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 passivity and corrosion behavior of several supersaturated aluminum alloys formed by cosputter deposition have been investigated. Several of these alloys exhibit superior resistance to localized attack in electrochemical polarization measurements and salt fog tests. X-ray photoelectron spectroscopy was used to examine the surface chemistry of the passive film as a function of applied potential for A1, AI-Ta, and AI-Zr alloys. The passive film that forms on each alloy becomes enriched in oxidized solute as the specimen is anodically polarized. In general, the oxidized solute protects the substrate by restricting the ingress of chloride and oxygen and thereby preventing or reducing localized attack and film growth, respectively. Of the solutes examined, Ta is the most effective in this regard; the passive film on A1-Ta alloys remains thin and protective at the most noble potentials. Breakdown occurs only as the potential drop across the film becomes great enough to allow the transport of chlorides.

The composition and thickness of the oxide/hydroxide film that forms on pure Al polarized in 0.05M Na2SO4 solutions with and without the addition of 1000 ppm Cl− under acidic, near-neutral, and alkaline conditions have been determined using x-ray photoelectron spectroscopy. Changes in the films due to the ex situ nature of the measurements have also been characterized. The results were plotted on surface behavior diagrams to follow the evolution of the surface during polarization and compared with the results predicted by the Pourbaix diagram. Although Al(OH)3 is the most stable species to form in near-neutral solutions under equilibrium conditions, the composition of the film at the corrosion potential under our experimental conditions was found to be that of the oxyhydroxide AlOOH at each pH. In the pH 7 and pH 10 solutions, this film thickens during anodic polarization (up to the pitting potential for the Cl−-containing solutions), but the composition is unchanged. In contrast, the Cl−-free, pH 2 solutions, an oxide film forms at the oxyhydroxide–metal interface and rapidly thickens with increasing anodic overpotentials. The original AlOOH layer then slowly dissolves so that the film becomes entirely Al2O3. During cathodic polarization, we observed growth of the oxyhydroxide film, especially at pH 2, which we attributed to cathodic corrosion.

The corrosion behavior of Al and six binary alloys (Al‐Mo, Al‐Cr, Al‐Ta, Al‐Zr, Al‐Cu, and Al‐Si) prepared by sputter deposition is studied by electrochemical polarization measurements in KCl solutions and under salt fog exposures.

The methodology of failure analysis and the use of XPS sputterdepth profiles in failure analysis is illustrated in three examples: a titanium adhesive bond, a multilayer optical filter, and an ohmic contact. XPS sputter-depth profiles supplement those obtained using AES and are essential to the failure analysis of a non-conducting sample or one that is easily damaged by an electron beam. XPS sputterdepth profiles also allow chemical state determination and improved quantification, especially for samples in which the Auger signals of different elements overlap.