Factors affecting the corrosion of metal products

Factors affecting the corrosion of metal products

There are many factors affecting the corrosion of metal products, both the characteristic factors of the metal products themselves and the storage environment factors of the metal products. The main aspects are now described as follows.
First, the impact of the characteristics of metal products on corrosion
1. Related to the type of metal products
Generally, the metal with a negative electrode potential is more likely to rust in the atmosphere. For example, the standard electrode potentials of iron and copper are -0.44V and +0.33V, respectively. Obviously, the electrode potential of iron is lower than that of copper, so iron is more likely to rust than copper in the atmosphere.
2. Related to impurities in metal products and other metal components added
Generally, the commonly used metal materials and their products are not pure metals but alloys of various compositions, and various kinds of inhomogeneities exist in composition, organization, physical state, surface state, etc., which further increases corrosion. The possibility. At the same time, industrial metal materials contain a certain amount of impurities. For example, industrial zinc contains iron. The influence of impurities on the corrosion of metals is not the same. Pure metals are relatively stable in the atmosphere or in the electrolyte, but as long as a small amount of impurities are present, the corrosion rate can be increased by several hundred times or even thousands of times. Different impurities have different effects on the same metal product.
If a certain amount of other metal elements are added to a certain metal, the electrode potential of the base metal can be changed. For example, when chromium is added to iron to reach a weight ratio of 11.7%, the electrode potential of the alloy will be increased to +0.2 V, and the rust of air, water vapor and other acid-base salts can be effectively resisted. In short, when there is a component or impurity in the metal product higher than the metal itself, it is easy to accelerate the corrosion of the product; if an element that is easily passivated (such as aluminum, chromium, silicon, etc. is added to the steel), the metal can be improved. Corrosion resistance.
3. Related to the plating of the surface of metal products
Some steel products are often plated with a protective metal coating on the surface to prevent rust. There are basically two types of metal plating: one is an anodic coating, that is, the electrode potential of the plated metal is negative compared to iron (such as galvanizing). The other is a cathode plating, that is, the electrode potential of the plated metal is positive compared to iron (such as nickel plating, copper, etc.). The protection of these two coatings is not exactly the same. Because there is pore or partial damage on the zinc layer of the anode coating, the steel still has a protective effect on the steel, because when rust occurs, zinc is the anode and iron is the cathode, and the result is rusted by zinc, and the iron is subjected to protection. The electrode potential of the cathode plating metal is higher than that of iron, so the result of rusting when the plating layer has pores or partial damage is that the base metal steel is rusted. Therefore, strictly speaking, the cathode coating can prevent steel corrosion only if there is no pores and the coating is not damaged and remains intact. Therefore, the rust-proof work of the coated metal product cannot be ignored during storage.
4. Related to the state of metal products
The non-uniformity of the physical state of metal products is also a factor affecting electrochemical corrosion. Metals often cause deformation non-uniformity and internal stress non-uniformity in various parts of the metal during machining. Generally, the most deformed part is the cathode. For example, in the bending of the iron plate and the rust of the rivet head, it is caused by this reason. In addition, it has been proved by experience that the stressed part is often an anode and is most susceptible to rust.
Metal surface conditions can also have an effect on electrochemical rust. The metal surface film is incomplete and has pores, so the metal surface potential under the pores is lower, which becomes the anode of the microbattery. In most cases, surface roughing parts are more rusted than surface finished parts. The effect of surface roughness on rust is much more pronounced in the atmosphere than in the electrolyte, mainly due to the large adsorption energy of the rough surface, the easy adsorption of moisture and dust, and the formation of oxygen-concentrated cells.
Some metals, such as aluminum, lead, copper, tin, antimony, etc., can form a dense, stable and stable protective film on the surface in the atmosphere, so that the metal does not continue to rust. The rust products generated by steel on the surface of the atmosphere are loose, have a porous structure, have no protective effect, and also have capillary adsorption, so steel is easily rusted in the atmosphere. The corrosion products of some metals are hygroscopic. For example, copper sulfate may be formed on the surface of copper in the atmosphere containing sulfur dioxide, and ferrous sulfate may be formed on the surface of iron. In the atmosphere containing chloride ions, metal chloride may be formed on the surface of the metal. These products The moisture absorption point is lower and thus more susceptible to rust.
Second, the impact of storage environment factors on the corrosion of metal products
The storage environment factor is the determinant of whether the product can be rusted during storage, and is therefore the main controlling factor for preventing corrosion of metal products in storage. The so-called environmental factors refer to the air temperature and humidity of the storage environment, as well as harmful gases and impurities in the air, such as carbon dioxide, sulfur dioxide, chloride, etc., as well as acids, alkalis, salts and other substances in contact with metal products are environmental factors.
1, the humidity of the air
Corrosion of metal products during storage is mainly due to moisture and rust. Moist atmospheric rust is an electrochemical process that takes place under a water film formed on the surface of a metal product. The speed of this process has a certain relationship with the thickness of the water film, as shown in Figure 6-3.

