Corrosion Resistance Evaluation of Some Stainless Steels Used in Manufacture of Hydraulic Turbine Runner Blades

The paper presents corrosion resistance testing results of three stainless steels that may be used in hydropower turbine blades manufacture. Two of these have a chemical composition close to that of some other stainless steels previously employed in producing these parts, being updated steel grades of the former ones. The third one is of a new conception, having a chemical composition close to that of a maraging steel. The three materials were produced in an induction furnace with cold copper crucible under vacuum and argon atmosphere in order to obtain improved mechanical and corrosion resistance characteristics as well as an inclusion - free structure. Quenching and tempering heat treatments were subsequently applied. Tests were carried out at room temperature in normally aerated 1N Na2SO4 and 3% NaCl solutions. Corrosion rates were calculated using the Tafel slope method. All steels have a passivation tendency in a chlorine-free aqueous medium. The newly conceived steel has a more pronounced anodic field as a result of a chromium content below 12%. However, the general corrosion behavior of this material is rebalanced by the content of about 10% Ni which leads to a mainly martensitic structure in quenched state. The corrosion rate values obtained for all samples enframe the three materials in highly and very highly corrosion resistant steels. Nevertheless it must be specified that in chlorine environments the overall corrosion rate is not a sensitive indicator of corrosion resistance performance due to the local depassivation process followed by corrosion pits.