The initial steps of localized corrosion of aluminum strained in aggressive solutions were studied by electron microscopy of oxide replicas. Aluminum samples were strained at a constant potential in the following deaerated solutions: 1M NaC1 (pH 7) ; 4M NaC1 (pH 11) ; and 1M NaNO8 (pH 7). No pitting was found on the aluminum strained in those solutions below the pitting potential; the slip bands showed the same substructure as that found on aluminum strained in air. At, and above, the pitting potential, pitting was found on the surface of the strained aluminum. Pitting nucleated on the whole surface in neutral NaC1 solutions. In 4M NaC1 solution (pH 11) and in 1M NaNO~ solution pitting nucleated preferentially on the slip bands, leading to semicylindrical pits. It was also concluded that mechanical film breakdown is not a sufficient condition for pit initiation. Another process, such as localized aci.dification, must be assumed to explain pit initiation at pitting potential.

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The corrosion behavior of Ni 270 in 1M H 3 PO4, NaH2 PO4 , Na2 HPO4 , and Na3 PO4 solutions, in the 25 to 100 C temperature range, was studied. Potentiokinetic and potentiostatic polarization curves were drawn in the four solutions. Anodic kinetics of Ni 270 increased with increasing temperature and increasing acidity of the solution.Corrosion potentials in 1 M H 3 PO4 and 1 M NaH 2 PO4 solutions were in the active corrosion range at all tested temperatures. Active to passive transition took place by successive steps at 25 and 50 C in both solutions. The first retardation in the anodic kinetics of Ni 270 seemed to be related to NiO formation and the second one to Ni 304 formation. At 75 and 100 C only one active peak was observed. Peak potential values were close to Ni/Ni 3 04 redox potentials for the 1M H 3 PO4 solution and to Ni/NiO redox potentials for the 1M NaH 2 PO4 solution.Passivation potential values for both solutions in the 25 to 100 C range were close to the corresponding Ni/Ni203 redox potentials.Potentiostatic polarization curves of Ni 270 in 1M Na 2 HPO4 solution showed that corrosion potentials were in the passive range at 25, 50, and 75 C. An active peak was obtained at 100 C, followed by a passive region. The corrosion potential of Ni 270 in the 1M Na 3 PO4 solution was in the passive zone in the 25 to 100 C temperature range.Film growth mechanism of the oxide film in the passive range was estimated from current density decay curves at constant potential. The initial state of anodic film growth follows a logarithmic or an inverse logarithmic law for most of the testing conditions. Film thickness values were also estimated in 1 M H 3 PO4 and 1 M NaH 2 PO4 solutions at 25 and 50 C.

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