What are the properties and application characteristics of nickel and nickel alloys

       Nickel is a metal with excellent corrosion resistance and good toughness. Nickel is basically not oxidized in the air below 500 degrees Celsius, and even if it is heated at 1000 degrees Celsius, the degree of oxidation is not serious. However, oxides are also produced when heated for a long time. Nickel has a certain hardness and excellent corrosion resistance. Nickel wire is generally used to prepare erosion, pump plunger shafts or decorative coatings. Nickel is also used for surfacing of mechanical parts. Pure nickel coating is mainly used for decoration. When other functional coatings are required, nickel-chromium alloys and Monel alloys are mainly used.

    1) Nickel-chromium alloy coating can be used as a heat-resistant, high-temperature oxidation-resistant and corrosion-resistant coating. A typical nickel-chromium alloy is 80% nickel and 20% chromium, but there are also 60% nickel, 16% chromium and the rest iron. Among them, 80Ni20Cr alloy is a commonly used material for thermal spraying. This alloy has good high temperature oxidation resistance, acid and alkali corrosion resistance, and is a typical material for preparing heat-resistant and corrosion-resistant coatings. Because the coating is dense and has good adhesion with the base material, it is usually used as the bonding bottom layer of the heat-resistant ceramic coating, which can not only increase the bonding strength of the coating, but also prevent the corrosion of the metal by high-temperature oxidation and corrosive gases. However, the alloy is not resistant to corrosion by hydrogen sulfide, sulfurous acid gas, salts and reducing gases under high temperature and humidity, and is also easily corroded in nitric acid and hydrochloric acid solutions. It can be widely used in boiler water-cooled tube wall (including water-cooled tube wall in heavy oil waste heat boiler and coal-fired boiler water-cooled tube wall) and heat exchanger tube wall to slow down the corrosion and erosion of boiler tube wall. For example, the 45CT nickel-chromium public alloy wire designed by the American TAFA company for spraying the protective coating of the boiler water wall can protect the boiler pipes and prolong its service life.

    Coatings can be prepared using arc spray and plasma spray processes.

    2) Nickel aluminum alloy

    The Ni-AI alloy wire used for arc spraying has a mass ratio of Ni:AI=95:5, which is similar to the nickel-clad aluminum self-bonding composite powder. Ni-AI full gold wire can replace Ni-AI composite wire in many aspects, and its metallurgical bonding performance is better than Ni-AI composite wire. When Ni-AI alloy wire is used for arc spraying, the arc strongly overheats the droplet, and the oxidation reaction of nickel and aluminum (rather than the formation of nickel aluminide) releases a lot of heat, which makes the droplet bond firmly between the impacted workpieces. .

    The nickel-aluminum alloy coating has good resistance to high temperature oxidation. Also has good corrosion resistance. The coating is resistant to attack by molten metal and has excellent corrosion resistance in red and white fuming nitric acid. However, the corrosion resistance of this coating in salt solutions is poor.

    The bonding strength of Ni-AI alloy coating sprayed with air is better than spraying with nitrogen or other inert gas. When sprayed with air, the bonding strength of the coating is very high, whether it is a sandblasted roughened substrate surface or a smooth substrate surface.

    3) Monel alloy

    Monel is a nickel-copper alloy with excellent corrosion resistance, containing 63%~70% nickel, 1.0%~3.0% iron, and the rest is copper. Its structure is a uniform nickel-based solid solution, with a typical macro hardness (HRB) of 50. The alloy not only has excellent high temperature strength, but also has almost unchanged mechanical properties when working below 400 degrees Celsius. It also has excellent corrosion resistance, it is stable in the atmosphere, steam, soil, fresh water, and flowing seawater, especially the corrosion of seawater and dilute sulfuric acid. Monel alloy also has good corrosion resistance in non-oxidizing acids (sulfuric acid, hydrochloric acid, phosphoric acid, hydrofluoric acid) and salt and alkali aqueous solutions. However, the corrosion resistance of sulfurous acid gas is low. Monel alloy coating is dense, uniform and has good machinability. It is mainly used for corrosion-resistant coatings of mechanical parts used in corrosive environments and to protect chemical equipment working in corrosive media such as sulfuric acid, hydrochloric acid, acetic acid, and phosphoric acid with low concentrations.

    The coating can be prepared by arc spraying and oxygen gas flame spraying.