In the figure, the area I (water film thickness = 1 to 10 nm) corresponds to the initial stage of adsorbing moisture. The metal surface has only a very thin adsorption film (only a few to several tens of molecular layers), which is a dry atmospheric corrosion. The corrosion rate is the smallest. Region II (water film thickness = 10 to 100 nm) corresponds to the rust (10 to 100 molecular layers) of the invisible water film on the metal surface. From the chemical history to the electrification process, the corrosion rate increases rapidly. In the area III (water film thickness = 100 ~ 1000nm), there is a visible water film on the metal surface, but as the thickness of the water film increases, it is difficult to diffuse oxygen through the water film to the metal surface (cathode control becomes the main factor) Therefore, the rust rate begins to decline. When the thickness of the water film is increased to the area IV (water film thickness > 1 mm), it is equivalent to the rust of the metal fully immersed in water.
Metal products in the air, the thickness of the surface water film is directly related to the relative humidity of the air. Only when the relative humidity of the air exceeds the critical humidity, the water film formed on the metal surface can meet the needs of the rust electrochemical process, so that the rusting speed is obviously accelerated, and the higher the humidity, the faster the rusting speed.
Generally, the critical humidity of metal corrosion is about 70%. The surface of the metal product is rough, the structure is complex, and the surface is adsorbed with salts, dust and dust, and the like, which can reduce the critical humidity of the rust. Figure 6-4 is a plot of the corrosion rate of copper in air containing sulfur dioxide versus air humidity.

Under the condition of exceeding the critical humidity, the electrochemical process of metal rust is accelerated with the increase of air humidity, which is related to the polarization of rusted cells. When the relative humidity is low, the degree of anodic polarization is higher. As the relative humidity increases, the degree of anodic polarization becomes smaller and the rusting speed becomes larger and larger, as shown in Figure 6-5: 1 and 5 are relative. The humidity is 100%; the relative humidity of 2 and 4 is 75%; and the relative humidity is 50%. When the relative humidity of the air increases from 50% to 100%, the anodic polarizability of zinc is reduced by a factor of 1000. The degree of anodic polarization decreases with the thinning of the water film under the visible water film; under the invisible water film, it decreases with the thickening of the water film. That is to say, the air humidity is increased, the degree of cathodic polarization is reduced, and the rusting speed is accelerated. The relationship between steel corrosion rate and air humidity is a logarithmic relationship and can be expressed as follows:
Vk=V0e-(h0-h)
Where: V-corrosion rate at this humidity
V0-corrosion rate under saturated humidity
h - actual humidity at this temperature
H0 - saturated humidity at this temperature
The formula shows that when the relative humidity is low, the rusting rate does not increase rapidly with the increase of relative humidity. However, when the relative humidity is close to the saturated humidity, the rusting speed increases quite rapidly. It can be seen that the relative humidity is an important factor affecting the corrosion of stored metal products. As long as the relative humidity of the storage environment is controlled below the critical humidity of the corrosion of metal products, the occurrence of corrosion can be effectively prevented.

2, the temperature of the air
The effect of temperature on corrosion is not isolated and is also affected by relative humidity. On the one hand, the increase of the ambient temperature will accelerate the corrosion rate of the metal products, because the thermal energy can accelerate the chemical reaction, and the temperature increase can reduce the polarization of the cathode, but the effect is only when the temperature is high and the temperature The rise does not occur when the water film on the surface of the metal product is dry. The effect of temperature in atmospheric corrosion can be seen from the velocity formula of atmospheric corrosion:
H-65
V = ----- × (1.054)t
10
Where: V--the rate of atmospheric corrosion
H--air relative humidity (%)
T--air temperature
When the relative humidity exceeds 65%, the increase in air temperature promotes the rust rate; however, when the temperature rises to 80 °C, the rust is inhibited due to the significant decrease in the solubility of oxygen in the water film.
In addition, sudden changes in temperature have a greater impact on the corrosion of metal products. When the temperature suddenly drops, dew condensation may occur on the surface of the metal product in the case of a large absolute humidity. When the metal products with lower temperature are moved into the environment with higher temperature, such as the metal products transported in winter, the temperature of the metal products often differs greatly from the temperature inside the library, and condensation is likely to occur after storage. Condensation will seriously accelerate the corrosion of metal products, so it must be noted.
Figure 6-6 shows the temperature difference that may occur when the relative humidity is different at a certain temperature. It can be seen from the figure that the relative humidity of the air temperature in the range of 5 to 50 ° C reaches 65% to 75%, and when the temperature suddenly drops by 6 ° C, condensation may occur. The greater the temperature change, the lower the relative humidity at which condensation may occur. The temperature difference between day and night in all regions of China exceeds 6 °C, and the temperature difference between day and night in some areas can reach above 15 °C. In this case, condensation may occur even if the relative humidity of the air is low. Therefore, the temperature inside the storage metal article should be kept relatively stable. Avoiding condensation is practical for rust prevention.

