Corrosion resistance of Nickel alloys in dilute and anhydrous hydrofluoric acid

Anhydrous hydrogen fluoride and hydrofluoric acid are major industrial process materials. Hydrofluoric acid is highly corrosive and extremely reactive, it is used in large magnitudes for pickling of stainless steels as well as other metals. Additionally, it is used in acid treatment of wells and glass etching. Other applications of hydrofluoric acid include production of aluminium fluoride and synthetic cryolite, fluorinated organics like aerosol propellants and plastics.

Anhydrous hydrogen fluoride is the basis of fluorocarbon industry that mainly includes coolants, fire extinguishing elements, ultrasonic cleaning fluids and fluorocarbon plastics.

Hydrofluoric acid in aqueous and anhydrous form is extremely hazardous that it severely affects eyes, lungs and mucous membranes.

Anhydrous hydrogen fluoride is made by the reaction of sulfuric acid and calcium fluoride. Commonly used materials for hydrofluoric acid service are carbon and alloy steels, stainless steels, aluminium, copper and nickel and nickel alloys.

Nickel and its alloys

Nickel 200 is less resistant than Monel 400 to aqueous  hydrofluoric acid. Oxygen has higher accelerating effect on corrosion of Nickel 200. In aqueous hydrofluoric acid, use of Nickel 200 is limited to air-free systems below 80oC. However there are cases of stress corrosion cracking of Nickel 200 in aqueous hydrofluoric acid, they seem to be associated to contaminants like cupric fluoride. Nickel 200 is highly resistant to hot anhydrous fluoride however it may be embrittled by sulfur compound contaminants.

Monel 400: Alloy 400 is widely used in hydrofluoric acid alkylation units and in the production of hydrofluoric acid. It has supreme resistance to liquid hydrofluoric acid over the whole concentration range in the absence of oxygen to minimum 150oC.

Monel 400 resists stress corrosion cracking when subjected to wet vapors of hydrofluoric acid in the presence of oxygen. Intergranular cracking occurs. In one test, alloy 400 received transgranular stress corrosion cracking in the vapour phase of dilute hydrofluoric acid solutions at temperatures up to 95oC. The cracking sensitivity was not based on the presence of oxygen and no cracking was noticed in the liquid phase.

The cracking mechanism was unknown until it was noticed that aqueous hydrofluoric acid solutions comprising significant concentrations of cupric chloride would result in fast cracking of stressed Monel 400. Nominally resistant nickel-copper composition corresponds to that Monel 400.

Stress corrosion cracking is often limited to the vapour phase rather than liquid is enrichment of a thin layer of aqueous hydrofluoric acid in the vapour with copper fluoride corrosion products. The availability of oxygen speeds up corrosion and develops CuF2 from CuF. The much greater dilution of corrosion products prevents reaching a critical concentration of CuF2 in the liquid phase.

Inconel 600: Alloy 600 resists corrosion in dilute aqueous hydrofluoric acid at ambient temperatures and anhydrous hydrogen fluoride. It is used in valves and other systems replacing alloy 400 to prevent the feasible stress corrosion cracking. It is commonly used for hot hydrofluoric acid vapors, preventing chemical resistance and providing good metallurgical stability.

High performance Nickel alloys: Hastelloy alloys like grade C276, B2 and G grades as well as Inconel 625 offer supreme resistance to aqueous and anhydrous hydrofluoric acid and to high temperature hydrofluoric acid vapors.