HIGH-TEMPERATURE ATTACK BY SO2 OF LASER BORONIZED IRON

Laser alloying of boron predeposited on iron samples led to Fe-Fe2B al loys, with composition and microstructure depending on the irradiation conditions. Three different surface alloys were prepared correspondin g to hypo-, hyper- and eutectic microstructures. This last kind of sur face alloy exhibited the best behaviour in static SO2 between 400 and 600-degrees-C. The parabolic rate law, tending to a 3rd and 4th power law at the highest temperatures was explained as the result of outward iron diffusion leading to a magnetite external sublayer and inward SO 2 and S2 transport, allowing the formation of a FeS-B2O3 internal subl ayer. In this sublayer, slowly reacting Fe2B transforms to B2O3 leadin g to an increased blocking during the course of the reaction.