Galvanic Factors in the Corrosion of Steel in Liquid Ammonia

As part of a program to evaluate the performance of steels in liquid ammonia, a study was made of the galvanic corrosion performance of ASTM A517 Grade F steel coupled to 3003 aluminum and exposed to metallurgical-grade ammonia. The influence of water additions and the contamination of the ammonia with air were also evaluated. The results of the investigation show that: (1) A517 Grade F steel and 3003 aluminum in the uncoupled condition, corrode at less than 1 mil penetration per year (mpy) regardless of the degree of contamination of the ammonia with air or water. (2) In the coupled condition, metallurgical-grade ammonia and air contaminated metallurgical-grade ammonia produced negligible corrosion (<1 mpy) of both the steel and the aluminum with surface-area fractions of 15/1, 1/1, and 1/15 steel to aluminum, respectively. In ammonia that contained water (with or without air), however, aluminum corroded sacrificially to protect the steel. Rates as high as 16 mpy for aluminum were noted at surface-area fractions of 15/1. (3) A decrease in surface-area fraction of steel to aluminum resulted in a decrease in the corrosion rate of the coupled aluminum. Under all conditions, the steel corroded at a rate of less than 1 mpy. (4) Galvanic coupling of aluminum to steel protected the steel from stress corrosion cracking in air contaminated metallurgical-grade ammonia. Overall, this study shows that aluminum components, such as baffles and dipsticks currently used in tank trucks that haul liquid ammonia, are not deleterious to the corrosion performance of the steel.