A theory for designing corrosion-resistant alloys

Release time: 2024-02-22

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 nanometre-thick passive oxidation protective film. If this film is damaged by scratches or abrasive wear, it will only have a small amount of metal dissolved. This is the main reason why stainless steel and other chromium-containing alloys, are used in critical applications including biomedical implants to nuclear reactor components. Elucidating the compositional dependence of this electrochemical behaviour 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 study, the researchers focused their attention on the permeation process that occurs during the initial stages of passivation, called primary passivation, which is a surface process that occurs in 10 ms or less. Based on the ionic radii of Cr3+, O2-, and body-centred cubic (bcc) Fe-Cr crystal structures, it was hypothesised that connected surface-Cr-O-Cr-bonds, also known as "mer" units, could evolve into Cr atoms separated by a third nearest-neighbour (NN) distance in the Fe-Cr lattice. Cr atoms separated by the third nearest neighbour (NN) distance in the Fe-Cr lattice. For face-centred cubic (fcc) Ni-Cr alloys, a similar argument suggests that Cr atoms can also be spaced up 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 linking the percolation phenomenon 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 hypothesised that these unconnected localised passive regions can be dissolved away, and the only way to prevent this from occurring is for these initial oxide nuclei to be continuous or to penetrate the surface of the alloy. the percolation thresholds for the bcc and fcc stochastic solid solutions, including up to the 3rd NN, are defined here as 0.095 and 0.061. Importantly, these thresholds set lower synthetic limits only for the Cr molar fraction required for passivation. At these thresholds, in order for major passivation to occur, Fe or Ni must be selectively dissolved to a depth of thousands of layers.

Here, one must recognise that the primary passivation process occurs on topological or rough surfaces, which are formed by electrochemical metal and chemical metal oxide dissolution. Figure 1a is a demonstration diagram showing the evolution of the surface of this alloy and how the initial alloy composition determines the depth of dissolution required for the formation of the primary passivation film, h. Figure 1b shows comparative results from kinetic Monte Carlo (KMC) simulations of a passive surface developed for a bcc Fe/17-at%-Cr alloy. Cr is enriched on rough surfaces when Fe is selectively dissolved. Sufficiently sized clusters of Cr atoms on the metal surface act as nucleation sites for -Cr-O-Cr-mer units, and Fe atoms bridging or immediately adjacent to these mer units form early mixed oxide nuclei. Since the neighbourhood of Fe atoms around small Cr clusters attenuates the Gibbs free energy of mer-unit formation, the electrochemical potential for passivating a particular size of Cr cluster will depend on its size.