Corrosion of MA754 and MA956 in a Commercial Aluminum Melter

Abstract

The University of North Dakota Energy & Environmental Research Center is working with Oak Ridge National Laboratory to test two oxide dispersion-strengthened alloys that could be used to construct very high-temperature heat recuperators for the aluminum-melting industry. For the initial tests, uncooled rings of MA754 and MA956 piping were exposed for 5½ months to gases leaving an aluminum melter furnace at 1200°–1290°C. The MA956 suffered spotty areas of severe corrosion and lost 25% of its weight. Scanning electron microscopy showed that there were small spots of alkali-rich corrosion products on the alloy surfaces, indicating the impact of droplets of fluxing agents. The corrosion products in these areas were mixed Fe, Cr, and Al oxides, which were depleted in Cr near the gas surface. However, Al concentrations in the remaining metal were typically between 3.5% and 4.0%, so there was a sufficient reservoir of Al remaining in the alloy to prevent simple breakaway corrosion which could have occurred if the Al were significantly depleted. The MA754 lost approximately 15% of its weight and showed void formation within 2 mm of the gas–metal surfaces. Within the porous area, the Cr had largely segregated into oxide precipitates up to 50 9m in diameter, leaving the remaining metal Ni-rich. Below the porous layer, the alloy composition was relatively unchanged. Remains of Na- and Al-rich particles that had impacted the surface sporadically were visible but had not obviously affected the surface scale as they had with the MA956.