Structural and anticorrosive properties of magnetron-sputtered Fe–Cr–Ni and Fe–Cr–Ni–Ta alloy films

Fe–Cr–Ni and Fe–Cr–Ni–Ta films were deposited on glass by magnetron sputtering using an AISI 316 stainless steel target, which for some experiments was supplemented with tantalum. XRD results indicated a fine crystalline structure of the sputtered Fe–Cr–Ni alloy film, which contained both martensitic and austenitic phases. The Fe–Cr–Ni–Ta alloy film had an amorphous structure. AFM images showed the morphology of both alloys to have characteristic pits, which had dimensions of the order of micrometres. The average depth of the pits in the tantalum-containing film was greater than that for the tantalum-free film (350–400 and 80–160 nm, respectively). The resistance to corrosion of both films was estimated using anodic currents at the passive region and breakdown potential. The sputtered Fe–Cr–Ni alloy had superior stability when compared to conventional stainless steel in both chloride and sulfate solutions (pH 6.5 and pH 3). The Fe–Cr–Ni–Ta alloy films exhibited higher stability than their tantalum-free counterpart; the tantalum inhibiting effect was more pronounced in an acid medium than in a neutral one. The breakdown potential of the sputtered alloy films was much higher than the values for bulk alloys in chloride solutions, while in sulfate solutions this difference was negligible. The superior anticorrosive stability of the sputtered alloys in chloride solutions was attributed to the high passivation capability of the oxide film due to its homogeneous and uniform structure.