Effect of surface conditions and strain hardening on the passivity breakdown of 304 stainless steel
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Matteo Chiesa Laboratory for Energy and Nano Science, Masdar Institute, Abu Dhabi 54224, United Arab Emirates; and Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 (Received 18 August 2011; accepted 30 December 2011)
Electrical and electrochemical properties of the passive layer formed on 304L austenitic stainless steel are investigated by means of both conductive atomic force microscopy in air and electrochemical atomic force microscopy in chloride-containing media. The maps of local electrical conductivity of the oxide overlayer exhibit different patterns depending on the surface conditions after mechanical or electrochemical polishing. In particular, the passive film covering strain-hardened regions reveals a higher electrical conductivity. The local enhancement of the electrical conduction is explained by local changes of the stoichiometry of the passive film. Moreover, the highly conductive regions lead to a local breakdown of the native oxide in chloride-containing media and favor the initiation of localized pits.
I. INTRODUCTION
The passivity of stainless steels (SSs) in ambient environments results from a thin oxide overlayer, called passive film, which acts as a barrier to the transport of ions, molecules, and electrons. In certain environments, especially in chloride-containing media, the passive film may locally breakdown leading to initiation of pits. This localized corrosion is the result of a combination of electrochemical and metallurgical factors including changes of alloying composition and the nature and distribution of the nonmetallic inclusions (carbide or sulfur precipitates), which has already been studied in depth.1,2–4 Despite the extensive literature in the field, the role that local changes in the passive layer structure play in the initiation of pitting corrosion in high-purity industrial SSs has been insufficiently investigated.5,6 Recent studies using scanning electrochemical microscopy (SECM) on Ti and Al as well as on 304 SS demonstrate that microscopic defect sites within the passive oxide film can act as efficient shunt pathways for electron transport.2,7–12 Local variations in thickness, stoichiometry, or crystallography of the thin oxide film are factors believed to influence the conductivity and the passivity breakdown.13,14 Moreover, surface conditions15,16 as well as strain hardening 17,18 are other factors believed to affect either the passive film structure or localized corrosion. a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2012.5 1580
J. Mater. Res., Vol. 27, No. 12, Jun 28, 2012
http://journals.cambridge.org
Downloaded: 15 Jan 2015
Point defects on the oxide structure as small as 10 nm are believed to be responsible for initiation of stable pits of corrosion as pointed out by Galvele19 by modeling study. More recently, Martin et al.20 have noticed, using in situ atomic force microscopy (AFM) in corrosive media, that some bumps of 5-nm-height are formed during passivation of
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