Stress-Corrosion Cracking and Galvanic Corrosion of Internal Bolts from a Multistage Water Injection Pump

Nineteen out of 26 bolts in a multistage water pump corroded and cracked after a short time in a severe working environment containing saline water, CO₂, and H₂S. The failed bolts and intact nuts were to be made from a special type of stainless steel as per ASTM A 193 B8S and A 194. However, the investigation (which included visual, macroscopic, metallographic, SEM, and chemical analysis) showed that austenitic stainless steel and a nickel-base alloy were used instead. The unspecified materials are more prone to corrosion, particularly galvanic corrosion, which proved to be the primary failure mechanism in the areas of the bolts directly exposed to the working environment. Corrosion damage on surfaces facing away from the work environment was caused primarily by chloride stress-corrosion cracking, aided by loose fitting threads. Thread gaps constitute a crevice where an aggressive chemistry is allowed to develop and attack local surfaces.