Performance of Ni-Cr-Mo-V turbine rotor steels in Na₂SO₄ solution

The stress corrosion cracking (SCC) susceptibility of different heats of steels from three turbine rotors in Na₂SO₄ solution was investigated as a function of applied potential and heat treatment. The electrochemical reactivity of the steels was studied by generating potentiodynamic polarization curves to determine the possible potential ranges for SCC. In both 0.01 and 1.0M Na₂SO₄ solutions at 25 and 100°C, none of the steels tested, in any of the heat treatment conditions, showed a well defined active to passive transition range. Tests using the slow strain rate technique in 1.0M Na₂SO₄ at 100°C showed that, at cathodic potentials, rotor steels in the embrittled condition showed the greatest susceptibility to environmentally assisted fracture, while a de-embrittlement treatment decreased this susceptibility. In general, rotor steels tested at the corrosion potential showed a higher resistance to environmentally assisted fracture, but it cannot necessarily be assumed from this that any of them are immune to cracking in concentrated sulphate solutions. The effect of heat treatment on the SCC susceptibility of the rotor steels studied was minimal.