Impact of Solution Chemistry on Pitting and Stress Corrosion Cracking of Steam Turbine Disc Steels Available to Purchase

A summary is given of recent measurements to evaluate the effect of solution chemistry variables on the likelihood of stress corrosion cracking of a steam turbine disc steel, 3NiCrMoV. Exposure tests were carried out for the disc steel in the form of cylindrical tensile specimens stressed to 90% of σ_0,2 and immersed at 90 °C for up to 22 months in deaerated pure water, aerated water, and aerated water containing 1.5 ppm chloride. Pitting was observed in all cases with the pit growth rate being greatest for the aerated water containing chloride. No cracking was observed in deaerated pure water but cracks initiated in aerated water. A minimum pit depth of about 50 μm was required for crack initiation. The percentage of pits with cracks at a specific depth in aerated solution was described by a Weibull function and seemed unaffected by chloride or exposure time.

In order to assess the impact of two-shifting operation (on-load to off-load cycling) of a steam turbine plant, the effect of oxygen excursions on the corrosion potential of a steam turbine disc steel was studied. Off-load, the liquid film on the turbine surface was assumed to be aerated water, but a range of environments, including deaerated water, deaerated 300 ppb, 600 ppb and 1.5 ppm Cl^- solution, and deaerated 300 ppb Cl^- + 300 ppb SO₄^2- solution, were considered to represent the range of possible condensate solution chemistry on-load. The measurements indicate a consistently higher mean corrosion potential compared with base-loading, although less marked in the presence of sulphate. The risk of increased pitting and stress corrosion cracking as a consequence of two-shifting would appear to depend critically on the concentration of sulphate in the condensate in service.