On the Hot Corrosion of Nickel at 700 A°C

The reaction of nickel with Na2SO4 was studied in an O-2-0.1SO(2) atmosphere at 700 A degrees 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 O-2-0.1SO(2). 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.