Thermochemical treatments for protection of steels in chemically aggressive atmospheres at high temperatures

Boronizing is a well-known thermochemical treatment process that has found applications in a variety of industries. Boride layers on Fe-alloys present high hardness values (similar to 1900 Hv), good wear properties and are resistant to various aggressive chemical environments. The mechanical behavior of boride layers on Fe-alloys has been well documented, but there seems to exist limited information on the chemical behavior of these coatings. We present in this paper the results of a systematic study of the chemical properties of boride layers, in aggressive environments that include dilute HCl, H3PO4, H2SO4 and HNO3 acid solutions and vapors of molten Al alloy and Zn metals in various temperature ranges. Results indicated that with the exception of HNO3 acid, the presence of boride layers on Fe-alloys, greatly improved their corrosion resistance. These results are discussed in terms of the potential applications they may have in various industrial sectors, including field tests carried out in fossil-fuel fired boilers