Fe-Cr and Ni-Cr binary alloys contain a sufficient proportion of chromium to act as typical corrosion-resistant metals due to the presence of a nanometer-thick passive oxidation protective film. If this film is damaged by scratches or abrasive wear, only a small amount of metal will dissolve. This is the main reason why stainless steel and other chromium-containing alloys are used in critical applications, ranging from biomedical implants to nuclear reactor components. Elucidating the compositional dependence of this electrochemical behavior has long been an open question in corrosion science.

With the advent of data mining, artificial intelligence, and increased computational power based on density functional theory (DFT), alloy families are being discovered at an increasing rate. However, there are no criteria for determining the formation of alloys with good service properties. Potential-pH diagrams constructed with DFT now assume thermodynamic equilibrium, but typically passive film growth is kinetically controlled; passivated films can be far from equilibrium, both in terms of crystal structure and composition.

In this research, the researchers focused on the percolation process occurring in the initial stages of passivation, termed primary passivation, a surface process occurring in 10 milliseconds or less. Based on the ionic radii of Cr3+, O2-, and the body-centered cubic (bcc) Fe-Cr crystal structure, it is hypothesized that connected surface -Cr-O-Cr bonds, also called "mer" units, can evolve from Cr atoms separated by the third nearest neighbor (NN) distance in the Fe-Cr lattice. For face-centered cubic (fcc) Ni-Cr alloys, a similar argument suggests that Cr atoms can also be spaced to the third NN distance, which is only slightly larger than the spacing of Cr atoms in the mer unit (0.016 nm). The key motivation for linking percolation phenomena to passivation is related to the formation of spatially isolated -Cr-O-Cr-mer units. Due to the selective dissolution of Fe or Ni during initial passivation, it is hypothesized that these unconnected localized passivation regions could be dissolved away, and the only way to prevent this is to make these initial oxide nuclei continuous or percolating across the alloy surface. The percolation thresholds for bcc and fcc random solid solutions, including up to the third NN, are defined here as 0.095 and 0.061, respectively. Importantly, these thresholds only set lower synthetic limits for the Cr mole fraction required for passivation. At these thresholds, for primary passivation to occur, Fe or Ni must selectively dissolve to depths of thousands of layers.

Here, it must be recognized that the primary passivation process occurs on a topographically or rough surface formed by electrochemical and chemical metal and metal oxide dissolution. Figure 1a is an illustrative diagram showing the evolution of this alloy surface and how the initial alloy composition determines the dissolution depth h required for primary passive film formation. Figure 1b shows comparative results from kinetic Monte Carlo (KMC) simulations developed for a bcc Fe/17-at %-Cr alloy. When Fe is selectively dissolved, Cr is enriched on the rough surface. Cr atom clusters of sufficient size on the metal surface act as nucleation sites for -Cr-O-Cr-mer units, and Fe atoms bridging or adjacent to these mer units form early mixed oxide nuclei. Since the Fe atomic neighborhood around small Cr clusters will weaken the Gibbs free energy of mer unit formation, the electrochemical potential for passivating a Cr cluster of a specific size will depend on its size.