Oxidation of Tungsten Disulfide

Oxidation of tungsten disulfide refers to the process in which tungsten disulfide reacts with oxygen to produce sulfur dioxide and tungsten trioxide when heated in air. As one product of the oxidation, tungsten trioxide can form a protective film during the lubrication process so as to reduce the wear of the metal surface.

Oxidation is a chemical reaction between a substance and oxygen. Slow reaction of substance and oxygen is called eremacausis while the intense oxidation is called burning.

When heated in air, tungsten disulfide reacts with oxygen to form sulfur dioxide and tungsten trioxide. Its initial rate of slow oxidation is 460°C, and the initial rate of fast oxidation is 802°C. At 427°C, it starts to lose sulfur. The oxidation of tungsten disulfide is affected by temperature and particle size. The higher temperature and the smaller tungsten disulfide particles lead to the more intense reaction.

There are two products of the oxidation of tungsten disulfide:

1.Sulfur dioxide

Sulfur dioxide (SO₂) is colorless transparent gas with a pungent smell. It is soluble in water, ethanol and ether. It has a property of bleaching and is often used to bleach pulp, wool as well as silk. As it can inhibit the growth of mold and bacteria, it is usually used as a preservative.

However, sulfur dioxide is one of the major pollutants in the atmosphere. It is produced by the burning of traditional fuels such as coal and oil. With the dissolution and oxidation, it can form sulfuric acid (the main component of acid rain), doing harm to the environment.

2.Tungsten trioxide

Tungsten trioxide (WO₃) is yellow powder, being insoluble in water, being soluble in alkali and being slightly soluble in acid. It is used for making tungsten wires, fireproof materials and alloys with high melting points.

During the lubrication, tungsten trioxide is formed by the slow oxidation of tungsten disulfide under high temperature. As a protective film, tungsten trioxide can inhibit the further oxidation and prevent the fast loss of tungsten disulfide. At the same time, it can protect the surface of metal with its low friction coefficient.

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