It has long been maintained that the ability of alloys to withstand a corrosive environment relies on the formation of a protective slow-growing oxide scale. Conceptually, this scale is the consequence of subdividing the oxidation process into an inner anode where metal is oxidized, an outer cathode where O2 is reduced, and an intervening oxide which offers electron conductivity. Additionally, in order to overcome the buildup of space-charge at the electrodes, a necessary condition for the sustained oxidation process comprises two opposite ionic fluxes: anions moving towards the anode and cations towards the cathode. Along this line, breakdown of the barrier oxide owing to the corrosive environment is the result of short-circuiting paths though the protective scale. Indeed, grain boundary diffusion comprises the archetypical “easy” pathway, while the effective barrier oxide is designed to support re-healing in response to formation of microscopic cracks owing to mechanical stresses in the oxide from e.g. thermal cycling. Minority components, e.g. Cr and/or Al attenuated by reactive elements constitute the protective component in alloys.
In biomass fueled power plants, early breakdown of the protective chromia scale on FeCr alloy is observed owing to the alkaline fuel causing chromia loss by formation of non-protective alkali chromates. Subsequently, presence of chlorides in the fuel has long been known to render the combustion conditions highly corrosive. A chlorine cycle has been proposed involving repeated cracking or transient pores formation through the iron oxide scale to explain the iron chloride FeCl2(s) found to accumulate in the metal/oxide interface.
In our study, we use atomistic modeling to unravel a viable route for the permeation of chloride through the iron oxide scale complementing the experimental work performed by our co-workers at the Center for High Temperature Corrosion (HTC) at Chalmers.