Abstract
The role of heterogeneous phases in the localized corrosion of materials is an emerging area of research. This work addresses the detrimental role MnS inclusions play in the localized corrosion of carbon steels. We report the results of surface and bulk characterization of MnS inclusions in 1018 carbon steel, using a high-resolution integrated Auger nanoprobe. It is shown that the surface morphology and composition of the inclusions are highly heterogeneous. MnS inclusions are found to be nonstoichiometric and to contain a highly O-enriched surface layer. Some inclusions are covered with a thin (approximately 5 nm) layer of Cu2S. The bulk composition of “MnS” inclusions is found to include 5–7% Fe and O, and these inclusions are believed to occupy Mn and S positions within the “MnS” lattice. Interfaces between “MnS” and host ferric grains are highly disordered. We hypothesize that pitting initiates and develops at these interfaces through a galvanic coupling between the strained and the unstrained ferrite grains.
Highlights
Numerous studies have demonstrated that pitting corrosion in steels is initiated at nonmetallic inclusions [1,2,3,4,5,6,7,8,9]; the exact mechanism behind the initiation and growth of these pits remains controversial [10,11,12,13]
It has been suggested that MnS inclusions play different roles in the pitting behavior of carbon and stainless steels [17]: in the case of stainless steels, sulfide inclusions are the sites of anodic activity, whereas in the case of carbon steel, inclusions are the sites of cathodic activity
Ar peaks observed in regions 1 and 2 of the Auger spectra are a result of light sputtering to remove some of the environmental hydrocarbon contaminants typically present on any given surface
Summary
Numerous studies have demonstrated that pitting corrosion in steels is initiated at nonmetallic (sulfide) inclusions [1,2,3,4,5,6,7,8,9]; the exact mechanism behind the initiation and growth of these pits remains controversial [10,11,12,13]. It was claimed that certain inclusions appear to be more corrosively active than others [17]. This distinction is associated with the presence of a finely dispersed. One proposed model of pit initiation in stainless steels involves the presence of a Cr-depleted region in the near vicinity of an inclusion [10,11,19]. This region causes a disruption in the passive layer, and pitting is initiated around the inclusion. It has been suggested that MnS inclusions play different roles in the pitting behavior of carbon and stainless steels [17]: in the case of stainless steels, sulfide inclusions are the sites of anodic activity, whereas in the case of carbon steel, inclusions are the sites of cathodic activity
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