There are many factors that affect the corrosion of metal products, both the characteristics of metal products itself, but also the impact of environmental factors of metal products. The main aspects are now described below.
First, the characteristics of metal products on the impact of corrosion 1, and the type of metal products in general, the greater the negative electrode potential of the metal in the atmosphere easier to rust. For example, the standard electrode potentials of iron and copper are −0.44 V and +0.33 V, respectively. Obviously, the electrode potential of iron is lower than that of copper. Therefore, iron is more likely to rust than copper in the atmosphere.
2. Commonly used metal materials and their products related to the impurities of metal products and other metal components are not pure metal but alloys of various components. There are various components in terms of composition, organization, physical state and surface state. All kinds of inhomogeneities increase the possibility of corrosion. At the same time, industrial metal materials contain a certain amount of impurities. Such as industrial zinc contains iron and so on. The effect of impurities in metals on corrosion is not completely the same. Pure metals are relatively stable in the atmosphere or in the electrolyte. However, as long as a small amount of impurities are present, the corrosion rate can increase several hundred times or even several thousand times. The influence of different impurities on the same metal product is also different.
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 a weight ratio of 11.7%, the electrode potential of the alloy will be increased to +0.2 V, and this will effectively resist the corrosion of air, water vapor and other acid-base salts. In short, when the metal product has a potential higher than the composition or impurity of the metal itself, it is easy to accelerate the corrosion of the product; if an easily passivated element (such as aluminum, chromium, silicon, etc. added to the steel) is added to the metal, the metal can be increased. The resistance to corrosion.
3. The coating on the surface of the metal product Some steel products are provided with a protective metal coating on the surface to prevent corrosion. There are basically two types of metal plating: one is anodic plating, in which the electrode potential of the plated metal is negative compared to iron (eg galvanized). The other is a cathodic coating in which the electrode potential of the plated metal is positive compared to iron (eg, nickel, copper, etc.). The protective effect of these two coatings is not exactly the same. Because there is porosity or local damage on the zinc layer of the anodic coating, there is still a protective effect on steel in the atmosphere because when rust occurs, zinc is the anode and iron is the cathode. As a result, zinc is rusted and iron is affected. protection. The electrode potential of the metal of the cathode coating is higher than that of iron, so that when the coating is porous or partially damaged, rust occurs and the base metal steel is rusted. Therefore, strictly speaking, the cathode coating can only prevent the steel from being rusted if there is no porosity and the coating remains intact without damage. Therefore, the corrosion-resistant work of the coated metal product cannot be ignored during storage.
4. The inhomogeneities in the physical state of metal products related to the state of metal products are also factors that affect electrochemical corrosion. Metals often cause non-uniformity of the deformation of each part of the metal and the non-uniformity of the internal stress during the machining process. Under normal circumstances, the part that deforms greatly is the cathode. For example, the corrosion of the iron plate and the rivet head is caused by this reason. In addition, it has been proved by experience that the stressed part is often an anode and is most likely to be rusted.
Metal surface conditions can also have an effect on electrochemical corrosion. If the metal surface film is incomplete and has pores, the surface potential of the metal under the pores will be lower and become the anode of the microbattery. In most cases, surface roughing parts are rusted faster than surface finishing parts. The effect of surface roughness on rust is much more pronounced in the atmosphere than in the electrolyte. This is mainly due to the fact that the rough surface has large adsorption energy, it is easy to adsorb moisture and dust, and oxygen concentration batteries are easily formed.
Some metals such as aluminum, lead, copper, tin, bismuth, etc. can generate a dense, stable protective film on the surface in the atmosphere, so that the metal does not continue to rust. The corrosion products produced on the surface of steel in the atmosphere have loose structure, porous structure, and no protection. At the same time, they also have capillary adsorption. Therefore, steel is easily rusted in the atmosphere. Some metal corrosion products are hygroscopic. For example, in the atmosphere containing sulfur dioxide, copper sulfate may be generated on the surface of the copper. Iron sulfate may be generated on the surface of the iron. Metal chlorides may form on the metal surface in the atmosphere containing chlorine ions. The lower moisture absorption point makes it easier to rust.
Second, the impact of the environmental factors of metal products on their corrosion Storage environment factors are the determinants of whether or not the product can rust in storage, and thus are the main control factors to prevent corrosion of metal products in storage. The so-called environmental factors refer to the air temperature and humidity of the storage environment, harmful gases and impurities in the air, such as carbon dioxide, sulfur dioxide, and chlorides. The acids, alkalis, and salts that are in contact with the metal products are all environmental factors.
1. Humidity of air The corrosion of metal products during storage is mainly damp and atmospheric corrosion. Wet atmospheric corrosion is an electrochemical process that occurs under the water film formed on the surface of a metal product. The speed of this process has a certain relationship with the water film thickness, as shown in Figure 6-3.
