Passivity and its breakdown on sputter-deposited amorphous Al-early transition metal alloys in 1 M HCl at 30°C

Abstract

The sputter-deposition method is successfully used for preparing amorphous AlTi, AlZr, AlNb, AlTa, AlMo and AlW alloys in wide composition ranges. Their corrosion behavior was investigated in 1 M HCl at 30°C. The results clearly reveal the difference in the alloying elements. When valve metals are alloyed, the passive films consist of a cation mixture of aluminum and other valve metals, their concentrations being not greatly different from the alloy compositions. These alloys are spontaneously passive. The corrosion rates of these alloys except for the AlTi alloys are considerably lower than that of aluminum metal. The corrosion rate of the AlTi alloys are almost the same as that of the aluminum metal regardless of the titanium content, because titanium is in the active state at the open circuit potential of the AlTi alloys. The AlTi, AlZr, AlNb and AlTa alloys suffer pitting by anodic polarization, but their pitting potentials are 350–1850 mV higher than the pitting potential of the aluminum metal. Alloying with molybdenum and tungsten decreases the corrosion rate of aluminum and raises remarkably the open circuit potential mostly due to decrease in the overpotential of hydrogen evolution. Anodic polarization of the AlMo alloys results in active dissolution possibly due to dissolution of molybdenum as molybdate, while that of the AlW alloys leads to coloration due to the formation of less protective corrosion product film consisting exclusively of the tungsten cation.