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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
£¨1£© 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. However, too thin water film is not enough to make the electrochemical corrosion of metal surface smooth. In this case, it is difficult to form effective ion transfer. When the relative humidity in the air reaches a certain height, so that a certain thickness of water film can be formed on the metal surface, the electrochemical corrosion rate will suddenly increase, and the relative humidity at this time is the critical relative humidity for a certain metal. The critical relative humidity of steel is 70%.
The relationship between water film thickness and metal corrosion rate is shown in the figure. When the water film on the surface of metal is very thin, which is equivalent to area I, the corrosion rate is very small. When the water film thickness is 100 ¦Ì m, the corrosion rate increases suddenly. When the water film thickness is about 1 um, the metal corrosion rate reaches the maximum (equivalent to area I). After that, the water film will continue to thicken. When the water film thickness exceeds 1 um, the corrosion rate decreases slightly (equivalent to Area III in the figure). When the water film thickness exceeds 1 um, it is equivalent to that the metal is completely immersed in water. Oxygen in the air dissolves into the water film and diffuses to the metal surface, which makes it difficult to supply oxygen for electrochemical corrosion, and the corrosion rate does not change with the thickness of water film (equivalent to Fig The corrosion rate was thus slowed down.
£¨2£© 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.
However, under special circumstances, it is difficult for oxygen to reach all parts of the metal surface, which will form a concentration difference battery. For example, on the overlapped metal surface, that is, when the metal surface is in close contact with another metal surface, although the edge of the contact surface is not rusted, there will be cloud like shadow or rust away from the edge, and the corrosion product is often gray or black. This is because the oxygen supply on the edge is easy to form a cathode, and it is difficult to supply oxygen at the depth, so it becomes an anode and causes corrosion. The corrosion product of steel products in this case is black iron oxide. This corrosion is called overlapping corrosion and is caused by the concentration difference of oxygen.
The corrosion caused by oxygen concentration difference can also be seen under the water drop of pure iron sheet, as shown in the figure. Due to the uneven aeration between the edge of the droplet and the central oxygen, the oxygen in the droplet center is not easy to reach, forming anode and corrosion. A little at the top of the cathode, then it stops at the edge. A ring of loose rust deposits can be seen between the edge and the center. This is caused by the concentration difference of oxygen, so it is called oxygen concentration corrosion or differential aeration corrosion.
£¨3£© 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 ¡æ. Although the solubility of oxygen in aqueous solution decreases with the increase of temperature, the corrosion rate of metal will be reduced, but the effect of oxygen solubility is not great in the range of atmospheric temperature. When the temperature drops sharply, the relative humidity increases greatly, even condensation, which promotes corrosion. For example, in areas or seasons with large temperature difference between day and night, the environment temperature drops greatly, and the metal surface is easy to condense water film and rust.
£¨4£© 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. The effects of various pollutants are described as follows:
(1) All kinds of corrosive pollutants, including various gases and salt particles, will dissolve in the droplets when water mist is formed in the air. Therefore, the water droplets or water film condensed on the metal surface are not pure water, even if their content in air is relatively small, the concentration of water soluble in water is still considerable. These corrosive active substances can play one or more of the following functions:
¢Ù All kinds of non-metallic oxides dissolve in water and become acids, which increase the concentration of H +.
¢Ú CI - destruction of passive film on metal surface.
¢Û Various electrolytes increase the conductivity of the thin water film.
¢Ü Reduce the critical relative humidity of metals.
Sulfur dioxide is one of the most harmful gases. The critical relative humidity of metals can be reduced from 70% to 50% when the atmosphere contains 0.01% S02. In humid atmosphere, a molecule of SO2 can make dozens of atoms of iron corrosion into oxide, and its reaction is the corrosion product ferric sulfate hydrolysis, then produce sulfuric acid, continue to promote corrosion. Sodium chloride will destroy the passive film on the metal surface. It and sulfuric acid remain on the metal surface, even if the metal surface is dry, it can also re absorb moisture and restore activity.
(2) Materials without chemical activity but with adsorption activity, such as carbon particles, have no corrosion activity. However, if SO2 is adsorbed, the concentration of SO2 on the metal surface will be greater than that in the air, thus accelerating metal corrosion, as shown in the figure.
(3) Non chemically active and non adsorptive substances, such as sand particles, form gaps in contact with the metal surface when they settle on the metal surface. In this gap, water is absorbed by capillary action, which promotes the formation of water film and keeps it. Oxygen concentration corrosion under water film is formed. The metal under the sand becomes an anode and is corroded, as shown in the figure.
The air pollution in the industrial area is the most serious, and the corrosion is more serious, followed by the ocean and the city, and the air pollution and corrosion in the countryside and mountain forest are relatively small.
2¡¢ Metal properties
£¨1£© 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. In metal products, the most common iron products are not corrosion resistant. At room temperature, the oxide film formed on its surface is loose and has no protective effect. On the contrary, the porous layer is more likely to accumulate water and reduce the critical relative humidity of steel, which is more conducive to the electrochemical corrosion.
£¨2£© 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. The microstructure of steel is heterogeneous. It has different phases, such as ferrite, cementite, and graphite phase in cast iron. These will form micro cells and cause chemical corrosion. After different heat treatment, the structure of steel is different, and the corrosion resistance is of course different. The residual stress of metal parts is uneven due to cold and hot processing. The parts with high stress are in high energy state, which is unstable, so it is easy to rust.
£¨3£© 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.
£¨4£© 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.
£¨5£© 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. If after pickling, neutralization and cleaning are not complete, a small amount of residual acid will remain on the surface of metal products, which will cause corrosion of the metal products. In the process of inspection, handling and assembly of metal products, the staff often contact with bare hands. The influence of sweat on hands often causes fingerprint corrosion on some metal products. This is because the sweat contains salt (sodium chloride) and organic acids.
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.