The Influence of Tungsten Alloying Additions on the Passivity of Aluminum

Transition metals such as Mo, W, Ta, Cr, and W can be used to significantly enhance the localized corrosion resistance of Al. 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 Al 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 Al‐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 and (or ). Previously, it was proposed that the enhanced passivity for sputter‐deposited Al‐Mo, Al‐Cr, and Al‐Ta alloys was a result of the oxidized solute in the film ( , , or ), 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 barrier‐layer mechanisms are not responsible for the dramatic enhancement in the passivity of Al 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.