(389ci) Multi-Scale Modelling of Stainless-Steel Corrosion By Ammonia Combustion

Stainless steel’s resistance to corrosion makes it a good building material for reactors. While the chromia layer resists oxidation in Earth’s atmosphere, ammonia combustion, which has been identified as a potential path to decarbonization, is highly corrosive and can cause stainless steel to degrade and lose its mechanical strength. This corrosion is a complex process depending on the conditions of the ammonia atmosphere and the type and composition of the steel itself. This work combines detailed microkinetic models with meso- and macro-scale diffusion models to simulate how stainless steel degrades under the different reacting conditions of ammonia combustion.

First, we use the Reaction Mechanism Generator (RMG) software to build a detailed kinetic mechanism of ammonia and oxygen reacting in the gas and surface phases. Next, we use the Cantera software package to simulate the reactor at different temperatures, pressures, and inlet compositions to determine the steady state surface coverages on the steel. Then, at the mesoscale, we gather phase equilibrium and chemical potential data from the literature to determine the phase of the steel as a function of time, distance from the surface, and species concentrations. This allows us to simulate phase changes in the steel as the chromium diffuses out and the oxygen and nitrogen diffuse in. The diffusion rates then become functions of the phase, and not just the temperature, as in the usual Arrhenius form. Finally, we use the method of lines to solve the differential equations governing the diffusion processes at the macroscopic scale. This shows how far the nitrogen and oxygen layers reach through the steel at a given time and reactor condition. This model could help determine the range of ammonia combustion conditions that will prevent stainless steel corrosion.