3. The role of oxygen in the air
Oxygen, like water, is a necessary factor for the corrosion of metals in the atmosphere. Without one of them, the metal will not be rusted. On the one hand, oxygen can dissolve and penetrate the water film on the metal surface. Taking iron as an example, the following corrosion reaction will occur on the metal surface:
O2 + H2O + 2e → 2OH-
Fe2+ ​​+ 2OH- → Fe(OH)2
2Fe(OH)2 + O2 + H2O → 2Fe(OH)3
On the other hand, in the process of atmospheric corrosion, the cathodic depolarization is mainly the depolarization of oxygen. That is, oxygen is the main depolarizer. The cathodic depolarizer continuously removes electrons from the cathode, allowing the cathode to be continuously subjected to electrons given by the anode, allowing the cathode process to continue and the metal to be continuously dissolved. Therefore, in the process of atmospheric corrosion, oxygen and water are important factors to promote corrosion.
4. Harmful gases and impurities in the air
In the air, it usually contains various industrial harmful gases and rusting impurities such as dust, which have a great influence on the corrosion of metal products.
(1) sulfur dioxide
Sulfur dioxide is the most harmful gas in the air pollutants that has the greatest impact on the corrosion of metal products.
In the air exposure test, the corrosion rate of metals such as steel, copper and zinc is approximately proportional to the sulfur dioxide content in the air. Figure 6-7 shows the effect of sulfur dioxide content on the corrosion rate of steel. The effect of sulfur dioxide in air on aluminum is special. The effect in dry air is small, but when the humidity is high (such as relative humidity above 98%) As long as the air contains a small amount of sulfur dioxide (0.01%), the corrosion rate will rise sharply. If the concentration of sulfur dioxide increases to 0.1%, the corrosion rate of aluminum can increase by 4 to 10 times; when the concentration of sulfur dioxide increases to 1%, it will corrode. The increase in speed slows down again. The sulphur dioxide in the air is generally more than 0.15%, and many of the more corrosion-resistant metal products are severely rusted in this environment.

The effect of sulfur dioxide on the corrosion of metal products is mainly due to the dissolution of sulfur dioxide in the water film on the surface of the metal to form sulfurous acid, which enhances the electrochemical action of corrosion. The acid can dissolve a plurality of oxide films having a protective surface on the metal surface, making the electrochemical process of the rust corrosion easier; and the acid hydrogen ions can accelerate the cathode process. Although the content of sulfur dioxide in the air is much smaller than the content of oxygen, since the solubility of sulfur dioxide in the electrolyte is more than 1000 times that of oxygen, when the sulfur dioxide content in the air is 0.015%, the concentration of sulfur dioxide in the electrolyte is equal to oxygen. concentration.
(2) Hydrogen sulfide
Hydrogen sulfide in dry air can only cause discoloration of some metal surfaces (copper, brass, silver and iron are obvious), but in the humid atmosphere, the promotion of copper and nickel, especially iron and magnesium corrosion, due to acidification of the liquid film Larger, and may cause corrosion of stainless steel.
(3) chloride
Both hydrogen chloride and chlorine in the industrial atmosphere have strong rusting effects on metal products because they dissolve in the water film to form hydrochloric acid. The ocean atmosphere often contains a certain amount of salt particles, which can promote the corrosion of steel and copper, aluminum and the like. The role of salt is mainly the role of chloride ions. Because of the small volume of chloride ions, it can penetrate the protective film on the metal surface, and the chloride ions are easily adsorbed on the metal oxide film, replacing the oxygen in the metal oxide film to form the medium soluble chloride. . The effect of the amount of salt in hydrogen chloride gas and air on metal corrosion is shown in Figures 6-8 and 6-9.
The composition of air and the nature of the pollutants are related to the area. Therefore, according to the influence of metal corrosion, the air is divided into industrial atmosphere, urban atmosphere, ocean atmosphere and rural atmosphere. Among them, the industrial atmosphere contains more harmful gases such as sulfur dioxide and hydrogen chloride, which has the greatest influence on metal corrosion. The marine atmosphere contains salt particles and has a high humidity, which promotes the corrosion of metal. The salt content in the ocean atmosphere It decreases with the increase of the distance from the coast, so the metal corrosion rate also decreases with the increase of the distance between the storage place and the coast. The rural atmosphere has the least impact.
In addition to the above factors affecting the corrosion of stored metal products, there are other factors, such as packaging materials, especially packaging materials that are in direct contact with metal products, which have a certain influence on the corrosion of metal, and some of the components of packaging paper. Contains considerable amounts of chloride and acid, and some also contain reducing sulfur. The agglomeration of the capillary of the wrapper also reduces the critical relative humidity of the rust of the metal product. In addition, some microorganisms promote the corrosion of metals. In wet conditions, iron bacteria can grow on steel and promote corrosion of steel. A small number of molds, such as Aspergillus oryzae, can promote the corrosion of aluminum. Under humid and hot conditions, instruments and meters often cause serious corrosion due to the proliferation of microorganisms (mainly molds). The rust of microbes on metal commodities is mainly due to the action of microbial metabolic products and the effects of sediments.