On the Hot Corrosion of Nickel at 700 °C

The reaction of nickel with Na2SO4 was studied in an O2–0.1SO2 atmosphere at 700 °C, conditions typical of type II hot corrosion. The interaction of NiO with the sulfate was thermodynamically analyzed and compared with the dissolution–precipitation process known to occur at higher temperatures. Here the dissolution stage was short-lived due to a positive solubility gradient for Ni2+. The relatively fast growth of a NiO layer above a nickel sulfide layer was attributed to the inward molecular transport of SO2–SO3 through the porous NiO. The influence of environmental variables was further assessed by reacting nickel with and without Na2SO4 in air and then with no deposit in O2–0.1SO2. The roles of the deposit and atmosphere on the boundary conditions affecting the oxidation–sulfidation process were discussed on the basis of the observed reaction paths.