Role of PVD oxide coatings on HK40 cast steel during short and long exposure to C-rich atmospheres

The role of Cr/Cr oxide thin film coatings on HK40 steel substrates during exposure to C-rich atmospheres was investigated. The coatings were produced via reactive magnetron sputtering as an alternative method to protect these materials against metal dusting. Coated and uncoated substrates were exposed at 1073 K to an Ar + CH4 atmosphere with residual oxygen for 10, 30, 60 min and 50 h. Analysis of the products formed on both samples indicated that attack in the uncoated samples involves both C and O from the atmosphere together with Cr, Fe, and Ni outward diffusion in which the dendritic structure plays an important role. The presence of even a residual amount of oxygen in the carburizing atmosphere had a remarkable effect on the corrosion mechanism in these samples as well. In the coated samples, the Cr/Cr oxide film minimized internal alloy attack not only by limiting carbon ingress, but also minimizing the role of oxygen and outward Cr (Fe and Ni) diffusion from the alloy.