Application of aluminum diffusion coatings to mitigate the KCl-induced high-temperature corrosion

Pack cementation was used to produce Fe_1−xAl and Fe₂Al₅ diffusion coatings on ferritic-martensitic steel P91 and a Ni₂Al₃ diffusion coating on pure nickel. The performance of diffusion coatings against high-temperature corrosion induced by potassium chloride (KCl) was evaluated by exposing the samples at 600 °C for 168 h in static lab air under KCl deposit. In addition, a salt-free experiment was performed for comparison. Microstructure, chemical and phase composition of the samples were analyzed with scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and X-ray diffractometry (XRD) before and after the exposures. It was found that all the diffusion coatings formed protective oxides under salt-free exposure in air. Under the salt deposit, Fe_1−xAl showed local failure while on large parts of the sample a protective layer had formed. Fe₂Al₅ was attacked over the entire surface and the dominant mode of attack was selective aluminum removal. Ni₂Al₃ showed excellent performance and no sign of attack was observed anywhere on the sample.