Commercially pure or low-alloy nickel has characteristics that are useful in several fields, notably chemical
processing and electronics. Nickel is highly resistant to various reducing chemicals and is unexcelled in resistance to caustic alkalis. Compared with nickel alloys, commercially pure nickel has high electrical and thermal conductivity.Nickel 200Nickel 200 is commercially pure (99.6%) wrought nickel. It has good mechanical properties and excellent resistance to many corrosive environments. Other useful features: high thermal and electrical conductivities, low gas content and low vapor pressure. The corrosion resistance of Nickel 200 makes it
particularly useful for maintaining product purity in the handling of foods, synthetic fibers, and caustic alkali’s; and also in structural applications where resistance to corrosion is a prime consideration.
Nickel 200 is normally limited to service at temperatures below 315°C. At higher temperatures Nickel 200 products can suffer from graphitization, which can result in, severely compromised properties. For service above 315°C, Nickel 201 is preferred.
The resistance of Nickel 200 to corrosion by distilled and natural waters is excellent. Nickel 200 effectively resists water containing hydrogen sulphide or carbon dioxide. Nickel 200 gives excellent service in seawater.
Nickel 200 can be used with Sulphuric acid at low or moderate temperatures.
According to the data available, Nickel 200 may be used in hydrochloric acid in concentrations up to 30%, at room temperature. Increasing temperature will accelerate corrosion. If oxidizing salts are present in any but very small amounts, corrosion will be increased. At less than 0.5% concentration, the material can be used satisfactorily up to 150°-205°C.
Nickel 200 has excellent resistance to anhydrous hydrofluoric acid even at elevated temperatures. In aqueous solutions, however, service is usually limited to below 80°C. Even at room temperature, 60-65%
commercial-grade acid has been found to severely corrode Nickel 200.
Nickel 200 has limited usefulness in commercial phosphoric acid solutions because they usually contain
impurities such as fluorides and ferric salts that accelerate corrosion. In hot or concentrated solutions, rates are usually too high for reasonable service life.
Nickel 200 should be used in nitric acid only in solutions of up to 0.5% concentration at room temperature.
In general, Nickel 200 has excellent resistance to organic acids of all concentrations.
The outstanding corrosion resistance characteristic of Nickel 200 is its resistance to caustic soda and other
alkali’s. (Ammonium hydroxide is an exception. Nickel 200 is not attacked by anhydrous ammonia or ammonium hydroxide in concentrations of 1%. Stronger concentrations can cause rapid attack.)
In caustic soda, Nickel 200 has excellent resistance to all concentrations up to and including the molten state. Below 50%, rates are negligible, even in boiling solutions. As concentration and temperature increase, corrosion rates increase very slowly. The chief factor contributing to the outstanding performance of Nickel 200 in highly concentrated caustic soda is a black protective film that forms during exposure.
This film – nickel oxide – results in a marked decrease in corrosion rates over long exposure under most conditions. Because the presence of chlorates in caustic increases corrosion rates significantly, every effort should be made to remove as much of them as possible.
The metal is not subject to stress-corrosion cracking in any of the chloride salts and has excellent resistance to all of the non-oxidizing halides. Oxidizing acid chlorides such as ferric, cupric and mercuric are very corrosive and should be used with alloy 200 only in low concentrations. Stannic chloride is less strongly oxidizing, and dilute solutions at atmospheric temperature are resisted. The maximum safe limit for use of Nickel 200 in oxidizing alkaline chlorides is 500 ppm available chlorine for continuous exposure. In bleaching, sodium silicate (1.4 specific gravity) can be used as an inhibitor to corrosion; as little as 0.5
ml/liter of bleach has been found to be effective. Some very reactive and corrosive chlorides – phosphorus oxychloride, phosphorus trichloride, nitrosyl chloride, benzyl chloride and benzoyl chloride – are commonly contained in Nickel 200.
It has excellent resistance to neutral and alkaline salt solutions.
Fluorine and chlorine
Although fluorine and chlorine are strong oxidizers that react with metal, Nickel 200 can be used successfully in such environments under certain conditions. At room temperature, Nickel 200 forms a protective fluoride film and is considered satisfactory for handling fluorine at low temperatures. Nickel 200 effectively resists dry chlorine at low temperatures. Hydrogen chloride (formed from hydrogen and chlorine) when dry behaves similarly toward the metal. In wet chlorine at low temperature or wet hydrogen chloride at temperatures below the dew point, Nickel 200’s performance is somewhat as in hydrochloric acid.