Mechanism of saccharin transformation to metal sulfides and effect of inclusions on corrosion susceptibility of electroplated CoFe magnetic films

The electroplated magnetic alloys (1.0T Ni80Fe20, 1.6T Ni45Fe55, 2.4T Co40Fe60), obtained in the presence of saccharin, and sputtered magnetic alloys of the same composition showed dramatically different corrosion properties at pH 5.9. The higher corrosion susceptibility of electroplated magnetic alloys, known for many years, was generally attributed to sulfur inclusions into the deposit. However, there was no direct evidence of the structure of sulfur-containing molecules included in deposit. We have analyzed electroplated, EP-CoFe, and sputtered, SP-CoFe, magnetic films using electrochemical, secondary ion mass spectroscopy, X-ray photoelectron spectroscopy (XPS), and high-pressure liquid chromatography (HPLC) techniques. The analysis of electroplated CoFe films obtained in the presence of saccharin revealed saccharin, benzamide, o-toluenbenzamide (HPLC) and metal sulfides (XPS) in EP-CoFe deposit. The proposed mechanism for saccharin transformation to metal sulfides involves four steps: (i) a reductive cleavage of C-S bond in saccharin giving rise to benzamido sulfinate, (ii) a desulfurization step leading to benzamide and sulfur dioxide, (iii) an electrochemical reduction of sulfur dioxide to hydrogen sulfide, and (iv) a reaction between H2S and M+2 (M = CO, Fe) to metal sulfides. The higher corrosion susceptibility of EP-CoFe magnetic alloys than SP-CoFe magnetic alloys is discussed in terms of the mechanism of sulfur-assisted corrosion. (c) 2006 The Electrochemical Society.