Nickel boride (NiB) as an inhibitor for an IGSCC of Alloy 600 and its applicability

Using an Alloy 600 tubing, slow strain rate tensile tests and polarization measurements have been performed in deaerated 40% sodium hydroxide and in aerated ammonia solutions (pH = ∼ 9.5) at 315 °C. Tensile samples strained at an open circuit potential showed a severe intergranular stress corrosion cracking in the NaOH solution and small intergranular cracks were observed on the gage section of the samples tested in the ammonia solution. When added to both the solutions, borides were found to suppress the IGSCC. From the polarization curves in the NaOH solution with and without the additives, the corrosion current densities were estimated. The corrosion current density of the alloy in the 40% NaOH solution was significantly decreased by the addition of CeB₆ and NiB to the solution, which was judged to be attributed to the reduced corrosion current density. In the experiments, NiB showed a greater effectiveness for a corrosion and SCC inhibition than CeB₆.