Mechanism of Pit Growth on Aluminum Under Open Circuit Conditions

Data on pit growth on aluminum under open circuit conditions have been analyzed and compared with the results under potentiostatic conditions. It was concluded that pit growth is ohmically controlled in both cases, which under ideal conditions results in a square root growth law. The influences of various factors such as the ohmic potential drop outside of the pit, the number of pits, time dependent electrolyte conductivity in the pit, the Tafel constant, and the cathodic current on the growth law are discussed. On the basis of the mechanism proposed, the influences of potential, electrolyte conductivity, temperature, and motion of the electrolyte on the pit growth rate can be understood. The relations derived from potentiostatic measurements permit the estimation of aluminum's useful life under open circuit conditions; they might also be helpful in failure analysis.