ABSTRACT
During heat treatment surface oxide layers, usually called heat tints, are formed on metallic materials. These oxide layers are composed of elements that have been selectively oxidized from the base metal; in the case of high-alloy materials principally chromium, nickel and iron. On austenitic stainless steels, it is well known that the region beneath the oxide layer is depleted in one or more of the elements that are involved in the scale formation. Consequently, reduced corrosion resistance is expected. It is also known that defects and stresses within the heat tint layer limit their protectiveness. Therefore, heat tint layers are usually removed by mechanical and/or chemical treatments to avoid corrosion issues during service. Nevertheless, the same understanding on heat tints formed during aging of precipitation hardenable Ni-based alloys is still lacking. Ni-based alloys generally have better corrosion resistance than stainless steels and the chemical composition of their surface oxide layers differ from those typically formed on stainless steels. In the present work the effect of heat tints on the pitting corrosion resistance of the Ni-based alloy UNS N07718 has been evaluated by means of electrochemical methods including cyclic potentiodynamic polarization tests and electrochemical noise measurements, and exposure tests in chloride-containing solutions.
