Abstract
The corrosion behavior of AISI 316 stainless steel (SS) in formic and acetic acids was investigated by measuring the corrosion weight loss, the polarization curve, the impedance at the steel/solution interface, and the variation of natural electrode potential (NEP) with time. It was found that the corrosion rate of AISI 316 SS in formic and acetic acids was markedly dependent on the concentration of the acid and temperature. The corrosion rate in aqueous solution of these acids was larger than in nonaqueous solution, and the corrosion behavior in nonaqueous solution was quite different from that in aqueous solution. It was believed that Cr and Mo mainly contribute to the corrosion resistance of AISI 316 SS in aqueous solution, while Ni contributes slightly. In nonaqueous solution of formic acid, Cr mainly contributes to corrosion resistance, and Mo and Fe contribute slightly; in nonaqueous solution of acetic acid, Cr mainly contributes to corrosion resistance, while Mo contributes slightly. To obtain information for the passive film on AISI 316 SS after immersion in formic acid, the distribution of the main compositional elements was analyzed by Auger electron spectroscopy (AES). From these results, it was found that the amount of Mo distributed on the surface of steel was larger, while the amount of Fe was smaller than that before immersion. It was also found that the amount of Cu in an aqueous system and Ni in a nonaqueous one was larger than that before immersion. In the case of formic acid, surface analysis of steel after immersion in acetic acid was conducted by ion micro analysis (IMA). In 50 and 100% acetic acids, some Cr was present on the steel surface. In 90% acetic acid, it was believed that selective corrosion did not occur because the distribution of the main compositional elements of the specimen surface after immersion did not change.
