Thermochemical Treatments for Protection of Steels in Chemically Aggressive Atmospheres at High Temperatures

Abstract

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 (∼ 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.