Designing of Corrosion Resistant Alloys via Percolation Theory

Canada and USA Scientists have reported on designing of corrosion-resistant alloys via percolation theory and published in Nature Materials on 01 February 2021.

Nickel–chromium, Iron–chromium binary alloys can serve as the prototypical corrosion-resistant metals owing to its presence of a nanometre-thick protective passive oxide film. The main key criterion for good passive behavior is the passive film should be compromised via a scratch or abrasive wear that can be reformed with a little metal dissolution. This could be a principal reason for the stainless steels and other chromium containing alloys that are used for critical applications which ranges from nuclear reactor components to biomedical implants. A long-standing unanswered question in corrosion science is the unravelling of the compositional dependence of the electrochemical behavior of the alloys [1, 2].

The discovery of the family of these alloys were increased its rate with the advent of artificial intelligence, data mining, and its computing power for density functional theory (DFT)- based calculations. Also it can be seen that, currently there are no any other criteria for the determination of these alloy compositions that can be expected to display a good passive film behavior. Construction of potential–pH diagrams are used via DFT with an assumption of thermodynamic equilibrium, both in terms of crystal structure and composition with the passive films that can be far from equilibrium. However, the growth of the passive film is kinetically controlled [3].

Yusi Xie et al. have developed a percolation theory of alloy passivation based on two-dimensional to three-dimensional effects which accounts for the selective dissolution. In this study, they have mainly focused on the attention on percolation processes that occur during the initial stage of passivation, also termed as primary passivation process over the surface. They have also been determined the quantity of metal dissolved. Further, validation of theory via both experimentally and kinetic Monte Carlo simulation studies have also been carried out. Hence, the results reveal a pathway to forward for the design of corrosion-resistant metallic alloys [4].

The passivation behaviour of nickel–chromium (Ni–Cr), iron–chromium (Fe–Cr) and stainless-steel alloys have been previously connected to the percolation sites. Based upon the ionic radii of Cr³⁺, O²⁻, the body-centred cubic (BCC) Fe–Cr crystal structure, was conjectured to connect the surface of –Cr–O–Cr– linkages, also termed as ‘mer’units, which could evolve for Cr atoms as separated by third nearest-neighbour (NN) distance in the Fe–Cr lattice. The key motivation factor for connecting the percolation phenomena is the passivity with the formation of spatially isolated –Cr–O–Cr– mer units. It can be reason out that the unconnected passive regions could be dissolved out as a result of the selective dissolution of Ni or Fe that occurs during primary passivation. Hence, the preventing way is only this incipient oxide nuclei were continuous or percolating across the alloy surfaces [5].

The percolation thresholds for BCC and FCC random solid solutions of third NN are termed as ‘p_c^3D’ {1,2,3} and the values are found to be 0.095 and 0.061 respectively. Also, these thresholds can only set for lower compositional bounds, where there is a requirement of Cr mole fraction for passivation. In order, to occur the primary passivation, these thresholds of Fe or Ni could be selectively dissolved over the depths which corresponds to thousands of monolayers. It is very essential to recognize that the process of primary passivation occurs over the topological or roughened surface which has evolved via chemical metal-oxide dissolution and electrochemical metals [6].

Cr have enriched on the roughened surface as Fe is selectively dissolved. The metallic surface of Cr atom clusters with sufficient size can serve as sites for the nucleation of –Cr–O–Cr–mer units. Also the Fe atoms were bridged or immediately adjacent to these mer units form an incipient mixed oxide nucleus. Moreover, the Fe atom neighbourhood will surrounds the small Cr clusters that can attenuate the Gibbs free energy for mer-unit formation. It will also depend on its size for the Cr cluster, particularly to passivate the electrochemical potential [6].

Primary passivation involves as the system ‘looks’ for mer-unit connectivity on the topological surfaces as the thickness direction selectively dissolves the Fe or Ni. Various experiments, DFT calculations, KMC simulations, Monte Carlo renormalization group (MC-RNG) are used to examine the assumptions and prediction of theory. Moreover this theory involves only percolation, and the Monte Carlo simulation which excludes the electronic effects that are related to how Fe might be attenuated with the Cr electronic structure, so that the oxidation of various sizes of Cr atom will be clustered [4].

Our SNB team have mainly emphasize this new research article to enrich our viewer’s knowledge about the concept on designing of corrosion-resistant alloys via percolation theory. Further they have reported that their results for Ni–Cr and Fe–Cr alloys were assumed to be ideal solid solutions. Almost, all these real alloys have showed some degree of non-ideality, defined interms of statistics of regular solutions with their tendency to its short-range order or clusters. Here, the main principle scenario is the amenable of first principle based quantum chemical calculations. However, a huge number of possible cluster configurations with a complete enumeration of alloying-cluster size effects is beyond today’s computing power. In addition, the alloy systems for the passivation can be tuned to first neighbours, ordering and clustering. This predicition is mainly to have important effects on compositional requirements for the corrosion protection. Thus, for alloy systems showing stronger ordering or clustering, these phenomena can be used as a ‘knob’ to tune the alloy passivation behavior.

References

E. McCafferty, Introduction to Corros Sci. (Springer, 2010).
M. Niinomi et al., Acta Biomater. 11 (2012) 3888.
L. Huang et al., Annu. Rev. Mater. Res. 49 (2019) 53.
Y. Xie et al., Nature Mater. (2021), DOI:10.1038/s41563-021-00920-9.
K. Sieradzki, R. C. Newman, J. Electrochem. Soc. 133 (1985) 1979.
V. K. S. Shante, S. Kirkpatrick, Adv. Phys. 20 (1971) 325.

----- Dr.Y. Sasikumar

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