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Which alloy is best for use in hydrofluoric acid applications?

Inconel 600 for acid processing

Anhydrous hydrogen fluoride and hydrofluoric acid are widely used in the industrial applications. Hydrofluoric acid is used in pickling of stainless steels, acid processing wells, glass etching, development of aluminium fluoride, synthetic cryolite for aluminium development, production of fluorinated organics like aerosol propellants, special purpose solvents, plastics, refrigerants and development of fluorides and fluoborates.

Aqueous hydrofluoric acid and anhydrous fluoride are dangerous chemicals to skin, eyes and lungs. The fluoride salts added to potable waters avoid tooth loss, are harmful in higher concentrations. The persistent burns and usual deep-seated ulcerations occur due to interaction between aqueous hydrofluoric acid and skin, and Iintake of concentrated vapors result into lung damage.

Ineffective materials

The metals unfit for hydrofluoric acid applications are categorized into- velocity, hydrogen sensitivity, component sensitive and notch sensitive.

Hot Gas: The corrosion analysis performed described that Nickel, Monel alloy 400 and Inconel alloy 600 offer significant resistance to gaseous anhydrous hydrogen fluoride at temperature limits of 600oC. However the corrosion rates were overstated and limited by short periods of 3 – 15 hours, various data factors were used to rank the materials.

Inconel Alloy 600 offered excellent performance in the hydrofluorination of metallic oxides at temperature limits of 370 – 590oC. Alloy 200 and cupronickel alloys are attacked by intergranular embrittlement due to sulphur materials at temperatures above 300oC or 570oF. Inconel alloy 600 prevents embrittlement and its use is recommended at these limits in gas hydrofluoric media. It should be kept in mind that various nickel alloys such as Hastelloy C276 and Inconel 625 attain metallurgical variations like aging or wide ordering at temperature limits above 400 – 500oC. The hardness and sensitivity to hydrogen embrittlement also arise. On the other hand alloy 600 remains unaffected.

Application materials

Carbon and alloy steels: Hardened carbon and alloy steel attain hydrogen embrittlement and steel plate and pipelines experience blistering and cracking in aqueous or anhydrous hydrofluoric acid. Carbon steels offer significant corrosion resistance in 64-100% hydrofluoric acid.

Stainless Steels: The service of stainless steel in hydrofluoric acid media is limited. Austenitic steels offer supreme resistance to attack in liquid anhydrous hydrogen fluoride at high temperatures.

Aluminum: Aluminum alloys are not fit for use in hydrofluoric acid applications, its damaging action is constant and creates a bright appearance to the metal. Hydrofluoric acid is used as an etchant in the development of designs on aluminum.

Copper: Normally, copper is sensitive to aeration and velocity factors in hydrofluoric acid concentration. Cupronickel 70/30 alloy offers good performance in aqueous and concentrated acids.

Nickel and its alloys: Nickel alloy 200 has lower resistance as compare to alloy 400 in aqueous hydrofluoric acid media. The use of Nickel 200 is limited to air-free equipments at temperatures below 80oC. It offers the highest resistance to hot anhydrous hydrogen fluoride.

Monel 400: The Monel alloy 400 is used in hydrofluoric acid alkylation systems and in the development and handling of hydrofluoric acid. It offers supreme resistance to liquid hydrofluoric acid to 150oC.

Inconel 600: Inconel alloy 600 prevents attack in dilute aqueous hydrofluoric acid at ambient temperatures as well as anhydrous hydrogen fluoride. It resists stress corrosion cracking in valves and other components.