Nitrogen and Water in High Temperature Corrosion - Insights from First Principle Calculations
Licentiate thesis, 2017
Service life-time of alloys used in high temperature applications is often limited by corrosion. FeCrAl(Re) alloys are partially designed to form a protective α-alumina oxide scale at elevated temperatures in order to mitigate further oxidation. The scale growth commonly follows a parabolic rate law, which under a Wagnerian setting depends on transport of charged species; ions, vacancies and electrons. Small amounts of reactive elements (Y, Zr, Hf) are added to the alloy to improve scale properties, while also inhibiting outward Al diffusion. The oxide growth thus depends on mobility of oxygen vacancies in the scale.
A FeCrAl(Re) alloy was exposed to a 95% N2, 5% H2, and low p(O2) environment, usually used for heat-treatment, forming a predominantly protective alumina scale with nodular inclusions of chromia. Under said conditions the chromia rich nodules permeate nitrogen. Density functional theory (DFT) calculations show that reducing processes, owing to the relative stability of chromia and alumina, lead to a maintained coverage of coordinatively unsaturated sites (CUS) on chromia surfaces, acting as N2 dissociation sites. Chromium oxy-nitrides were shown metastable, offering a path for nitrogen into the alloy. H2O acts as the main oxidant in this environment. A quasi-Wagnerian context was explored in which hydrogen was preferentially disposed as H- in oxygen vacancies in hydroxylated alumina grain boundaries.