Chemical and structural effects of phosphorus on the corrosion behavior of ion beam mixed Fe-Cr-P alloys

An experimental program was conducted to determine the mechanisms by which phosphorus affects the corrosion and passi~ation behavior of F e Cr P alloys, "lb idcnti~ separately the ell~cts of structure and chemistry on the corrosion behavior, thin tilms of Fe--ItK'r-xP 10 < x < 35 at.%) were I~-epared by ion beam mixing. Films with a phosphorus content greater than approximately 20 at.% were found to be entirely amorphous, i~evitrilication of the amorphous phase was accomplished by heating the samples to 450 '(" in an inert environment, Standard polarization tests of the alloys in sulfuric acid twith and without (.'1 I indicated that the lilms containing phosphorus were more corrosion r¢~,'istant than Fe I(K'r, at I~th active and passive potentials, There was a monotonic relationship betw~n the amount of phosphorus in the alloy and the corrosion resistance, with the ol~en-circuit corrosion rate of Fe I (K'r 35P nearly four orders of magnitude lower than that of Fe I(K'r. Devitrilication of the alloys had no significant effect on the corrosion rate, indicating that the primary eflL-ct of phosphorus is chemical in nature, and n'~t structural, The passive oxides were depth-prolilcd using X-ray photoelectron spectroscopy, which indicated that phosphorus was a primary constituent, as phosphate, The pr~sem.'¢ of phosphate in the passive oxides reduced the overall corrosion rate directly, by suppressing anodic dissolution. The presence of phosphorus did enhance chromium enrichment in the oxide, but that was not thought to he the primary mechanism by which phosphorus increased the corrosion resistance.