Hydrofluoric Acid is extremely aggressive and attacks most metals and glass. The approach to the selection of stainless steel is similar to that for hydrochloric acid.
Commercially concentrated acid is around 40wt. %. Plastics are normally considered for handling hydrofluoric acid.
The common stainless steel types, 304 stainless steel and 316 stainless steel should be considered non-resistant to hydrofluoric acid at any concentration and temperature.
Corrosion resistance of stainless steel
The iso-corrosion diagram 0.1 mm/year lines are represented for stainless steel 316 types (red) and a 6% molybdenum austenitic types (green)
Higher grades of stainless steel can have limited resistance, up to around 2% maximum at ambient temperatures, but may suffer local attack, mainly as crevice and pitting corrosion, even at such low concentrations.
Uses for hydrofluoric acid with stainless steel
Hydrofluoric acid is used, along with nitric acid, in stainless steel pickling solution and paste preparations. Its main function is to loosen and help remove scale deposits. Incorrect use can be the cause of pitting problems if the solutions or pastes are left in contact with the stainless steel too long. It is very important that the manufacturers / suppliers instructions are followed to avoid damage to stainless steel products when using products containing hydrofluoric acid.
Source: Zhejiang Yaang Pipe Industry Co., Limited (www.yaang.com)