Chemical and structural effects of phosphorus on the corrosion behavior of ion beam mixed iron-chromium-phosphorus alloys.

Amorphous metals often exhibit unusual resistance to degradation in the corrosive aqueous environments, even when crystalline alloys of similar composition corrode rapidly. Most amorphous metals are produced by combining metals and metalloids, and the chemical effects of the metalloid cannot be neglected. In this study, a novel sample preparation technique, ion beam mixing, was used to produce a variety of thin films of metallic glasses of the Fe-Cr-P family in order to study the mechanisms of corrosion resistance of those alloys. Films of Fe-10Cr-xP $(0\le\times\le35$ at%) were produced by multilayer evaporation of iron, chromium, and iron phosphide, and ion beam mixing used to homogenize the layers and induce the amorphous phase transformation. Films with 25 and 35 at% phosphorus were entirely amorphous, while films with 15 at% were mixtures of crystalline and amorphous phases, and films with 0 and 8 at% were entirely crystalline. The amorphous films could be devitrified by heat treating. Electrochemical tests were performed to study the corrosion behavior of the alloys. The films were 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 (XPS) indicated that the presence of phosphorus encouraged chromium enrichment in the passive oxide, but that the major mechanism of protection was the development of a phosphate-rich oxide. The phosphate may act as a chemical shield, preventing anodic dissolution of underlying metal.PhDNuclear EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/105519/1/9135585.pdfDescription of 9135585.pdf : Restricted to UM users only