Pitting corrosion of sheets of a nickel-base superalloy

Abstract

Cold rolled sheets of a nickel-base superalloy are used in building gas turbine engines. After rolling to the final gauge, the sheets were annealed at 11808C. Pickling of the sheets was carried out to remove the scale and the oxidized layer. It was observed that there was heavy incidence of pitting with perforations at several locations due to pits penetrating through the thickness. The sheets got rejected, resulting in heavy loss. The superalloy contains 25wt% chromium, 15 wt% tungsten. Tungsten provides high resistance to pitting corrosion. With such a high level of W, the grade has a high Pitting Resistance Equivalent. Further it is a single-phase material. Pitting attack is thus totally unexpected. The failure was analyzed. Pickling process for this material normally involves the usage of salt bath, sulfuric acid, nitric acid, and/ or hydrofluoric-nitric acid baths. In the case under study, satisfactory pickling was not achieved this way; discoloration and deposits were persistently present on the sheet surface. In order to improve the surface quality, the sheets were dipped in hydrochloric acid bath for a long time. Hydrochloric acid bath was in aged condition; by that point of time, the bath went through extensive usage for pickling of different grades of steels. Consequently there was significant accumulation of iron salts in the bath. It is known that ferric and chloride ions cause significant acceleration in pitting. The bath was stagnant and the time of immersion of the sheets in the bath was long. The material failed under the combination of these adverse conditions. The paper brings out the details of the failure analysis carried out