ABSTRACT

At concentrations below 60%, wet hydrofluoric acid (HF) is extremely corrosive to steels, stainless steels and reactive metals, such as titanium, zirconium, and tantalum. In fact, only a few metallic materials will withstand wet HF at temperatures above ambient. Among these are the nickelcopper (Ni-Cu) and nickel-chromium-molybdenum (Ni-Cr-Mo) alloys. Previous work has shown that, even with these materials, there are complicating factors. For example, under certain conditions, internal attack and stress corrosion cracking (SCC) are possible with the Ni-Cr-Mo alloys, and the Ni- Cu materials can suffer intergranular attack when exposed to wet HF vapors.

The purpose of this work was to study further the response of the Ni-Cr-Mo alloys to HF, in

particular their external corrosion rates, susceptibility to internal attack and susceptibility to HF-induced SCC, as a function of alloy composition. As a side experiment, one of the alloys was tested in two microstructural conditions, i.e. solution annealed (the usual condition for materials of this type) and long-range ordered (this being a means of strengthening the alloy in question).

The study of external corrosion rates over wide ranges of concentration and temperature revealed a strong beneficial influence of molybdenum content. However, tungsten, which is used as a partial replacement for molybdenum in some Ni-Cr-Mo alloys, appears to render the alloys more prone to internal attack.

With regard to HF-induced SCC of the Ni-Cr-Mo alloys, this study suggests that only certain alloys (i.e.those containing tungsten) exhibit classical SCC. It was also discovered that high external corrosion rates inhibit HF-induced SCC, presumably due to rapid progression of the external attack front.

With regard to the effects of long-range ordering, these were only evident at the highest test temperatures, where the ordered structure exhibited much higher external corrosion rates than the annealed structure.

INTRODUCTION

Of the common halogen acids, hydrofluoric is the most difficult to deal with. Not only is it extremely hazardous, but also it attacks glass, titanium, zirconium, and tantalum, in addition to ferrous alloys. Nickel alloys are among the few metallic material options for wet hydrofluoric acid service. The mechanism by which the nickel alloys are protected is not well understood, but is believed to involve the formation of insoluble fluoride salts on their surfaces.1-3

However, it is now well known that nickel alloys and austenitic stainless steels are susceptible to Environmentally Assisted Cracking (EAC) when exposed in wet hot hydrofluoric acid. 4 Yet nickel alloys are less susceptible than stainless steels to cracking and all nickel alloys are not equally susceptible.5 Among the different types of corrosion mechanism which the alloys may encounter are: uneven corrosion or serration of the surface, stress corrosion cracking, internal attack, transgranular or intergranular attack, dealloying, internal voids, pits or cavities, and so forth, depending on the environment conditions (concentration, temperature, velocity, impurities, and liquid or vapor state of the acid) and also on the chemical composition of the alloy. Also, it is worth mentioning that the internal attack is unique to HF since other mineral acids, such as hydrochloric, sulfuric, nitric, phosphoric, etc., do not promote internal attack in the alloys.

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