Transition metals such as Mo, W, Ta, Cr, and W can be used to significantly enhance the localized corrosion resistance of A1. Since these elements exhibit very low solubilities in aluminum, a nonequilibrium alloying method, such as sputter deposition, must be used to produce single-phase alloys. The addition of approximately 9 atom percent W to A1 can shift its pitting potential in the positive direction as much as 2600 mV. Heat-treatment of these nonequilibrium alloys results in the precipitation of a second phase that is detrimental, but not catastrophic, to corrosion performance. Surface analysis of the AI-W passive film formed as a function of applied anodic potential reveals that it remains thin throughout the polarization sequence and contains very little oxidized solute, regardless of the applied potential. The oxidized W present in the film is in the form of WO2 and WO3 (or WOg2). Previously, it was proposed that the enhanced passivity for sputter-deposited A1-Mo, A1-Cr, and A1-Ta alloys was a result of the oxidized solute in the film (MoO~ 2, CrOOH, or Ta20~), making the film less susceptible to chloride attack by electrostatic-repulsion, oxide-structure modification, or oxidized-solute barrier-layer formation mechanisms. The results of this investigation reveal that the electrostatic-repulsion or oxidized-solute barrierlayer mechanisms are not responsible for the dramatic enhancement in the passivity of A1 with the addition of W. Instead, the small amount of oxidized W in the passive film may interact synergistically with the hydrated aluminum oxide structure to form a more protective film.

9 The Electrochemical Society, Inc. 951 metal interface. Almost the same diffusion coefficients were observed for E/p and E/a coatings. The pore resistance of the samples has initially a high value of 10 I~ ~ and decreases linearly with time. This phenomena was attributed to diffusion to the solute into the epoxy coating. AcknowledgmentsThe authors appreciate the financial support of this project by the Dow Chemical Company and the Offshore Technology Research Center. One of us (M.. A. A.) received a student fellowship from the Saudi Arabian Cultural Mission to the U.S.A.ABSTRACT Sputter-deposited AI-W alloys exhibit considerably enhanced resistance to pitting corrosion over a range of pHs extending from pH 0 to 9.6. Surface analysis showed that although very little oxidized W is found in the passive film at near-neutral pHs, at pH 3 there are comparable amounts of oxidized W and A1. A review of the different mechanisms proposed to explain the passivity of this class of alloys suggests that the pitting resistance of A1-W is likely to result from inhibition and repassivation of pits due to the stability of oxidized W in low-pH environments as described by the solute-rich interphase model (SRIM). ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 128.122.253.228 Downloaded on 2015-06-04 to IP ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 128.122.253.228 Downloaded on 2015-06-04 to IP

The electrochemical behavior and surface chemistry of sputter-deposited non-equilibrium stainless AI-Ta alloys have been investigated. These alloys exhibit enhanced passivity over a pH range of 2-12 even though the passive film chemistry varies considerably over this range. This enhanced passivity can be explained by the solute-rich interphase mechanism (SRIM), which states that formation and passivation of occluded cells are controlled by localized concentrations of solute. The higher concentrations of solute at the metaloxide interface and around occluded cells stabilize the passive-film from continued CI-attack and dissolution.