ENVIRONMENTAL FACTORS AFFECTING CRITICAL PITTING POTENTIAL OF ALUMINUM

The critical pitting potential determined potentiostatically of pure A1 in 0.1N NaCl is −0.40V (SHE). This value is not greatly sensitive to temperature (0–40°C), to small alloying additions of Mn or Mg, nor to thickness of oxide film produced by anodizing. The value is more active the higher the Cl− concentration, but becomes more noble with additions to NaCl of nitrates, chromates, acetates, benzoates, or sulfates. The latter act as pitting inhibitors which are effective in the order as listed. Dissolved Cu+ + in trace amounts deposit on the Al surface as Cu which then acts as an efficient cathode, shifting the corrosion potential of aluminum to the critical pitting potential. Trace amounts of Fe + + + and Pd + + act similarly. Anodized surfaces effectively retard penetration of the oxide by Cu++ thereby delaying onset of pitting, but they are not similarly effective when Al is coupled to Cu. The mechanism of pitting is interpreted in terms of competitive adsorption of Cl− with oxygen for sites on the metal surface. Extraneous anions compete in turn with Cl− ions, making it necessary to shift the potential in the positive direction in order for Cl− to adsorb followed by pit initiation. Similarities in adsorption parameters of various anions other than Cl− on Al and 18-8 stainless steel are pointed out. © 1969, The Electrochemical Society, Inc. All rights reserved.