Effect of Electrode Potential on the Hydrogen-Induced IGSCC of Alloy 600 in an Aqueous Solution at 350 C

Slow strain rate tests (SSRTs) were conducted to study the effect of electrode potential on the intergranular stress corrosion cracking (IGSCC) of Alloy 600 in deaerated 0.01 M H3BO3 + 0.001 M LiOH solution at two different partial pressures of molecular hydrogen at 350 C, using tensile specimens with a cold-pressed hump in the middle of the gage length. IGSCC was found to occur at electrode potentials more negative than a threshold value. No IGSCC occurred at potentials more positive than this threshold value. The intergranular fracture developed at sites in the tensile specimen located close to the cold-pressed hump. To characterize the protective properties of oxide films on the alloy surface, AC impedance measurements were made under the same environmental conditions as above. The results have shown that IGSCC of Alloy 600 occurs only under such environmental conditions, under which a sufficiently large amount of hydrogen is absorbed by the strained alloy. From the AC impedance measurements it follows that the protective properties of oxide films on the alloy surface depend upon the partial pressure of hydrogen in the system.