Corrosion of Several Typical Alloys in Concentrated Sodium Hydroxide Solution with Chloride Between 180°C and 240°C

The corrosion rates of two nickel-based alloys, Alloy 400 (UNS N04400) and Alloy C-276 (UNS N10276), and two stainless steels, Type 316L (UNS 31603) and Type 304 (UNS S30400), in a concentrated sodium hydroxide (NaOH) solution (45 mass%) containing sodium chloride (NaCl, 15 g/L) between 180°C to 240°C were studied. Methods, such as weight variation, color changes, scanning electron microscopy (SEM), and energy spectrum analysis of the samples during tests were conducted to evaluate the corrosion of those alloy samples. Results showed that the corrosion rate of Alloy C-276 in high-alkali solution with chloride at high temperature is the lowest, about 0.022 mm/y to 0.073 mm/y, then Alloy 400, about 0.09 mm/y to 0.60 mm/y, and finally Types 304 and 316L, about 1.0 mm/y to 16.4 mm/y. Thus, the corrosion rate of Alloy C-276 is 14 to 180 times lower than that of mild steel used in the “Bayer” process. The energy spectrum analysis showed that after testing, Alloy C-276 had a very thin oxide layer. Alloy 400 can exhibit pitting corrosion when some S²⁻ anions exist in the solution and forms an oxide film when concentrated Cl⁻ exists in the solution. For Types 304 and 316L, cracks formed on their surface, showing that they have a high tendency to stress corrosion.