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.

A study was performed to investigate the changes that take place in the chemistry of the passive film that forms on A1 alloys containing either Mo or Cr, as a function of solute concentration and applied potential in 0.1N KC1. The results show that the mechanisms by which the solute protects the Al alloy differ. Molybdenum forms a film containing MoO4 -2 that impedes the ingress and movement of the C1-anion in the film. In contrast, Cr forms a CrOOH barrier layer that inhibits the oxidation of the A1 substrate and restricts the C1-anions from reaching the metal/film interface. The results show that pitting of A1-Mo alloys occurs when sufficient MoO4 2 is replaced by hydrated Mo § compounds, whereas pitting of the A1-Cr alloys occurs when Cr +3 is oxidized to the more soluble Cr § state.Aluminum and its alloys are known for their poor resistance to localized attack and, in particular, for pitting in aqueous chloride-containing environments. The formation of pits is caused by the breakdown of the passive film in the presence of chlorides due to its interaction and chemical reaction with the chloride anion. This results in the formation of a more soluble, and therefore less protective, film which eventually leads to the rapid localized dissolution of A1 metal and the formation of a pit.Elements like Mo or Cr promote passivity in A1 if they can be retained in solid solution without the formation of precipitates which can serve as active microgalvanic cells. Retention of these additions in solid solution can be accomplished by several nonequilibrium alloying techniques (1-4). Since the corrosion behavior of A1 is dependent upon the protective properties of the passive film, the presence of these elements in the film improves the protective nature of the film by decreasing its susceptibility to pitting attack.Changes in the passive film can be studied by monitoring the products of the reactions. X-ray photoelectron spectroscopy (XPS) is an ex situ surface-sensitive technique that has been widely used to study the chemistry of passive films (4)(5)(6)(7)(8)(9)(10)(11)(12). This technique provides a quantitative analysis of the passive film and yields information regarding the oxidation state of the elemental species present, thereby providing insight into the nature of the chemical changes that occur during passivation and film breakdown in the presence of chlorides.The purpose of this paper is to study the changes occurring in the passive film on sputter-deposited A1 alloys containing either Mo and Cr as a function of applied potential from the open-circuit potential (Eoc) to the breakdown, or pitting, potential (Eb). The changes in the surface chemistry, are used to study the mechanism by which these alloying additions affect the passive behavior of A1 in aqueous chloride environments.

The electrochemical response of Al alloys containing various concentrations of Mo in solid solution was studied using potentiodynamic polarization. The evolution of the chemical composition of the passive film during exposure to deaerated 0.1N normalKCl was analyzed using x‐ray photoelectron spectroscopy. Exposure of Al‐Mo alloys to the chloride solution results in an increase in the corrosion potential and a decrease in the cathodic Tafel slope. The breakdown potential is also found to increase with higher concentrations of Mo in the alloy. These results correlate with an increase in the amount of Mo in the oxide film during polarization, even though the concentration of Mo in the passive film is well below that in the alloy. This work suggests that the oxidized Mo in the passive film inhibits the nucleation of pits in the alloy by improving the integrity of the passive film.