NACE CP3 - Chpt 1. Mechanisms of Corrosion Exam Questions and Answers

Corrosion Zone - ANSWER-Fe0 -> Fe++ + 2e- (soluble - reaction can keep occuring) Immunity to Passive - ANSWER-Fe0 + 2H2O -> Fe(OH)3 + 2H+ + 2e- (insoluble - builds up at the interface) Corrosion to Passive - ANSWER-Fe++ + 3H2O -> Fe(OH)3 +3H+ + 1e- More Active (+ or -) - ANSWER-Electronegative - Less Noble Less Active (+ or -) - ANSWER-Electropositive - More Noble Nerst Equation - ANSWER-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 - ANSWER-Reduction Reaction Anode Reaction - ANSWER-Oxidation Reaction SHE to CSE - ANSWER-+316 mV SHE to SSC (SJ) - ANSWER-+256 mV SHE to ZRE - ANSWER--800 mV SSC to CSE - ANSWER-+60 mV In non-aerated soils and waters the typical reduction reaction is ... - ANSWER-Reduction of Hydrogen H+ + e- -> Ho In aerated soils and waters the typical reduction reaction is ... - ANSWER-Reduction of Oxygen O2 + 2H2O + 4e- -> 4OH- Corrosion Current in a Corrosion Cell - ANSWER-Direction of positive charges flow. Anode to Cathode through the electrolyte and from cathode to anode in the metalic path. Define Polarization - ANSWER-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 - ANSWER-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 - ANSWER-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 - ANSWER-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 - ANSWER-Increased OH- (increased pH) Decreased agitation Decreased aeration Decreased temperature Decreased surface area Increased film forming ions Anode Polarization Increase - ANSWER-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 - ANSWER-Decreased moisture Decreased temperature Increased metal path resistance Decreased ion concentration More resistant path depolarizes both the anode and cathode. 7 Types of Corrosion - ANSWER-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 Which type of Corrosion CANNOT be protected by Cathodic Protection? - ANSWER-Hydrogen-induced cracking and fatigue cracking Pourbaix Diagram - ANSWER-The potential E developed across the interface is a function of the metal involved and the pH Then there is a thermodynamic tendency for the metal to corrode by one or more of the three reactions. Exchange Current Density (Tafel Equastion)