Sputter deposition of a corrosion-resistant amorphous metallic coating

Starting with corrosion-resistant amorphous Fe₃₂Ni₃₆Cr₁₄P₁₂B₆ alloy material, rf sputter deposition has been successfully used to deposit amorphous thin films very similar in composition onto low-carbon (i.e., 1008) steel. The effects that varying sputter deposition parameters has on a film's corrosion resistance, microstructure, and chemical composition have been examined. Optical, scanning, and transmission electron microscopy, Auger depth profiling, and x-ray diffraction were used to characterize the microstructure and composition of the films, while the corrosion resistance was determined by anodic polarization in basic and acidic solutions. A ∼4000 Å thick amorphous film sputtered at ambient temperature onto a 0.05 μm polished 1008 steel substrate improved the corrosion resistance of the steel in a buffered borate solution by lowering the steel's critical current density by two orders of magnitude and by raising its corrosion potential by ∼0.4 V. Bias voltage sputtering was required to produce a film with properties that could withstand a sulfuric acid solution. For example, a film sputtered at – 70 V at ambient temperature onto a steel substrate passivated in sulfuric acid solution, whereas the steel was completely active in this solution without the sputtered film. Passive current densities in this case were ∼2x10²μA/cm². In both solutions the improved corrosion resistance was exhibited by films with lower oxygen content and a denser microstructure. Thus a direct correlation between corrosion resistance, microstructure, and composition is shown.