Effects of Cr and Free C on the Structure and the Corrosion Resistance of Chromium Diffusion Coatings on Cr Alloy Steels

Abstract

The effects of C and Cr on the chromium diffusion layer formation by pack cementation process on Cr alloy steels ( Cr : 2.25 mass%-12 mass%) were examined. The following results were obtained.
(1)The structure of chromium diffusion coating on alloys which contained free C consisted of two layers independent of Cr content on alloys. The outer layer was chromium carbide film consisting of Cr₂₃C₆ and Cr₇C₃. The inner layer was Cr diffusion layer whose Cr content ranged from 25 mass% to 13 mass%.
(2) The thickness of Cr diffusion coating significantly depended on the free C and Cr content in alloy. The thickness of chromium carbide film, increased with increasing C content in alloy. On the other hand, the thickness of Cr diffusion layer increased with decreasing C content and increasing Cr content in alloy.
(3) The outer chromium carbide film and inner Cr diffusion layer exhibited good oxidation resistance in super heated steam environment due to the formation of Cr₂O₃ scale on both layers.