidation

4

• For good oxidation resistance the oxide should
  be adherent to the surface
• Adherence of the oxide f(the volume of the
  oxide formed the volume of metal
  consumed in the oxidation) f(Pilling-Bedworth
  ratio)
• PB lt 1 ? tensile stresses in oxide film ?
  brittle oxide cracks
• PB gt 1 ? compressive stresses in oxide film ?
  uniformly cover metal surface and is protective
• PB gtgt 1 ? too much compressive stresses in oxide
  film ? oxide cracks

5

• If the metal is subjected to alternate heating
  and cooling cycles ? the relative thermal
  expansion of the oxide vs metal determines the
  stability of the oxide layer
• Oxides are prone to thermal spalling and can
  crack on rapid heating or cooling
• If the oxide layer is volatile (e.g. Mo and W
  at high temperatures) ? no protection

6
Progress of oxidation after forming the oxide
layer diffusion controlled ? activation
energy for oxidation is activation energy for
diffusion through the oxide layer
Oxygen anions
Metal Cations
Oxidation occurs at air-oxide interface
Oxidation occurs at metal-oxide interface

• Diffusivity f(nature of the oxide layer,
  defect structure of the oxide)
• If PB gtgt 1 and reaction occurs at the M-O
  interface ? expansion cannot be accommodated

7
Oxidation resistant materials

• As oxidation of most metals cannot be avoided
  the key is to form a protective oxide layer on
  the surface
• The oxide layer should offer a high resistance
  to the diffusion of the species controlling the
  oxidation
• The electrical conductivity of the oxide is a
  measure of the diffusivity of the ions (a
  stoichiometric oxide will have a low diffusivity)

• Alloying the base metal can improve the
  oxidation resistance
• E.g. the oxidation resistance of Fe can be
  improved by alloying with Cr, Al, Ni
• Al, Ti have a protective oxide film and usually
  do not need any alloying

8
Diffusion in Ionic crystals

• Schottky and Frenkel defects (defects in thermal
  equilibrium) assist the diffusion process
• If Frenkel defects dominate ? the cation
  interstitial of the Frenkel defect carries
  the diffusion flux
• If Schottky defects dominate ? the cation
  vacancy carries the diffusion flux
• Other defects in ionic crystals ? impurities and
  off-stoichiometry ? Cd2 in NaCl crystal
  generates a cation vacancy ? ?s diffusivity ?
  Non-stoichiometric ZnO ? Excess Zn2 ? ?
  diffusivity of Zn2 ? Non-stoichiometric FeO ?
  cation vacancies ? ? diffusivity of Fe2
• Electrical conductivity ? Diffusivity

Frenkel defect
Schottky defect

• Cation (being smaller get displaced to
  interstitial voids
• E.g. AgI, CaF2

• Pair of anion and cation vacancies
• E.g. Alkali halides

9
Alloying of Fe with Cr

• A protective Cr2O3 layer forms on the surface of
  Fe ?(Cr2O3) 0.001 ?(Fe2O3)
• Upto 10 Cr alloyed steel is used in oil
  refinery components
• Cr gt 12 ? stainless steels ? oxidation
  resistance upto 1000oC ? turbine blades,
  furnace parts, valves for IC engines
• Cr gt 17 ? oxidation resistance above 1000oC
• 18-8 stainless steel (18Cr, 8Ni) ? excellent
  corrosion resistance
• Kanthal (24 Cr, 5.5Al, 2Co) ? furnace
  windings (1300oC)

Other oxidation resistant alloys

• Nichrome (80Ni, 20Cr) ? excellent oxidation
  resistance
• Inconel (76Ni, 16Cr, 7Fe)

10
Corrosion

• THE ELECTRODE POTENTIAL
• When an electrode (e.g. Fe) is immersed in a
  solvent (e.g. H2O) some metal ions leave the
  electrode and ve charge builds up in the
  electrode
• The solvent becomes ve and the opposing
  electrical layers lead to a dynamic equilibrium
  wherein there is no further (net) dissolution of
  the electrode
• The potential developed by the electrode in
  equilibrium is a property of the metal of
  electrode ? the electrode potential
• The electrode potential is measured with the
  electrode in contact with a solution containing
  an unit concentration of the ions of the same
  metal with the standard hydrogen electrode as the
  counter electrode (whose potential is taken to be
  zero)

Metalions
-ve
ve
11
Standard electrode potential of metals
Standard potential at 25oC
Increasing propensity to dissolve
12

• Alloys used in service are complex and so are
  the electrolytes (difficult to define in
  terms of M) (the environment provides the
  electrolyte

• Metals and alloys are arranged in a qualitative
  scale which gives a measure of the tendency
  to corrode ? The Galvanic Series

Galvanic series
More reactive
13
Galvanic Cell
e? flow
Anode Zn (?0.76)
Cathode Cu (0.34)
Cu2 2e? ? Cu Reduction
Zn ? Zn2 2e? oxidation
or 2H 2e? ? H2 or O2 2H2O 4e? ? 4OH?
Zn will corrode at the expense of Cu
14
Anodic/cathodic electrodes
Anodic/cathodic phases at the microstructural
level
Differences in the concentration of the Metal ion
How can galvanic cells form?
Differences in the concentration of oxygen
Difference in the residual stress levels
15

• Different phases (even of the same metal) can
  form a galvanic couple at the
  microstructural level (In steel Cementite is
  noble as compared to Ferrite)
• Galvanic cell may be set up due to concentration
  differences of the metal ion in the
  electrolyte ? A concentration cell Metal ion
  deficient ? anodic Metal ion excess ?
  cathodic
• A concentration cell can form due to differences
  in oxygen concentration Oxygen deficient region
  ? anodic Oxygen rich region ? cathodic
• A galvanic cell can form due to different
  residual stresses in the same metal Stressed
  region more active ? anodic Stress free region
  ? cathodic

O2 2H2O 4e? ? 4OH?
16
Polarization

• Anodic and Cathodic reactions lead to
  concentration differences near the
  electrodes
• This leads to variation in cathode and anode
  potentials (towards each other) ? Polarization

Vcathode
IR drop through the electrolyte
Potential (V) ?
Vcathode
Steady state current
Current (I) ?
17
Passivation

• Iron dissolves in dilute nitric acid, but not in
  concentrated nitric acid
• The concentrated acid oxidizes the surface of
  iron and produces a thin protective oxide
  layer (dilute acid is not able to do so)

• ? potential of a metal electrode ? ? in current
  density (I/A)
• On current density reaching a critical value ?
  fall in current density (then remains constant)
  ? Passivation

18
Prevention of Corrosion

• Basic goal ? ? protect the metal ? avoid
  localized corrosion
• When possible chose a nobler metal
• Avoid electrical / physical contact between
  metals with very different electrode potentials
  (avoid formation of a galvanic couple)
• If dissimilar metals are in contact make sure
  that the anodic metal has a larger surface area
  / volume
• In case of microstructural level galvanic
  couple, try to use a course microstructure
  (where possible) to reduce number of galvanic
  cells formed
• Modify the base metal by alloying
• Protect the surface by various means
• Modify the fluid in contact with the metal ?
  Remove a cathodic reactant (e.g. water) ? Add
  inhibitors which from a protective layer
• Cathodic protection ? Use a sacrificial anode
  (as a coating or in electrical contact) ? Use an
  external dc source in connection with a
  inert/expendable electrode