NACE CP3 - Chpt 1. Mechanisms of Corrosion All Answers Correct

NACE CP3 - Chpt 1. Mechanisms of Corrosion All Answers Correct Corrosion Zone ️Fe0 -> Fe++ + 2e- (soluble - reaction can keep occuring) Immunity to Passive ️Fe0 + 2H2O -> Fe(OH)3 + 2H+ + 2e- (insoluble - builds 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.