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

A Fattah-Alhosseini,N Attarzadeh

doi:10.4061/2011/521384

A Fattah-Alhosseini, N Attarzadeh

Open Access

https://doi.org/10.4061/2011/521384

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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.

Highlights

Transpassivation is a phenomenon in which a passivated metal starts rapid dissolution if the electrode potential of the metal becomes too positive [1]
Assuming that the transport of metal vacancies is slower than the rate of their annihilation at the metal/film interface, they accumulate at the film/solution interface
Specimens were fabricated from 1 cm diameter rods of AISI 321 stainless steel; the nominal composition is given in the Table 1

Summary

Introduction

Transpassivation is a phenomenon in which a passivated metal starts rapid dissolution if the electrode potential of the metal becomes too positive [1]. The SCA assumes that transpassive dissolution is a complex process including several reaction stages: formation and growth of a passive film via generation, transport and annihilation of ionic point defects, continuous changes in the stoichiometry of this film and especially of the first atomic layers adjacent to the solution, charge transfer reactions at the film/solution interface, and transport of reaction products in the bulk solution. According to this model the passive film was represented as an n-i-p junction structure with injection of oxygen vacancies from the metal substrate during film growth and metal vacancies from the solution during film dissolution. It seems that an investigation into transpassivation mechanism of stainless steels will be of scientific and practical interest

Experimental

Results and Discussion

V versus SCE

Conclusions