Surface characterisation and corrosion behaviour of SiC-coated AISI 321 stainless steel

The influence of SiC-coatings on the corrosion properties of AISI 321 austenitic stainless steel (Fe/Cr18/Ni8/Ti) in 1N H2SO4 was studied. SiC-coatings of various thicknesses (100-800 nm) were prepared at room temperature (RT) or at temperatures up to 750 degrees C by ion beam sputtering of a SiC-target. The growing films could be continuously bombarded with a 160 keV Ar+ ion beam and the role of this Dynamic Ion Mixing (DIM) on both the structure and corrosion resistance was investigated. The chemical composition of the coatings was determined by Rutherford Backscattering (RBS) using 1.8 MeV alpha-particles and by Nuclear Reaction Analysis (NRA). Transmission Electron Microscopy (TEM) observations were also performed for the determination of the microstructural state of the samples. The characterisation shows, that for all the deposition temperatures the DIM-treatment promotes the crystallisation of the beta-SiC phase. It was also found, that the corrosion resistance of the stainless steel is considerably improved when the coatings are deposited by DIM regardless of the deposition temperature. The possible mechanisms are discussed and it is suggested that the interface mixing along with the coating densification effect are responsible for the improvement of the corrosion resistance.