Molecular dynamics of the early stages of high-temperature corrosion

Sung-Yup Kim,Christopher D Taylor

doi:10.1103/physrevmaterials.5.113402

Abstract

We performed molecular dynamics simulations of the high-temperature oxidation of metal alloys composed of Al, Cr, and Fe and compared their behavior with that of pure Fe. The metal alloy elements (Al and Cr) segregated to the surface during oxidation, producing a lower stress gradient at the metal/environment interface compared to pure Fe. We have found that the lowered stress gradients produced in the alloy material appear to play a key role in the development of corrosion. Interfaces with lower stress gradients have reduced rates of ${\mathrm{H}}_{2}\mathrm{O}$ adsorption, especially for the ferritic (bcc) alloys. The diffusivity of oxygen and hydrogen drops more rapidly for the interfaces with reduced stress gradients. The stress gradient is also diminished when the gas pressure is increased, indicating that the Fe-Cr-Al alloy system is more resistant to oxidation than pure Fe at higher pressures. Therefore, we conclude that the lower stress gradients at the alloy/environment interface reduce the stress concentration and can slow down the rate of the initial oxide scale growth. We also compared bcc and fcc alloys with pure Fe based on our three evaluation criteria (peak stress, stress gradient, and summation of stress in the oxide scale). We found that the alloys have lower values under the three criteria compared to pure Fe. The bcc alloy has the best score under a water rich environment and the fcc alloy is proven to be better for peak stress and summation of stress in the oxide scale under an oxygen rich environment. For surface segregation to occur, we find that a minimum content of Al or Cr in the near-surface region must be achieved. We also learned that the role of Al is more important than that of Cr in terms of corrosion resistant behavior at relatively higher temperatures for the Fe-Cr-Al ternary alloys.

Highlights

Metals and alloys react with the surrounding environment during high temperature service, resulting in high temperature corrosion
The metal alloy elements (Al and Cr) segregated to the surface during oxidation producing a lower stress gradient at the metal/environment interface compared to pure Fe
We performed molecular dynamics simulations to investigate the high temperature corrosion behavior of metal alloys and pure Fe exposed to exhaust gases

Summary

Introduction

Metals and alloys react with the surrounding environment during high temperature service, resulting in high temperature corrosion. High temperature corrosion reactions initiate by molecular adsorption of exhaust gases on to the material surface. Depending on the alloying elements present in the surface and near-surface region, certain reactions and adsorption processes will be favored. An incipient oxide scale is formed, and, once the competing surface chemistry resolves to establish the most favored corrosion product at the nanometer scale, the film will grow due to mass transport and continual reaction of the scale surface with the environment. Once the film reaches a certain thickness, the rate of this process depends on the defect transport kinetics (interstitials and/or vacancies), and the transport kinetics will be affected by the crystal structure, microstructural aspects (e.g. oxide grain size or amorphous characteristics) and composition of the corrosion product[1,2]. High temperature corrosion is a complex multiphysics and multiscale phenomenon which involves the intersection of thermodynamic stability, chemical reaction kinetics, molecular diffusion and mechanical properties, etc

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