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International Journal of Electrochemistry
Volume 2011, Article ID 521384, 9 pages
http://dx.doi.org/10.4061/2011/521384
Research Article

The Mechanism of Transpassive Dissolution of AISI 321 Stainless Steel in Sulphuric Acid Solution

1Faculty of Engineering, Bu-Ali Sina University (BASU), Hamedan 65178-38695, Iran
2Department of Materials Engineering, Isfahan University of Technology, Isfahan 84156-83111, Iran

Received 9 August 2010; Accepted 21 October 2010

Academic Editor: Angela Molina

Copyright © 2011 A. Fattah-Alhosseini and N. Attarzadeh. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

The transpassive dissolution mechanism of AISI 321 stainless steel in a 0.5 M sulphate solution was studied using electrochemical impedance spectroscopy (EIS). On the basis of the experimental results, surface charge approach, and point defect model, a kinetic model of the transpassive dissolution process is proposed. The transpassive film is modeled as a highly doped n-type semiconductor—insulator-p-type semiconductor structure. Injection of negative defects at the transpassive film/solution interface results in their accumulation as a negative surface charge. It alters the nonstationary transpassive film growth rate controlled by the transport of positive defects (oxygen vacancies). The model describes the process as dissolution of Cr as Cr (VI) and Fe as Fe (III) through the transpassive film via parallel reaction paths.