The composition depth-profile of anodic oxide films on iron at various potentials in a boric acid-sodium borate solution of pH 8.4 was measured by the sequential use of Auger electron spectroscopy (AES) and sputter-etching. It was found that the anodic oxide film consisted mainly of oxygen and iron with minor impurities such as boron, sulphur and carbon. The composition ratio of boron or sulphur to oxygen exhibited a maximum value at a certain depth in the outer layer of the film and the maximum value of ( B O) increased with increasing potential up to 500 mV (SCE). The film formation mechanism is discussed from the observed dependence of boron content in the film.
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Acknowledgement--This work was conducted in a Japan-USA collaborative research project supported by JSPS (Japan Society of Promotion of Science) and NSF (National Science Foundation in USA). The authors wish to acknowledge the financial support of JSPS and NSF.