Effect of Organic Acids on the IGSCC of Sensitized AISI 304 Stainless Steel in High Temperature Aqueous Solutions

The effect of low molecular weight carboxylic acids on the inter-granular stress corrosion cracking (IGSCC) of sensitized AISI⁽¹⁾ 304 stainless steel (SS) in high temperature (275 C) water was investigated using slow strain rate tests (SSRTs). Significant differences were found by comparing the effect of formic, acetic, and oxalic acids at a concentration of 4.35 × 10⁻⁴ M over a wide range of oxygen contents (0.005 to 8.0 ppm). At intermediate dissolved oxygen concentrations (0.2 and 0.6 ppm), formic and oxalic acids suppressed IGSCC, whereas acetic acid accelerated it. The IGSCC behavior was correlated with the corrosion potential of the alloy. The presence of formic and oxalic acids shifted the potential below the critical potential for IGSCC in pure water. It is suggested that the potential shift is caused by the enhanced reduction of oxygen on the alloy surface, prompted by the adsorption and electro-oxidation of the acids or intermediate products of their decomposition, such as carbon monoxide. However, the possibility of an homogeneous reaction in the liquid phase cannot be disregarded.