Behavior of Slurry Aluminized Austenitic Stainless Steels under Steam at 650 and 700 A degrees C

摘要

Slurry aluminide coatings were deposited on IN-800HT and HR3C austenitic stainless steels. The additive layers were based on B2-(Fe,Ni)Al and contained tensile cracks in a greater extent in HR3C than in IN-800HT. The oxidation kinetics of the aluminized and uncoated steels were then evaluated under 1 bar of steam at 650 and 700 A degrees C till 2000 h. The overall mass gain of the Al-coated IN-800HT was about half that of the uncoated substrate at 650 A degrees C. The oxidation kinetics of the Al-coated IN-800HT were parabolic, but a sub-parabolic regime was found in the uncoated counterparts. In the former, alpha-Al2O3 grew while the latter developed a very thin scale that could not be accurately characterized. The Al-coated HR3C accumulated a greater specific mass gain than the uncoated steel at 700 A degrees C. However, it is believed that the presence of cracks and of a rougher surface of the coating is responsible for the greater mass gains. Indeed, both the Al-coated and the uncoated HR3C substrates followed sub-parabolic oxidation rate indicative of a very thin oxide scale. The scales were composed of alpha-Al2O3 in the Al-coated material, while Fe3O4, Fe2O3 and Cr2O3 were found in the uncoated HR3C. No evidence of Cr loss by evaporation of CrO2(OH)(2) or of hydrogen permeation was found under the experimental conditions studied here. The aluminide coatings appeared though stable from a metallurgical point of view after the 2000 h of exposure to steam.