High Temperature Sulfidation Behavior of Laser Alloying with Al Flame Sprayed Coating on Mild Steel

A mild steel substrate was coated with Al wire by flame spraying and irradiated with CO2 laser to produce surface modification layer. The isothermal sulfidation behavior on CO2 laser surface alloyed layers were investigated at high temperature(1123k) in sulfidation environment (Ps2= 10-3 Pa) for 48 hours. Generally, surface alloyed layers were uniformed and their porosities were little. These structures were α Fe-Al, since Al concentration was analyzed from 36.3 to 48.5 percentages. The sulfidation kinetics of surface alloyed layers were shown to follow parabolic rate law during isothermal sulfidation, however the sulfidation rate of FA13(13 mm/min, 36% Al) specimen was approximately decreased from one-fifth lower than FA25(25 mm/min, 39% Al) and FA50(50 mm/min, 48% Al) specimen. Interface of FA50 surface alloyed layer was existed to the Al2S3 sulfide and Al2O3 oxide. By calculating partial pressure of impurity oxygen contained in H2S/H2 gas, the Al2O3 oxide formation could be explained using Fe-Al-S-O thermodynamical stability diagram