APPLICATION OF ELECTRON SPECTROSCOPY FOR CHEMICAL ANALYSIS TO THE PASSIVE FILM OF AQUEOUSLY CORRODED STAINLESS STEEL
Abstract
The work reported dealt with two aspects of corrosion study. The first was the study of the nature of the passive surface film formed on aqueously corroded stainless steels. The second was the use of Electron Spectroscopy for Chemical Analysis, ESCA, to study corrosion problems. The very thin surface film reported to be on the order of 10 to 80 angstroms thick, had been difficult to study. Since ESCA had not been widely applied to metallurgical systems, the sample preparation, ESCA manipulations and data handling procedures were established as part of this project. Samples from commercially manufactured alloys were aqueously corroded under potentiostatic control. The main experiments were carried out on 304 stainless steel in 1N H(,2)SO(,4) solutions with 0.25N KCl. These samples were then analyzed on ESCA and with alternating argon gas sputtering and analyses, the composition of the passive film was studied. The surface film was found to be on the order of 20 to 35 angstroms thick for samples tested at lower potentials in the passive region. The composition profiles indicated a monotonic gradient for both iron and chromium in the film, while nickel showed an unexpected gradient. The concentration of chromium, greatly enhanced at the surface, varied with the corrosion potential in the passive region. This indicated, as predicted, a change in the film itself as the potential at which it was formed was increased toward the transpassive region. The use of both 2P and 3P data from ESCA showed the absence of a layered structure in the film. The success of the project was shown in the complete composition profiles which were calculated for the main samples. These samples were corroded at +0.200 Vsce and +0.500 Vsce in the passive region. The suitability of ESCA for corrosion studies was clearly evident. One of the questions answered by this project was the applicability of ultra-high vacuum equipment to the study of aqueously corroded samples. The correlation of these experiments with predicted film compositions indicated that the approach was suitable and that under appropriate conditions a one to one correlation of ESCA results to the passive film can be made.
Degree
Ph.D.
Subject Area
Metallurgy
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