Anomalous Surface Segregation Profiles in Ferritic FeCr Stainless Steel

TL;DR

This study uses Monte Carlo simulations to explore the atomistic thermodynamics of FeCr stainless steel surfaces, revealing a complex chromium enrichment behavior that influences corrosion resistance and nanomaterial design.

Contribution

It provides a detailed atomistic thermodynamic analysis of surface segregation in FeCr alloys, highlighting the non-linear chromium enrichment and its underlying mechanisms.

Findings

Strong surface Cr enrichment at low bulk concentrations

Optimum surface Cr enrichment around 12 atomic percent

Surface composition decreases sharply beyond this concentration

Abstract

The iron-chromium alloy and its derivatives are widely used for their remarkable resistance to corrosion, which only occurs in a narrow concentration range around 9 to 13 atomic percent chromium. Although known to be due to chromium enrichment of a few atoms thick layer at the surfaces, the understanding of its complex atomistic origin has been a remaining challenge. We report an investigation of the thermodynamics of such surfaces at the atomic scale by means of Monte Carlo simulations. We use a Hamiltonian which provides a parameterization of previous ab initio results and successfully describes the alloy's unusual thermodynamics. We report a strong enrichment in Cr of the surfaces for low bulk concentrations, with a narrow optimum around 12 atomic percent chromium, beyond which the surface composition decreases drastically. This behavior is explained by a synergy between (i) the complex phase separation in the bulk alloy, (ii) local phase transitions that tune the layers closest to the surface to an iron-rich state and inhibit the bulk phase separation in this region, and (iii) its compensation by a strong and non-linear enrichment in Cr of the next few layers. Implications with respect to the design of prospective nanomaterials are briefly discussed.