Effects of galvanostatic treatments on hot corrosion of Ni

Abstract

Several electrochemical polarization tests have been developed to assess the hot corrosion resistance of gas turbine materials [1-3]. Shores [4] has developed a screening test based on the corrosion current density estimated from the anodic part of the polarization curve after galvanostatic polarization. The corrosion current density of some super-alloys determined by this method correlated well with the penetration rates measured by the burner rig test. Erodos et al. [5] proposed an electrochemical screening test based on the measurement of activation potential after a galvanostatic pretreatment similar to that by Shores. The activation potential after galvanostatic polarization was reported to have a linear correlation with the depth of corrosion determined metallo-graphically after the test. However, these works [4, 5] were conducted in air containing no oxidents such as SO2 or SO3, different from the gas turbine operating environments, and the work by Shores was criticized for low repeatibility [5]. This letter describes the results of a study on the effect of galvanostatic treatment on the corrosion current density of Ni and its scatter measured by the conventional Tafel method, in molten Na2SO 4 in contact with gas atmospheres containing Ar, 02 and SO3. Commercial Ni wire of 99.995% purity was used in this investigation. Some specimens were preoxidized for times of 10 to 180 min in still air at 900 °C. The reference electrode was made of pure Ag wire dipped into a 10 m/o Ag2SO4-Na2SO 4 solution contained in a mullite tube, and the counter electrode was made of pure Au wire. Specimens were exposed to a reagent grade molten Na2SO 4 at 910 °C in Ar, air and Pt-catalysed 0.2% SO2-O 2 gas atmospheres. The flow rate of gases except the air used in static condition was 400 ml/min. For the determination of corrosion current density I~or~, potentials were scanned at a rate of 0.3 mV/s from E~o~r 250 mV to Ecor~ + 250 mV, using a Princeton Applied Research model 273 potentiostat. For the galvanostatic treatment, currents were applied to the specimens for 2 h either at a density of 5 mA cm -2 for the cathodic treatment or at +5 mAcm -2 for the anodic treatment. The results of hot corrosion tests of clean Ni specimens in various conditions are summarized in Table I. Values of Icor~ are remarkably dependent upon the gas atomospheres in contact with the melt. The highest value of Ico~r was observed in specimens tested in Pt-catalysed 0.2% SO2-O2 gas containing both SO3 and O2, and the lowest in Ar gas