Comparative study of the alumina-scale integrity on MA 956 and PM 2000 alloys

The present work analyzes the oxidation kinetics of MA 956 and PM 2000 alloys at 900 and 1100 degreesC for exposure times up to 1000 hr. Special emphasis was placed on a comparison of the alumina-scale integrity formed at 1100 degreesC by means of electrochemical tests at room temperature, which have been shown to be very reliable methods to detect the presence of microdefects within oxide scales. To check whether a preoxidation treatment makes these materials corrosion resistant against aggressive fluids, an electrolyte containing chloride ions was chosen. The mass gain of MA 956 was found to be slightly lower than that of PM 2000 up to 200 hr exposure at 1100 degreesC and for the M whole exposure range at 900 degreesC. A subparabolic time dependence (n=0.3) of the oxide growth rate was determined for both alloys at both temperatures. On the other hand, the electrochemical-impenance spectroscopy (EIS) and anodic-polarization tests performed on preoxidized alloys (1100 degreesC/100 hr) revealed goon room-temperature corrosion behavior for both alloys, the corrosion resistance and polarization values being somewhat higher for preoxidized PM 2000. Consideration of these results and those of both surface and cross-section examinations of the scale, the better room-temperature corrosion behavior of preoxidized PM 2000 denotes the formation of a denser and mechanically more stable alumina scale containing a lower number of microdefects. This could result from the higher aluminum content of this alloy and the lower density of chemical heterogeneities within the scale. The higher mass gain of PM 2000 could be related to the higher concentration of oxide nodules on top of the alumina scale, as deduced from SEM examination.