Corrosion by Acids and with Complexing Agents

In general, the susceptibility of metals M to aqueous corrosion is expected to correlate inversely with the E° values for the reduction of M + (aq) to M(s) the less positive E° is, the greater is the tendency of M to corrode in aerated water. Factors that can upset predictions based on E° include the presence of protective films passivation), overpotential effects (Sections 15.4 and 16.6), effects of complexing agents, and incursion of a cathode reaction other than O2 reduction or H2 evolution. 

Stabilization of nickel(II) and copper(ll) by complexing with EDTA or NTA similarly increases the vulnerability of these normally corrosion-resistant metals to corrosion by aerated waters. For this reason, EDTA or NTA solutions should not be used in copper or high nickel alloy equipment. 

Corrosion of metals by aqueous acids with hydrogen evolution is usually rapid and fairly uniform across the surface [general corrosion) since the reductive dissolution of the oxide film that helps maintain the distinction between anodic and cathodic sites is favored by low pH (reaction 16.9). Thus, although local anodic and cathodic areas persist, pitting becomes less important than overall loss of metal. If the oxide film is sufficiently insoluble in acids and is also resistant to reductive dissolution, as with titanium or stainless ( 11% Cr) steels, the metal may remain unaffected by aqueous acids, except at quite negative Eh values. In cases where the cathodic discharge of hydrogen ions 

If oxidizing anions are present, the main cathodic reaction in aqueous corrosion may be neither oxygen absorption nor hydrogen evolution but 

Copper metal therefore dissolves readily in relatively dilute aqueous nitric acid (see Section 15.2) to give sky-blue Cu (aq) and nitric oxide gas  

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