Comparison of performance of Alloys 400 and C2000 in HF acid
Hydrofluoric acid is water based hydrogen fluoride solution that is commonly used in different types of industrial applications commonly in the pickling solutions in metal industry, production of chlorofluorocarbon compounds, alkylation agent for gasoline and etching in glass industry. Hydrofluoric acid has wide applications in the production of semiconductors and microelectronics during wet chemical cleaning of silicon wafers. This acid is also used as a reducing agent and however it is chemically found to be weaker than sulfuric and hydrochloric acids though it is severely corrosive. Hydrofluric acid is specifically toxic and causes health risks.
The corrosion by this acid does not target to uniform thinning although to localized penetration under the surface of metal in the form of thin pits, and intergranualr corrosion.
The selection of application materials to use with hydrofluoric acid is not much. The conventionally used materials for example glass, titanium and tantalum are quickly corroded by this acid. Hydrofluoric acid dissolves most oxides and hence any alloy that is found to be immune by passive oxide layer for resistance to corrosion is attacked while using hydrofluoric acid.
Hydrofluoric acid when used in concentrations above 64% can be used with carbon steel, although at lower concentrations, the acid corrosion of steel occurs quickly. Steels also experience embrittlement in presence of hydrofluoric acid. As nickel based super alloys offer suitable performance over a wide range of acid concentrations and temperatures, they are a suitable choice for use with wet hydrofloruic acid. For instance, it has been found that in solutions of mixtures of HF and HNO3, Hastelloy G30 offers excellent service. A commonly used Nickel –Copper alloy Monel 400 although is an excellent material but it is corroded in the presence of air or oxidizing salts. Hastelloy N06200 is also known for offering suitable performance in hydrofluoric acid. It is found that these nickel alloys offer corrosion resistance in wet HF acid is by virtue of the development of insoluble fluoride salt layer on their surface.
The recommended alloys for use in hydrofluoric acid are Hastelloy bar C2000 and Monel 400. Their performance is evaluated in exposure for 240 hours in vapor phase of 20% hydrofluoric acid at 79oC and 93oC. Monel 400 is found to be attacked with the increase in test duration. In the vapor phase, its corrosion rate was high and independent of time. When the acid concentration is increased from 1 – 20%, the corrosion rate of both alloys increases, although it was more noticeable in Monel 400 particularly in vapor phase. Both alloys show similar levels of corrosion at 38oC and 52oC although with the increase in temperature to 79oC and 93oC, Hastelloy C2000 is found to be more suitable for use in both liquid and vapor phases. On the other hand, Monel 400 was corroded at all temperatures in vapor phases becoming unsuitable for use in vapor phase of hydrofluoric acid.