Corrosion, including metal and nonmetal, is a process of loss and destruction under the action of surrounding media (water, air, acid, alkali, salt, solvent, etc.). Metal corrosion refers to the destruction of metals under the chemical or electrochemical action of surrounding media, and often under the joint action of physical, mechanical or biological factors. The influencing factors of metal corrosion are as follows

1 .Environmental factors

A.  Humidity
The humidity that affects metal corrosion is actually relative humidity. Because the relative humidity of air is related to the formation of water film on metal and the thickness of water film formed. In clean air, when the relative humidity reaches 100%, water molecules will condense on the metal surface and become water droplets and water films. However, due to the adsorption of water on the metal surface and the irregularity of the metal surface, when the relative humidity is relatively low, a certain amount of water has been adsorbed on the metal surface to form a water film. 

B. Oxygen
The main corrosion process in neutral medium is oxygen depolarization. In the reaction formula O2 (cathode area) = > O2 + 2H2O + 4E -- > 40H -, oxygen is the main electron acceptor in the cathode. Therefore, without oxygen, metal corrosion will not occur. Some data show that in the deoxidized sea water, the iron nail will not lose its luster after soaking for decades. There is a lot of oxygen in the atmosphere, and the water film adsorbed on the metal surface is very thin, which makes it easy for oxygen to dissolve and diffuse to the cathode area of the metal surface. Oxygen plays an important role in metal corrosion.

C. Temperature
The rate of general chemical reaction increases with the increase of temperature. In atmospheric corrosion of metals, when the relative humidity is above the critical state, the reaction rate increases with the increase of temperature. Similar to the general chemical reaction, the corrosion rate doubled when the temperature increased by 10 ℃. 

D. Air pollutants
In addition to air and water vapor, the atmosphere also contains a variety of pollution impurities. The impurities of gas include SO2, nitrogen oxides, CO2, HCl, etc. The ocean atmosphere contains salty particles. In industrial areas, the amount of solid dust falling per square kilometer per month amounts to tens of tons. These dusts include corrosive and noncorrosive types. Although non corrosive dust can not directly corrode metal, it can promote metal corrosion indirectly. 

2.Metal properties

A. Types of metals
Different types of metals have different corrosion trends. The more expensive metals are corrosion resistant because of their thermodynamic stability. There are not many such metals as gold, silver, platinum, iridium, etc. they exist in nature in the state of elements and do not deteriorate due to the influence of atmospheric environmental factors. Aluminum, chromium, titanium and other metals in natural conditions, due to the role of atmospheric oxygen, the surface can quickly form a stable passive film and corrosion resistance. 

B. Factors in steel
The corrosion resistance of steel can be obtained when the content of corrosion resistant alloy elements reaches a certain value. If the chromium content of various stainless steels exceeds 13%, a stable passive film can be obtained on the surface of the steel, which has good corrosion resistance in the atmosphere. When more than 2% copper is contained in copper steel, a protective rust layer can be formed on the surface of copper steel, which shows certain corrosion resistance. 

C. Contact of dissimilar metals
The contact of dissimilar metals promotes the corrosion of active metals. For example, for copper steel assemblies, the corrosion of steel is promoted. When zinc contacts with steel, the steel is protected, but the corrosion of zinc is accelerated. At this time, zinc becomes the so-called sacrificial anode. When dissimilar metals contact with each other, if the surface area of the metal used as cathode is large and that of the metal used as anode is small, the corrosion is more serious.

D. Metal surface state
The metal surface is rough, easy to absorb moisture and form water film, and easy to accumulate dust, so it is easier to be corroded than the metal surface is smooth. Metal products with complex shape, concave, gap, groove, small hole, especially blind hole can significantly reduce the vapor pressure of water film, thus reducing the critical relative humidity of forming water film, so it is easier to promote the electrochemical corrosion of metal.

E. Process factors
Metal products start from raw materials, through a variety of processing into products. There are a variety of hot and cold J processing, assembly and management of the various processes, such as inspection, handling, etc. Contact with all kinds of personnel, processing, handling machinery and appliances, these factors are related to the corrosion of metal products. Sometimes the raw materials have been seriously rusted, and the corroded parts can not be completely removed during the processing, resulting in the corrosion of the finished products. Process materials used in processing, if not completely removed after processing, will also cause corrosion of metal products. 

3.Human factors
Due to some negligence in management, corrosion can also be caused. For example, dirt mixed in the working fluid, unclean handling equipment, long turnover period of workpieces, poor anti rust packaging management of semi-finished products warehouse, and unclean workplace will all have an impact on metal corrosion.