Effect of boron on the IGSCC behavior of alloy 600 in a caustic solution

The effect of the addition of candidate inhibitors on the IGSCC of Alloy 600 in a caustic solution was investigated. The SCC susceptibility was evaluated by slow strain rate tensile (SSRT) tests and the electrochemical properties of the alloy in the solutions containing several candidate inhibitors were measured using potentiodynamic polarization measurements. In the reference solution, 40% NaOH, severe intergranular cracks were observed on the surface after the SSRT tests. It was turned out that the addition of CeO2 [Ce4+], (CH3COOH)3Ce [Ce3+], H3BO3 to the reference solution had no effect on the IGSCC of the alloy. However, CeB6, NiB, and FeB2, that commonly contain boron, significantly suppressed the IGSCC. The surface oxide examination by Auger electron microscopy showed that boron was deposited into the oxide when the borides were added to the reference solution. The corrosion current density at the corrosion potential determined from the polarization results was significantly decreased in the boride added solutions where the SCC was suppressed. These results suggest that boron may play a key role in the inhibition of the IGSCC of the alloy in the caustic solution.