ALUMINUM-BASED PROTECTIVE COATINGS PRODUCED ON AISI-304 STAINLESS-STEEL

The diffusion of Aluminium is one of the most promising methods to bui ld superficial coatings for stainless steel protection. Heat treatment s at 800 degrees C performed on rods of AISI 304 steel, Aluminium coat ed by means of electrodeposition, displayed the possibility of forming intermetallic compounds. Depending on the duration of the heat treatm ent and on the cooling kinetics, these compounds can be continuous. At high cooling kinetics (water cooling) a two-phase structure composed of gamma' (Ni3Al) and gamma (substitutional solid solution) is obtaine d. At slow cooling rates (in furnace), a substitutional solid solution and some precipitates of gamma' (Ni3Al) and beta (NiAl) can be observ ed. At intermediate cooling rates (oil cooling and air cooling), only the substitutional solution and the gamma' phase are present. Using fu rnace cooling from 800 degrees C until 500 degrees C, permanence at th is temperature for 192 h and cooling furnace, the two-phase structure obtained is composed of the substitutional solid solution and the beta (NiAl) phase. The existence of these phases and their composition hav e been reported by X-ray diffraction patterns and microanalysis. The p ossibility of forming a natural composite, constituted by a hard phase of aluminides diffused in a substitutional solid solution, has an imp ortant consequence on the mechanical and protection properties of thes e coatings. Moreover, the diffusion of Al improves the adhesion of coa tings.