CORROSION OF STAINLESS-STEELS AND NICKEL-BASE ALLOYS IN SOLUTIONS OF NITRIC-ACID AND HYDROFLUORIC-ACID

Reactions involving nitric acid may always result in the contamination of this acid with fluorides. In highly concentrated nitric acid, the presence of small amounts of HF will substantially reduce the corrosion of metallic materials. Mixtures consisting of hydrofluoric acid and hypo-azeotropic nitric acid on the other hand will strongly attack: the metal loss will markedly increase with increasing HNO3 and HF concentrations as well as with rising temperatures. The investigation covered 12 stainless steel grades and nickel-base alloys. With constant HNO3 content, corrosion rates will rise linearly when increasing the HF concentration. With constant HF concentration (0.25 M), corrosion rates will increase rapidly with increasing nitric acid concentration (from 0.3 M to 14.8 M). This can best be described by superimposing a linear function and a hyperbolic function that is reflecting the change in the HNO3 content. Alloys containing as much chromium as possible (up to 46 wt.%) will exhibit the best corrosion resistance. Alloy NiCr30FeMo (Hastelloy alloy G-30) proved to be well suitable in this investigation.