Low Temperature Hot Corrosion of Cobalt-Base Alloys: Part II. Reaction Mechanism

This paper presents a mechanism of low-temperature hot corrosion that is based on rapid dissolution of the more noble metal or metal oxide in liquid salts. It is proposed that the rapid degradation of cobalt-base alloys results from dissolution of cobalt or cobalt oxides on the surface, which prevents the formation of a protective Cr₂O₃ or A1₂O₃ film. The reaction occurs in two stages: (a) an initial stage, during which an Na₂SO₄-CoSO₄ liquid forms on the surface, and (b) a propagation stage, during which SO₃ migrates inward and cobalt outward through the molten salt. At longer times, cobalt dissolves at the scale/salt interface and forms Co₃O₄ and/or CoSO₄(s) in different regions of the reaction product. The mechanisms of transport of various reactants and products through the liquid salt and the effects of their relative transport rates on the reaction product morphology have been considered.