Corrosion Zone
Fe0 -> Fe++ + 2e- (soluble - reaction can keep occuring)
Immunity to Passive
Fe0 + 2H2O -> Fe(OH)3 + 2H+ + 2e- (insoluble - builds
... [Show More] up at the interface)
Corrosion to Passive
Fe++ + 3H2O -> Fe(OH)3 +3H+ + 1e-
More Active (+ or -)
Electronegative - Less Noble
Less Active (+ or -)
Electropositive - More Noble
Nerst Equation
A metal electrode potential is a function of the metal ion activity, which is related to metal ion concentration.
As metal ion concentration increases (M^n+) the potential becomes more electropositive.
At 1 molar solution of metal ions.
Em = ....... ln(a^Mn+/a^Mo)
Cathode Reaction
Reduction Reaction
Anode Reaction
Oxidation Reaction
SHE to CSE
+316 mV
SHE to SSC (SJ)
+256 mV
SHE to ZRE
-800 mV
SSC to CSE
+60 mV
In non-aerated soils and waters the typical reduction reaction is ...
Reduction of Hydrogen
H+ + e- -> Ho
In aerated soils and waters the typical reduction reaction is ...
Reduction of Oxygen
O2 + 2H2O + 4e- -> 4OH-
Corrosion Current in a Corrosion Cell
Direction of positive charges flow.
Anode to Cathode through the electrolyte and from cathode to anode in the metalic path.
Define Polarization
The electrical energy used up in the transfer of charge across the respective metal/electrolyte interfaces. Build up of product.
Increase in polarization is a slowing down of the charge transfer reaction.
Faraday's Law
Material lost at the anode or deposited at the cathode is a function of
atomic weight of the metal
number of charges transferred
corrosion current
Cathode Depolarization Increase
Anything that increases the rate of charge transfer across the interface.
Increased H+
Increased aeration
Increased agitation
Increased temperature
Increased surface area
Increase iac
Anode Depolarization Increase
Anything that increases the rate of the oxidation reaction.
Increased H+ (for steel)
Increased temperature
Increased Cl-
Increased agitation
increased surface area
increased iac
Cathode Polarization Increase
Increased OH- (increased pH)
Decreased agitation
Decreased aeration
Decreased temperature
Decreased surface area
Increased film forming ions
Anode Polarization Increase
Oxidation reaction at the anode is inhibited.
Increased OH- (increased pH)
Decreased agitation
Decreased temperature
Decreased surface area
increased passivating ions
Increased M+ concentrations
Increase in Resistance in a Corrosion Cell
Decreased moisture
Decreased temperature
Increased metal path resistance
Decreased ion concentration
More resistant path depolarizes both the anode and cathode.
7 Types of Corrosion
Uniform Corrosion
Pitting Corrosion
Crevice Corrosion
Galvanic Corrosion
Environmentally Induced Cracking
Dealloying and Dezincification - one of the alloying metals is more active.
Erosion-Corrosion and Fretting [Show Less]