Which metals are most severely corroded by hydrofluoric acid
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Considering the corrosion characteristics of hydrofluoric acid discussed earlier, the following metals are most susceptible to its corrosion: chromium, aluminum, and zinc. The passivation layer on the surface of these metals is directly destroyed by fluoride ions, preventing the formation of a stable protective fluoride layer; as a result, they dissolve rapidly in hydrofluoric acid at normal concentrations, making them metals that are extremely prone to corrosion. Titanium and titanium alloys: The TiO₂ passivation layer on the surface of titanium reacts with fluoride ions to form soluble fluorotitanates. When the F⁻ concentration in hydrofluoric acid exceeds 0.0005M, the passivation layer is completely destroyed, leading to severe pitting; it is therefore almost impossible to use these materials stably in an environment containing hydrofluoric acid. Ordinary austenitic stainless steels (such as 18-8 type 304 and 316 stainless steels). In the passivation film of these stainless steels, Cr₂O₃ is converted into the soluble compound CrF₃ by fluoride ions; as a result, the film thins out and becomes porous rapidly. These steels are subject to slow corrosion even at room temperature, and severe pitting occurs in high-concentration hydrofluoric acid. In industrial applications, these materials are generally not used when in contact with hydrofluoric acid. Steel in low-concentration hydrofluoric acid: In low-concentration hydrofluoric acid with a mass fraction of less than 5%, a stable iron fluoride passivation layer cannot form on the steel surface, resulting in continuous and rapid corrosion. This is precisely why hydrofluoric acid, as a low-concentration pickling agent, can effectively dissolve rust. Precious metals such as gold and platinum, as well as special nickel-copper alloys like Monel, can maintain good corrosion resistance in most hydrofluoric acid environments, with extremely low levels of corrosion.
Thank you to everyone above for their detailed sharing, especially the additional information on Monel and Hastelloy – it’s very useful. In addition to the commonly used corrosion-resistant materials, I would like to add that when selecting materials in practice, it is necessary to consider not only the corrosion rate but also how the material performs under specific temperature, flow rate, and contamination conditions. For example, Hastelloy C-276 sees a decline in its performance under high temperature and pressure, while Monel may also experience problems in hydrofluoric acid containing oxidizing agents such as Fe3+. Additionally, in the event of non-standard operating conditions, it is recommended to conduct pilot tests first or consult professional corrosion databases (such as NACE standards), given that the costs of materials and processing are not low. Of course, there are also differences among material batches from different suppliers; for specific applications, it is recommended to consult a materials engineer or corrosion expert to make the final decision