Effects of Carbon Contents in Steels on Alloy Layer Growth during Hot-dip Aluminum Coating

Abstract

Hot-dip aluminum coating of hypo-eutectoid steels containing 0.05-0.88 mass% carbon were performed, and the alloy layers formed in the coating were investigated. In the hot-dip aluminum coating at immersion temperatures ranging from 700 to 850°C, the alloy layers on the steels consisted of a single phase of the intermetallic compound Fe₂Al₅. The thickness of the alloy layer increased in proportion to the increasing square root of the immersion time (t^1/2) for immersion temperatures lower than 800°C for the whole base steel. On the other hand, for immersion temperatures higher than 800°C, the thickness of the alloy layer on the 0.05 mass% C steel and 0.45 mass% C steels exhibited a negative deviation from the linear relationship. The growth rate constant k decreased as the carbon concentration of the base steel increased up to 0.8%, above which k had a constant value. The reaction activation energies for the base steel in this study were approximately 70-80 kJ/mol. The alloy layer/base steel interfaces were serrated, and the serration width decreased with increasing carbon concentration of the base steel. In addition, the serration width had a larger value in the immersion temperature range wherein the pro-eutectoid ferrite content in the base steel was larger.