Aggressive corrosion of C-276 nickel superalloy in chloride/sulphate eutectic salt

Abstract

Molten salts are often used to store thermal energy in concentrating solar power plants. The need for higher temperature thermal energy storage systems has prompted investigation into salts and structural alloys for use at higher temperatures. A eutectic mixture of 68.05 wt% Na2SO4: 31.95 wt% NaCl salt is a potential phase change material for such systems, with low cost and high latent heat of fusion. This paper investigates corrosion of Inconel C-276 nickel alloy in this salt under air and argon atmospheres. It was found that this alloy was aggressively corroded in air at a rate calculated as 64 mm/year, with complex and unique corrosion structure observed. When tested in argon, this corrosion was even more severe, with the sample completely consumed in 5 days. This result was attributed to a basic salt corrosion mechanism which attacked the chromium and iron, whilst sulphur attacked the nickel. The high molybdenum and tungsten content of C-276 was not present in significant concentration in the corrosion product, possibly because these elements formed chloride-based compounds which evaporated. This reinforces that sulphur and oxygen containing salts can be highly corrosive, even to corrosion resistant nickel superalloys under inert atmospheres.