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Main factors causing metal rust

Update:12 Jan 2019
Summary:

1) Atmospheric relative humidity At the same temperatur […]

1) Atmospheric relative humidity

At the same temperature, the percentage of the water vapor content of the atmosphere to its saturated water vapor content is called relative humidity. Below a certain relative humidity, the metal rust rate is small, and above this relative humidity, the rust rate increases sharply. This relative humidity is called critical humidity. The critical humidity of many metals is between 50% and 80%, and that of steel is about 75%. Atmospheric relative humidity has the greatest impact on metal corrosion. When the atmospheric humidity is higher than the critical humidity, a water film or water droplets appear on the metal surface. If the harmful impurities contained in the atmosphere dissolve the soil water film and water droplets, the electrolyte becomes an electrolyte, which increases corrosion.

2) Temperature and humidity

The relationship between atmospheric temperature and humidity affects metal corrosion. This has the following main conditions: First, the water vapor content of the atmosphere increases with the increase of temperature; second, the high temperature causes the corrosion to increase, especially in humid environments, the higher the temperature, the faster the corrosion rate. When the relative humidity is low, the effect of temperature on corrosion is not obvious, but when it is higher than the critical humidity, the amount of corrosion increases sharply with the increase of temperature. In addition, if there is a temperature difference between the atmosphere and the metal, condensed water is formed on the surface of the metal having a low temperature, and the metal is also rusted.

3) Corrosive gases

Corrosive gases in polluted air have the greatest impact on corrosion of metals with sulfur dioxide, especially for steel, copper and their alloys. The sulphur dioxide in the atmosphere is mainly derived from the combustion of coal. At the same time, the combustion product carbon dioxide also has a corrosive effect. Corrosive gases are mixed in the atmosphere around the plant. Such as hydrogen sulfide, ammonia gas, hydrochloric acid gas, etc. are all factors that promote metal corrosion.

In addition, if there is a temperature difference between the atmosphere and the metal, condensed water is formed on the surface of the metal having a low temperature, and the hardware tool is also rusted.

4) Other factors

The atmosphere contains a lot of dust, such as smoke, coal ash, chloride and other acids, alkalis, salt particles, etc., some of which are corrosive, or condensation of water droplets, but also corrosion factors of hardware tools, such as chloride It is considered to be the ''dead enemy' of corrosive metals.