Effects of Variations of Carbon, Sulfur and Phosphorus on the Corrosion Behavior of Alloy 600

Corrosion tests were conducted on a series of Ni-Cr-Fe heats with C, S, and P varied within the composition range of alloy 600. The materials were in several conditions of heat treatment. Stress corrosion cracking (SCC) tests were conducted in undeaerated and deaerated water and deaerated 50% NaOH, all at 316 C (600 F). Intergranular corrosion tests were conducted in boiling 65% nitric acid and ferric sulfate-sulfuric acid solutions. In 8000-hour deaerated water tests, all materials were resistant to stress corrosion cracking (SCC). In undeaerated water and in deaerated 50% NaOH, all materials showed intergranular attack. Corrosion rates in boiling 65% nitric acid and ferric sulfate-50% sulfuric acid increased with increased carbon and sulfur levels. Some of the test coupons corroded uniformly and intergranularly during the ferric sulfate-50% sulfuric acid exposure. Reducing the sulfuric acid concentration and increasing the ferric sulfate concentration caused intergranular corrosion of carbon and phosphorus containing materials, while sulfur acted to decrease the deleterious effects of carbon and phosphorus. In addition, a grain size effect was revealed in the ferric sulfate-50% sulfuric acid exposure. Structural investigations showed carbon and phosphorus could be redistributed by heat treatment and were generally associated with M7C3 and Ti(C,N) particles. High sulfur heats contained sulfur-rich particles which were located both within the grain and on grain boundaries. The way in which these residual elements influence intergranular corrosion is discussed.