Effect of nitric acid contamination on mild steel corrosion in hydrofluoric acid at 25°C

Abstract

HNO₃ contamination increased the corrosion rates of mild steel storage tanks at an HF plant containing 70% HF acid from 0.5 to nearly 30 mm/year. The steel had been pre-treated by exposure to 70% HF for up to 24 h, to form a protective scale, which restricted the corrosion rate (CR) to <1 mm/year. This study simulated the corrosion effects on a laboratory scale, to determine the corrosion rates and mechanisms of 0.1% ≤ HNO₃ ≤ 1% (mass per volume), after the nitric acid had been consumed. Mass loss, apparent corrosion rates, acid consumption, scale formation, pit corrosion and depth profiles were measured. The HNO₃ selectively dissolved the protective oxide-fluoride scale, which then allowed HF access to the underlying metal and continuously corroded the exposed steel until failure occurred by perforation.