The rate of corrosion of welded and unwelded chromium-nickel steels, Uniloy 19-9DL and 19-9DX and welded alloy 321 by fuming nitric acid in the liquid and the gas phase was measured with and without hydrofluoric acid added as a corrosion inhibitor. Corrosion by this system is of interest because of its widespread use as a rocket propellant. The effect of heat treatment on the corrosion process was measured by performing tests on one set of samples annealed at 1800 degrees F and air-cooled and on another series of samples that had been work-hardened and stress-relieved at 1200 degrees F. In general, samples in the latter condition had the least corrosion resistance. It was found that 0.6 weight percent hydrofluoric acid in fuming nitric acid inhibits both liquid- and gas-phase corrosion in all cases in which uncorroded metal was exposed to the acid. Most of the measurements were conducted at 130 and 160 degrees F although a few measurements were made at 100 degrees F to determine the effect of temperature on the corrosion process. Inhibition was also effective with temperature fluctuating between 70 and 160 degrees F. When the metal had previously undergone intergranular corrosion, however, the presence of hydrofluoric acid in fuming nitric acid aggravated the corrosive attack.
4.3.2
