Effect of Plastic Deformation on Pitting Mechanism of SS304
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INTRODUCTION
THE activation-controlled corrosion reaction is purely driven by thermodynamics, where additional strain energy is expected to enhance general corrosion. It has been widely reported that stresses (either internal or external) and strain accelerate the general corrosion in different alloy/environment systems.[1–3] On the other hand, a protective oxide film or a passive film forms on the metal surface in certain alloys/environment systems to slow down or prevent further oxidation. The passive film acts as a physical barrier which suppresses general corrosion. Hence, the breakdown of passive film may lead to localized corrosion. In stainless steel systems where a passive film forms on the metal surfaces, surface defects formed due to deformation are suspected to promote localized corrosion, e.g., pitting corrosion. Strain-induced corrosion failures have caught much attention for steels in various chemical environments.[4–13] Much pertinent prior work was focused to address the stress/strain-related pitting corrosion in different material systems. One important deformation-influenced pitinitiation phenomenon is related to the presence of MnS inclusions. The MnS inclusion is one of the most common types of inclusions which can act as crackinitiation sites[14] or pit-initiation sites[15–17] in steels. Even though various mechanisms have been proposed
GAOXIANG WU and PREET M. SINGH are with the Georgia Institute of Technology, North Ave NW, Atlanta, GA 30332 and also with the Atlanta. Contact e-mail: [email protected] Manuscript submitted February 11, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
to understand the MnS-promoted pit initiation, the consensus involving preferential dissolution of MnS as an initial step has been reached.[15–17] The MnS inclusion at the surface also causes inhomogeneity of passivefilm composition, where the passive film covering the local MnS area forms a Cr-depleted zone and thereby is less protective. Plastic deformation like cold-rolling can deform these inclusions into stringers, and increase the chance of forming a crevice-like structure between the inclusions and the matrix. Therefore, cold-rolling has been widely found to facilitate pit initiation in aggressive environments for inclusion-containing stainless steels.[15,18] With the modern steel-making processes, MnS inclusions are minimized in the microstructure, hence their influence on pitting corrosion became far less significant. It was then argued that the local plasticity is likely to promote the pitting corrosion in stainless steels. Plastic deformation in stainless steels results in a very significant amount of dislocations, twinning, and possible martensitic transformation.[8,10,13,19–25] The strain energy due to plastic deformation is stored as the internal stress associated with the defects and microstructural changes. The relationship between the martensitic transformation in austenitic stainless steels (ASSs) and the pitting corrosion has been studied by numerous researchers, yet no unified conclusion was reach
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