Chemical and Structural Effects of Phosphorus on the Corrosion Behavior of Ion Beam Mixed Fe‐Cr‐P Alloys

Abstract

Ion beam mixing has been used to produce a variety of thin films of Fe‐Cr‐P metallic glasses to study the relative importance of alloy chemistry and amorphicity on the corrosion resistance of those alloys. Films of were produced by multilayer evaporation of iron, chromium, and iron phosphide, and ion beam mixed to homogenize the layers and induce the amorphous phase transformation. Films with 25 and 35 a/o phosphorus were entirely amorphous, films with 15 a/o were mixtures of crystalline and amorphous phases, and films with 0 and 8 a/o were entirely crystalline (P in solid solution bcc Fe‐Cr). The amorphous films were devitrified by heat‐treating. The alloy films produced in this study exhibited remarkable corrosion resistance in sulfuric acid solutions, with and without chloride additions. The addition of large amounts of phosphorus to Fe‐10Cr lowered the passive current density by nearly two orders of magnitude. The active dissolution regime was eliminated by the addition of phosphorus, lowering the open‐circuit corrosion rate by four orders of magnitude, primarily by suppressing the anodic dissolution reaction. Devitrification of the amorphous phase had no significant effect on the corrosion behavior of the resistant alloys. Examination of the passive oxides by x‐ray photoelectron spectroscopy indicated that the presence of phosphorus increased chromium enrichment in the passive oxide, but that the major mechanism of protection was the development of a phosphate‐rich oxide which may act as a chemical shield to prevent anodic dissolution of the underlying metal.