Inhibiting Effect of Hydrofluoric Acid In Fuming Nitric Acid on Liquid and Gas Phase Corrosion of Several Metals

Abstract

Abstract Measurements were made at 130 degrees F of the extent of corrosion of several metals exposed to the liquid and the gas phase of thermally stable nitric acid containing 11 to 13 weight percent nitrogen dioxide and 2 to 4 weight percent water with and without hydrofluoric acid added as a corrosion inhibitor. Liquid-phase corrosion of the following metals was readily inhibited by hydrofluoric acid in fuming nitric acid of this composition: aluminum alloys 2S-0 (1100), 14S-T6 (2014-T6), 17S-T4 (2017-T4), 24S-T4 (2024-T4), 61S-T6 (6061-T6), and 75S-T6 (7075-T6); aluminum 2S-0 welded to 356; and chromium-nickel steels 302, 303, 304, 321, 347, Armco 17-7PH, and Uniloy 19-9DL and 19-9DX. Carbon steel C1020 and chromium alloy steels 4130, 410, 430, and 446 having intact natural metal oxide films, which were formed in moist air, were inhibited by hydrofluoric acid. Corrosive attack of these steels without the oxide film, however, was aggravated by the presence of hydrofluoric acid in fuming nitric acid, the corrosion becoming more extensive the lower the chromium content of the steel. Commercially pure lead was inhibited by hydrofluoric acid, whereas pure chromium, Nickel-A, tantalum, and titanium alloys 75A and 130A were either only slightly inhibited or showed corrosion rates which actually were increased by presence of hydrofluoric acid in fuming nitric acid. Inhibition of gas-phase corrosion was found to occur readily in the case of the following metals tested: steels 303, 410, 430, and 446 and aluminum 61S-T6. Gas-phase corrosion of steels 4130 and 1020 was usually aggravated by hydrofluoric acid. Exposure of aluminum 61S-T6 and stainless steel 347 to fuming nitric acid with a repeated cycling of temperature between 70 and 160 degrees F was found not to impair the inhibiting effect of hydrofluoric acid on gas and liquid-phase corrosion of these metals. A few tests of the liquid and gas-phase corrosion of aluminum 61S-T6 and steels C1020 and 347 by nitrogen dioxide at 130 degrees F were also made, and this medium was found in general to be much less corrosive than fuming nitric acid. 4.3.2