The area I in the figure (water film thickness = 1 to 10 nm) is equivalent to the initial stage of adsorption of water, and the metal surface has only a very thin adsorption film (only a few to several tens of molecular layers). At this time, it is a dry atmosphere corrosion. The rust rate is the smallest. Area II (water film thickness = 10 to 100 nm) corresponds to the presence of invisible water film corrosion (10 to 100 molecular layers) on the surface of the metal. The transition from the chemical history to the electrochemical process results in a rapid increase in the corrosion rate. In region III (water film thickness = 100 to 1000 nm), there is a visible water film on the metal surface. However, as the thickness of the water film increases, it becomes difficult for oxygen to diffuse through the water film to the metal surface (cathode control becomes the main factor). As a result, the corrosion rate begins to decline. When the water film thickness increases to the area IV (water film thickness> 1mm), it corresponds to the corrosion of the metal fully immersed in water.
Metal products in the air, the surface water film thickness and the relative humidity of the air are directly related. Only when the relative humidity of the air exceeds the critical humidity, the water film formed on the metal surface can meet the need of corrosion electrochemical process, so that the corrosion speed is obviously accelerated, and the higher the humidity is, the faster the corrosion speed is.
The critical humidity of general metal corrosion is about 70%. The rough surface of metal products, complex structure, the surface adsorption of salts, dust and other harmful gases, can reduce the critical humidity rust. Figure 6-4 shows the relationship between corrosion speed and air humidity of copper in air containing sulfur dioxide.
Under the condition of exceeding the critical humidity, the reason why the electrochemical process of metal corrosion increases with the increase of the air humidity is related to the polarization of the rusted battery. When the relative humidity is low, the degree of anodic polarization is high. As the relative humidity increases, the degree of anodic polarization becomes smaller and the corrosion rate becomes larger, as shown in Figure 6-5: 1 and 5 Humidity is 100%; 2 and 4 relative humidity is 75%; 3 Relative humidity is 50%. As the relative humidity of the air increases from 50% to 100%, the anodic polarization of zinc decreases by a factor of 1,000. The degree of anodic polarization decreases with water film thinning under the visible water film and decreases with water film thickening under the invisible water film. In other words, when the air humidity increases, the degree of cathodic polarization decreases, and the rust speed increases. The relationship between steel corrosion speed 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 at saturation humidity h - Actual humidity at this temperature h0 - Saturation humidity formula at this temperature shows that when relative humidity is low, with relative humidity With increasing humidity, the corrosion rate does not increase quickly. However, near saturation humidity, even if the relative humidity does not increase much, the corrosion rate increases quite rapidly. It can be seen that the relative humidity is an important factor affecting the corrosion of the stored metal products. As long as the relative humidity of the storage environment is controlled below the critical humidity of the metal products, corrosion can be effectively prevented.
2. The effect of air temperature and temperature on corrosion is not isolated, but it is also affected by relative humidity. On the one hand, an increase in the temperature of the storage environment will accelerate the corrosion rate of the metal products, because the heat energy can accelerate the progress of the chemical reaction, while the temperature increase can reduce the polarization of the cathode, but this effect is only when the temperature is high and the temperature is The increase did not show up until the water film on the metal surface dried up. The role of air temperature in atmospheric corrosion can be seen from the formula of atmospheric corrosion rate:
H-65
V = ----- × (1.054)t
10
Where: V - atmospheric corrosion rate H - relative air humidity (%)
T--air temperature When the relative humidity exceeds 65%, the increase of air temperature shows only a promotion effect on the corrosion rate; but when the temperature rises to 80°C, the corrosion is inhibited due to the obvious decrease of 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 decreases, dew condensation may occur on the surface of the metal product under conditions of relatively high absolute humidity. When a metal product with a relatively low temperature is moved into a high-temperature environment, such as a metal product that is transported in winter, its own temperature often differs greatly from the temperature inside the warehouse, and dew condensation is likely to occur after storage. Condensation can severely accelerate the corrosion of metal products and must be noticed.
Figure 6-6 shows the temperature difference that may occur when the relative humidity is different at a certain temperature. As can be seen from the figure, the relative humidity in the range of 5 to 50°C in the air temperature reaches 65% to 75%. When the temperature suddenly drops by 6°C, condensation may occur. The greater the temperature at which the temperature changes, the lower the relative humidity at which dew condensation may occur. The temperature difference between day and night in all regions of China exceeds 6°C. In some regions, the temperature difference between day and night can reach 15°C or more. In this case, condensation may occur even if the relative humidity of the air is low. Therefore, the temperature of the interior of the stored metal product should be kept relatively stable. It is of practical significance to prevent condensation from occurring.
3. The Role of Oxygen in the Air Oxygen, like water, is a necessary factor for the corrosion of metals in the atmosphere. In the absence of any 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 depolarization of the cathode is mainly the depolarization of oxygen. That is, oxygen is the main depolarizer. The cathodic depolarizer continuously removes electrons from the cathode, so that the cathode can continuously receive electrons from the anode, allowing the cathode process to continue and the metal to be continuously dissolved. Therefore, in the atmospheric corrosion process, oxygen and water are the important factors that promote corrosion.
