NACE CP3 - Chpt 1. Mechanisms of Corrosion questions and answers 100% correct

NACE CP3 - Chpt 1. Mechanisms of Corrosion questions and answers 100% correctCorrosion 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. 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