Innovative slurry coating concepts for aluminizing of an austenitic steel in chlorine and sulfur containing atmosphere

Three new concepts for AlSi-slurry diffusion coatings manufactured in air and in aggressive, chlorine and sulfur containing atmosphere on alloy 800 H have been studied. These concepts were: an oxygen getter concept with fine Al-powder, a eutectic concept with Ge and a barrier concept with a glass-ceramic. Particularly the oxygen getter concept is based on the high reactivity of the fine Al-powder to form a dense and stable oxide layer. The eutectic concept is established via the facilitated diffusion of Al in the liquid phase. The barrier concept aims at the formation of a dense glass-ceramic layer. All three studied concepts were based on the idea of reducing the partial pressure of oxygen and other critical gases (Cl, S) within the Al-containing slurry near the substrate surface via a protective second layer. For the heat treatment several hold times were intentionally chosen to simulate the heating up process of waste to energy plants. Due to the slow heating up, an extended time is allowed for the oxidation or corrosion of the underlying substrate as well as the aluminum powder of the slurry before the diffusion occurs. The application of slurry manufacturing route in air with all three studied concepts leads to homogeneous diffusion coatings. The barrier concept provides the best results in aggressive atmosphere, whereas both other concepts lead to inhomogeneous coating structures. The presence of Ge changes the coating microstructure due to the fast diffusion of Ge into the base material. A model of the microstructural evolution of the coating in Cl- and S-containing atmosphere is proposed.