4. In the air, harmful gases and impurities in the air usually contain various industrial harmful gases and rust impurities such as dust, which have a great impact on the corrosion of metal products.
(1) Sulfur Dioxide Sulfur dioxide is the most harmful gas in the air pollutants that affect the corrosion of metal products.
In air exposure tests, the rust rates of metals such as steel, copper, and zinc are approximately proportional to the amount of sulfur dioxide in the air. Figure 6-7 shows the influence of sulfur dioxide content on the corrosion rate of steel. The effect of sulfur dioxide content in air on aluminum is very special. The effect of dry air is small, but when the humidity is high (such as relative humidity above 98%). As long as the air contains trace amounts 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 4 to 10 times; when the concentration of sulfur dioxide increases to 1%, the corrosion will occur. The speed increases slowly. Sulfur dioxide in the air generally exceeds 0.15%, and many relatively corrosion-resistant metal products are likely to be heavily rusted in this environment.
The effect of sulfur dioxide on the corrosion of metal products is mainly that sulfur dioxide is dissolved in the water film on the metal surface to generate sulfurous acid, which enhances the electrochemical effect of corrosion. Acids can dissolve oxide films with protective effects on many metal surfaces, making it easier for electrochemically etched cathodic processes; at the same time, acid hydrogen ions can accelerate the cathodic process. Although the content of SO2 in air is much smaller than that of oxygen, the solubility of SO2 in the electrolyte is more than 1000 times greater than that of oxygen. Therefore, when the content of SO2 in air is 0.015%, the concentration of SO2 in the electrolyte is equal to oxygen. concentration.
(2) Hydrogen sulphide Hydrogen sulphide in dry air can only cause surface discoloration of some metals (copper, brass, silver and iron are more obvious), but in the humid atmosphere, due to acidification of the liquid film and copper nickel, especially iron and Magnesium corrosion has a large promoting effect and may cause corrosion of stainless steel.
(3) Chloride Industry Atmospheric hydrogen chloride and chlorine gases have a strong rust effect on metal products because they dissolve in the water film to form hydrochloric acid. Marine air often contains a certain amount of salt particles, which can promote the corrosion of steel, copper and aluminum. The role of salt is mainly the role of chloride ions, because the volume of chloride ions is very small, can penetrate the protective film on the metal surface, while chlorine ions are easily adsorbed on the metal oxide film, replacing the oxygen in the metal oxide film to generate soluble chloride . The influence of the hydrogen chloride gas and the salt content in the air on the metal corrosion is shown in Figure 6-8 and Figure 6-9.
The composition of air and the nature of pollutants are related to the region. Therefore, according to the influence of metal corrosion, the air is divided into industrial atmosphere, urban atmosphere, ocean atmosphere and rural atmosphere. Among them, industrial atmosphere contains more harmful gases such as sulfur dioxide and hydrogen chloride, which have the greatest impact on metal corrosion; sea salt contains salt particles and high humidity, and the promotion of metal corrosion is also very obvious, and the salt content in the marine atmosphere As the distance from the coast decreases, the metal corrosion rate often decreases as the distance from the storage site to the coast increases. The rural atmosphere has the least impact.
In addition to the above factors affecting the corrosion of the stored metal products, there are other factors, such as packaging materials, especially packaging materials in direct contact with the metal products have a certain impact on the corrosion of the metal, and some packaging paper components Contains considerable amounts of chloride ions and acids, and some contain reducing sulfur. The condensation of the capillaries of the packaging paper can also reduce the critical relative humidity of the rust of the metal product. In addition, certain microorganisms have a catalytic effect on the corrosion of metals. In wet conditions, iron bacteria can grow on steel and promote the corrosion of steel. A few molds, such as Aspergillus can promote the corrosion of aluminum. Under humid and hot conditions, instrumentation often causes severe corrosion due to the propagation of microorganisms (mainly molds). The corrosion of microorganisms by metal products is mainly due to the effects of microbial metabolites and the influence of sediments.
Rhein Lighting has developed a line of LED waterproof strip light. The 120V light strip is dimmable and work with smart controller. It has multiple colors such as white, red, blue, yellow, green and RGB etc. The waterproof strip light has clear PVC tubing, which is flexible, transparent and durable and it can be shaped, formed or bent in any angle to perfectly fit the space being decorated.
The IP67 led rope light has come with different options of lengths in a reel, which has 50ft, 100ft and 150ft. The 120v Led Strip Light can be cut in every 1.64ft/0.5m along the cutting marks, without damaging the rest strips.
The Led Strip Light is ETL/CE/ROHS certified, which is Eco-Friendly, No splash and UV, without mercury and no damage to human and environment.
Rhein Lighting is specialized in the design and manufacture of led panel downlight, led slim down light, backlit Led Panel Light, led rope light, led strip light, led plant light and Led Grow Light etc.
Outdoor Led Strip Lights Waterproof,Waterproof Color Changing Led Strip Lights,Led Strips Waterproof,Submersible Led Light Strips
Rhein Lighting Technology Co.,Ltd. , https://www.rheinlighting.com