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Technical
section of Nicprotect Z300TM
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Corrosion Process Fundamentals
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Corrosion
is basically an electrochemical process of
complex kinetics |
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In
which anodic metal dissolution
takes place at temperatures when
water is in liquid form. |
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Involving
cells of macroscopic dimensions
with very distinct anodic and
Cathodic areas |
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The
rate of anodic metal dissolution depends on |
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Surface
structure of the substrate |
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Density
of steps & kinks of the surface
orientation of crystal faces exposed
to the electrolyte |
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Dislocations
and grain boundaries in the metal |
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Segregation
of impurities from metal |
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Chemisorptions
of various substances from the
electrolyte which change the structure
of the interphase metal/electrolyte
- catalyzing or inhibiting the
metal dissolution |
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Close
correlation between kinetics and structure
of the metal surface |
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Dissolution
of divalent metal ions from kink sites in
mono atomic steps: |
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Surface
concentration of kinks depend on: |
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Electrode
potential |
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Crystallographic
orientation |
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Lattice
defects at metal surface |
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Chemisorption
of species from electrolyte |
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Metal
ion dissolution also retards due to MeOH+
formation |
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Thermodynamics: |
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Oxidation
means change in free energy (G) |
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 G
= G
(products) - G
(reactants) |
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where
G
is defined as standard
free energy (Gibbs
Functions) |
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Standard
free energy change for all metal
oxides is negative (means oxides
are stable) whereas metals are
not hence oxidation will always
occur. |
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Go
(kJ/mole) is related to standard
heat of reaction Ho
and So
standard change in entropy and
absolute T as under: |
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Go
=
Ho - T
So |
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For
spontaneous change,
the free energy (G)
of the system must
decrease (G
<0). |
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Selected
values of Std. enthalpies, free
energies and entropies |
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Progressively
stable substances |
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More
Stable
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Substance
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H
enthalpy kJ/mol
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G
energy kJ/mol
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S
entropy kJ/mol
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Fe
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0
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0
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27
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FeO
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267
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244
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54
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Fe2O3
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822
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741
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90
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Fe3O4
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1118
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1015
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146
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Electrochemistry |
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Free
energy provides driving force
for corrosion |
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Chemical
potential is augmented when electrically
charges species are involved. |
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Metal
surface in contact with aqueous
medium/film develops both anodic
& cathodic sites. |
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Basic
Reactions: |
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FOR
CARBON STEELS: |
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Important:
Total oxidation rate=Total reduction rate
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Electrolytic
or Galvanic Cell |
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e
represents the electron flow thro' external
short circuit. |
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Corrosion
in aerated water: |
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A
& C denote anodic and cathodic areas |
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Atmospheric
Corrosion: |
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Discontinuous,
complex system involving metal, corrosion
products, surface electrolyte and the atmosphere.
Takes place in corrosion cells. |
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K=
ne1 tnVk(n) |
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where
K is accumulated corrosion effect, tn is time
of wetness, Vk is average corrosion rate during
the individual period of wetness |
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Wetness
time is critical as it's the period
for which metal is covered by
the water fill |
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Wetness
depends on relative humidity,
rain, dew or fog (frequency &
duration), temperature, wind speed
& sunshine |
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Total
wetness time is defined as the
time metal is moist due to adsorption
of water vapor plus the period
surface is covered by a phase
layer of water |
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Composition
of surface electrolyte: |
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The
electrolytic film contains deposits
of |
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Various
species from the atmosphere |
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O2,
SOx,
NOx,
Chlorides,
carbonaceous
mat1erial |
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Various
corrosion products |
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Fe(OH)2,
Fe(OH)3,
a, b and c FeOOH,
Fe3O4 |
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The
electrolyte thus would have |
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Saturated oxygen - significant
corrosion impact |
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H2SO4 (SO2+H2O+1/2O2)
- significant corrosion impact |
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NO2, HNO3
-not much impact |
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Chlorides - significant corrosion
impact |
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Corrosion
products somewhat retard the corrosion
process |
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Corrosion
Mechanism: |
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Clean atmosphere 20-50 A thick
oxide film formation (inner layer
of Fe3O4
& outer layer of polycrystalline
Fe2O3) |
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Further
initiation of corrosion through
adsorption of SO2and water vapours
as also hygroscopic salts like
chlorides/Sulphate and carbonaceous
material forming corrosive electrolyte |
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Propagation
of process through the galvanic
cells formed |
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Pourbaix
Diagram: |
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Rudimentary
diagram showing domains:C : Corrosion
domain, P : Passive domain, I
: Immune domain |
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Pourbaix
diagram for Iron : |
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