An Exploratory Study of Inhibition of Intergranular Stress Corrosion Cracking in Sensitized Type 304 Stainless Steel

Abstract

The possibility of preventing intergranular stress corrosion cracking (IGSCC) by the addition of inhibitors was investigated in the present study as an exploratory study. The specimen adopted was the standard ASTM U-bend specimen, the cracking of which was monitored by means of intermittent microscopy. The standard environment selected was acidified sodium chloride at 100 C. The resulting SCC cracks resemble those of service IGSCC failures. The effectiveness of candidate inhibitors is reported as bar charts. None of the traditional inhibitors, including the high temperature organic inhibitors successfully used to mitigate corrosion in deep oil wells, was found to be effective. It was found that the combination of cadmium and sulfate acts synergistically to prevent not only IGSCC but all other forms of corrosion. Some success was achieved in analyzing the effectiveness of this inhibitor: It does not change markedly the cathodic half of the overall corrosion process, but it appears to interfere with the anodic part of the reaction. Kinetic experiments indicated that this inhibition is probably not due primarily to effects on the kinetics of regrowth of a protective oxide layer, but is possibly due to adsorption by both cadmium and sulfate ions, perhaps supplemented by complexing of chloride ions by cadmium. Metallic lead is sometimes used to prevent SCC in stainless steel in the chemical industry. During the present work, experiments were conducted to see whether the effectiveness of this approach is due to: (1) inhibitive characteristics of lead corrosion products or (2) changing the potential of the stainless steel. The results showed that the mechanism of inhibition was by depressing the potential (about 150 mV). Supplementary basic studies of polarization, pit initiation, scrape potential, and electron beam analysis were performed to search for better understanding of the mechanisms by which inhibitors act.