Corrosion & Environmental Degradation		Table of Contents
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	·Two metals in contact in a moist atmosphere can form a galvanic cell, resulting in the destruction of one metal by an oxidation reaction: M -> M n+ + ne This metal is the anode of the cell, and the metal ions go into the atmospheric-film electrolyte on the sample. · At the cathode, a reduction reaction must take place at the same rate using the electrons liberated in the oxidation process. In general, the concentration of metal ions in the electrolyte will be low, and so some other reaction will control the reduction process. ·If the atmospheric electrolyte is acidic (acid rain!), the reduction process might release hydrogen in the reaction:         2H+(solution) + 2e -> H2 (gas) ·An alternative reduction reaction can occur due to oxygen dissolved in the electrolyte surface-film: 2H2O + O2 (solution) + 4e (cathode) -> 4 (OH)- (solution) ·The hydroxyl ions produced in this second reduction reaction may react with the metal ions contributed by the oxidation reaction and produce a metal hydroxide.                            M2+ (solution) + 2(OH)- (solution) -> M(OH)2 (solution) ·If hydroxide precipitates on the cathode, it may form a deposit that hinders the cathode reactions. In some cases, the film may prevent further cathodic reaction terminating the corrosion (passivation). In a fluid flow, the hydroxide may be removed from the cathode region, permitting the corrosion reaction to continue.