Aqueous solutions of hydrofluoric acid (HF) are among the most difficult to deal with. Not only can such solutions attack glass, but also alloys of elements such as titanium and zirconium, which are normally protected by oxide films, are rendered useless by hydrofluoric acid. In fact, select nickel alloys are among the few metallic materials resistant to hot hydrofluoric acid solutions, and even they have restricted temperature and concentration capabilities. Moreover, hydrofluoric acid can induce other forms of corrosion, such as internal attack and stress corrosion cracking (both of which will be discussed in later sections of this manual).
Hydrofluoric acid is also unusual (for a reducing acid) in that it can induce the formation of pseudo-passive films on nickel-based alloys. Indeed, the outstanding performance of MONEL® 400 alloy (from the nickel-copper family of materials) has been attributed to the formation of protective fluoride films on exposed surfaces.
As to industrial uses of hydrofluoric acid, it is found in the etching and cleaning of metals and ceramics, acid treating of oil and gas wells, and extractive metallurgy. Although beyond the scope of this manual, anhydrous hydrogen fluoride is used in the manufacture of the industrially important fluoro-chemicals (Jennings 2006). While all acids pose a safety issue, hydrofluoric acid is by far the most dangerous. No skin should be exposed when handling solutions of the acid, and fumes should be avoided.
Since glass is attacked by hydrofluoric acid, laboratory corrosion tests are carried out at Haynes International in TEFLON® flasks, with TEFLON® condensing systems. Due to the dangers of HF, tests (of 96 h duration) are normally carried out without interruption (tests in other acids involve four 24 h test periods, with interruptions for cleaning and weighing of samples, and replenishment of solutions, if needed).
Within the realm of corrosion-resistant nickel- and cobalt-based alloys, the nickel-copper alloys are the most commonly utilized for industrial applications involving hot, pure, aqueous solutions of hydrofluoric acid. However, they are negatively affected by the presence of oxygen. In such circumstances, the nickel-chromium-molybdenum alloys are used, although they are restricted to lower operating temperatures.
An iso-corrosion diagram for MONEL® 400 alloy in aqueous solutions of hydrofluoric acid is shown in Crum et al, 1999. Essentially, it indicates that the alloy generally exhibits rates of less than 0.5 mm/y at all concentrations up to 100 wt.%, and at temperatures up to boiling. Jennings 2006 states that commercial aqueous hydrofluoric acid is available at concentrations of 49 and 70%; this is consistent with the experimental procedure of Crum et al, 1999, which states that their aqueous hydrofluoric acid tests involved solutions prepared from a concentration of 49 wt.%. It does not explain how the higher concentrations were attained, nor does it give the duration of the tests, which experience at Haynes International suggests is important. Tests of 24 h duration gave variable results, suggesting an incubation period, during which the pseudo-passive fluoride films are becoming established. Tests of 240 h (without interruption) resulted in concentration changes, due to the escape of hydrogen fluoride gas at the thermometer seals in the TEFLON® system.
A more important graph generated by Crum et al is reproduced in the following figure. It compares the positions of the 0.5 (or more precisely 0.51) mm/y lines of many nickel alloys on the iso-corrosion diagram for hydrofluoric acid. In particular, it indicates that the nickel-chromium-molybdenum alloys fall within a fairly narrow performance band, along with pure nickel, and well above that for the nickel-chromium, nickel-chromium-iron, and nickel-iron-chromium materials.