Silicon surface treatment via CVD of inner steel tube surfaces

Abstract

Inner surfaces of steel tubes should be protected against corrosion and oxidation by a coating in order to assure optimised application in power plants and the chemical industry. Since the silicon content in chromium alloyed steels improves the oxidation protection. Chemical vapour deposition (CVD) of Si has been investigated for martensitic 9% chromium steel (T91). The surface treatment was performed as a combined process of CVD at atmospheric pressure with heat treatment in a chemical atmosphere with high frequency heating (HF), automated with a Simatic PCS7. The Si out of SiCI 4 diffused into the steel lattice of the substrate surface. As a result, the Si-content near the surface layer increased as a ferrite layer which did not transform to martensite during heat treatment. The excess of Si was converted to Si 3 N 4 using NH 3 . This improves the anti-oxidation behaviour. The physical and chemical properties of the modified surfaces were investigated and characterised using the SEM, EDX, electron microprobe and X-ray diffraction methods. The deformation capacity of a coated specimen was proven by means of hot tensile tests. It could be determined that in the ferritic layer Fe is substituted by Si with a maximum concentration of approx. 5%, however the Cr content was almost constant.