Modelling Oxide Growth and Morphology Evolution During Alloy Oxidation and Corrosion

Ramanathan, Rohit

doi:https://doi.org/10.21985/n2-spd4-en51

Work

Modelling Oxide Growth and Morphology Evolution During Alloy Oxidation and Corrosion

Public

Download PDF

Download All Files (.zip)

A series of theories and models are developed and used to investigate the growth of protective oxide films on metal and alloy surfaces for cases in which Wagner's classical model of oxidation does not hold. First, irreversible thermodynamics is applied to formulate a model for the outward growth of rocksalt oxides on metal surfaces in which local equilibrium of the oxide interfaces is not assumed and no \textit{a priori} assumption of a rate limiting step is made. The model is formulated in three dimensions and capable of describing shape changes in the oxide due to interfacial energy and curvature. For comparison with existing oxidation models, the implications of this new model are thoroughly examined in one dimension, and the spatial distribution of free energy dissipation during oxidation is discussed for a series of limiting cases. The newly formulated model accounts for the observed transition from linear to parabolic kinetics during oxidation, resolving a shortcoming of Wagner's classical description of oxidation. Expressions are also developed for the non-equilibrium defect concentrations at the interfaces during the early stages of oxidation when departures from local interfacial equilibrium are expected. The stabilizing influence of surface energy on the evolution of non-planar interfaces is briefly discussed. The non-equilibrium oxidation model is then extended to the binary alloy case in order to describe the recently discovered phenomenon of solute capture. Specifically, the capture of trivalent B cations in a nominally rocksalt oxide AO during the oxidation of a binary A-B alloy is discussed. This has relevance to the Ni-Cr and Co-Cr systems in which significant incorporation of Cr in a rocksalt oxide that would ordinarily be presumed to be NiO or CoO has been observed. The kinetic conditions under which solute capture at a \textit{stationary} metal-oxide interface can occur are elucidated, and are shown to be markedly different from the conditions under which solute trapping during rapid solidification occurs. It is found that it is possible to form a pure rocksalt oxide \ce{B_{2/3}O} at high growth velocity, and that there is a non-dimensional parameter $\mathsf{M}$ that controls whether capture of A or B atoms occurs. Following this, a model of NiO island growth and coalescence on the surface a Ni-5Cr (wt.\%) alloy is then presented and solved numerically to compute the evolution of the island size distributions during oxidation at low $P_\mathrm{O_2}$ prior to full coverage of the alloy surface. Explicit comparison is made between the model results and STM observations of the size distribution evolution. It is found that a size independent growth rate best captures the observed evolution which is attributed to a small diffusion screening length. Coalescence is needed to account for the skew in the observed distributions toward large radii, and also to account for the observed broadening of the distribution and the decrease in the number of islands with time. The simulation results are in agreement with the experimental results. Finally, the morphological stability of passive oxide films during aqueous corrosion is discussed. A revised version of Macdonald's Point Defect Model is formulated in which the electrostatic potential description is replaced with a boundary value problem, and the transport boundary conditions are modified to include interfacial energy and curvature. Steady-state film thicknesses are computed in one dimension and are found to scale linearly with the applied anode potential, in agreement with experiments. A linear stability analysis is used to examine whether small perturbations in the shape of the film are stable or unstable. It is found that dissolution leads to an instability at the oxidation-solution interface if the dissolution mechanism is anodic. The instability is driven by an enhanced electric field in the Helmholtz layer of the solution in concave regions of the film. We find that the growth rate of instability increases as the surface energy of the film decreases. This suggests that chloride adsorption to passive film and the subsequent decrease in the surface energy can exacerbate this instability and contribute to pitting breakdown.

Creator