Titanium carbonitrides, corrosion

Under applied anodic potential the corrosion of stainless steels in molten chlorides is electrochemical in nature. At the initial period the exchange reaction between steel components and alkali metal cations takes place in parallel with the electrochemical process. It was found that titanium in steels forms stable carbonitride species that do not dissolve during anodic oxidation. Preliminary thermal treatment of austenite steels has an effect on anodic dissolution processes. 

Alloy 800 series are derived from Incoloy 800, originally developed in the 1950s by the International Nickel Company [18]. AUoy 800 series have high nickel and chromium contents which give them superior resistance to corrosion in many kinds of environments (Table 17.2). Titanium and aluminum additions lead to the precipitation of different carbonitrides which enhance the material properties. But this Ti + A1 content also leads to the precipitation of hard y Ni3(Ti,Al) phase which enhances creep resistance but decreases the material ductility at high temperature. This is why the Ti + A1 content has been limited in some specifications (800 HT, 800 SPH). On the other hand, annealed material was historically differentiated from solution annealed material, with the terms grade 1 annealed at approximately 980°C, and grade 2 solution annealed at approximately 1150°C [19]. 

---