XPS, ellipsometry and electrochemical studies on the passivity of Alloy-800

Electrochemical studies on Alloy-800, a candidate material for steam genera ting tubing for power plants, has been studied by the potentiodynamic polar ization technique in deaerated 1 N H2SO4 and 3.5% NaCl solutions. Thickness and refractive index of the films developed on the alloy in these media we re computed using ellipsometry, while the compositions of the films were de termined by surface analysis at the end of the polarization scan. In acid t he alloy presented a more noble potential (153 mV SCE) compared with the ch loride solution (-14 mV SCE). The passive current density was of 10(-5) cm( -2) in acid which was about one order less than that in the chloride soluti on. The film developed in acid was seen to be composed of protective oxides such as Cr2O3 and Fe3O4 while that developed in the chloride solution was seen to be composed of defective oxides such as Fe2O3 and Fe (OH). with a t race of NiCl2. The defective oxide film grows to a larger thickness and als o exhibits a larger passive current density. Prefilming the alloy in acid p rior to polarization in chloride solution results in a lower passive curren t density.