Corrosion failure process and countermeasures of tinplate three-piece cans
1. Tin cans corrosion phenomenon
The corrosion of metal packaging products occurs because the material is corrosive Electrochemical instability in the material caused. The main corrosion resistant materials for tinplate three-piece cans are can coatings, tinplate tin and iron layers, and top and bottom lids with coatings. Since the metal packaging products have a certain shelf life, the tinplate cans can meet the production requirements when the corrosion life of the cans is greater than the shelf life, and can guarantee the quality and safety of the food and beverage during the shelf life. Excessive corrosion margin is an excess of quality, increasing the product. Economic costs. In order to simultaneously take into account the requirements for a qualified design life and economy, the production of three-piece cans of tinplate has precise requirements for raw materials and processes to maintain the quality of the products.
Experimental research shows that tinplate three-piece can coatings, tin coating And iron layer is the main corrosion protective barrier of the tank, stable raw materials and reasonable technology can meet the requirements of the corrosion resistance of most of the solid tank products. At the same time, related research found that the corrosion time of the cans of some products occurred earlier. Due to the type of corrosion and the location of the corrosion, the development speed is very different. Some solid cans have produced rust in a few weeks. Even after several months of corrosion, corrosive punctures occur, and some solid-can corrosion can continue until after the shelf life. During the production and storage of tin cans, it is often found that the corrosion of the cans occurs before the shelf life of the cans is reached. The main types of corrosion are divided into two categories: uniform corrosion and local corrosion. The local corrosion phenomenon is a great threat to the quality of the tank, and it can lead to corrosion and perforation of the tank during the shelf life.
1.1 Uniform corrosion
Uniform corrosion, also known as general corrosion, is distributed throughout the entire gold On the surface, the corrosion rate of all parts of the metal surface is approximately the same, the surface of the metal is relatively evenly thinned, and there is no obvious difference in the corrosion morphology of the metal surface. Such corrosion occurs on all surfaces and is easy to find and control. The more common corrosion phenomenon in tinplate can corrosion is uniform corrosion, which mostly occurs at the top of the can body in the necking area, the bottom of the can body shrinkage deformation area and the weld seam coating area.
1.2 Local corrosion
Local corrosion, also called uneven corrosion, due to uneven electrochemical performance One, such as dissimilar metals, surface defects, concentration differences, stress concentrations, or environmental inhomogeneities, causes localized batteries to corrode. The local corrosion of the cathode and anode can be distinguished, and the localized corrosion is concentrated at a specific location and occurs rapidly. The material is rapidly corroded and destroyed, and local corrosion of the tin can causes perforation leakage. The local corrosion shows diversity characteristics. According to the damage morphology caused by local corrosion, such corrosion can be classified into galvanic corrosion, pitting corrosion, seam erosion, intergranular corrosion, wear erosion, stress corrosion, fatigue corrosion, or selective corrosion. Types of.
The local corrosion of tinplate cans is mostly concentrated in the weld area or the bottom of the tank. On the cover expansion ring, the bottom corrosion of the tank is the main area of ​​the corrosion hole. As shown in the figure below, the corrosion hole occurs in the center of the black uniform corrosion area. Compared with the uniform corrosion area, the area of ​​the corrosion hole area is very small, which is a typical local corrosion. Phenomenon that corrosion continues to develop can lead to corrosive perforation of the tank.
2. Analysis of Causes for Corrosion of Tinplate Cans
2.1 can body corrosion protection system
The corrosion protection system of tinplate cans consists of an organic coating/tin layer/iron layer triple protective structure. The initial stage of solid tank corrosion is mainly coating protection, and the middle tin layer protection is the main protection. In the later period, the protection is based on iron material and the shelf life is guaranteed. Corrosion protection of the inner tank, coating protection plays a major role. During the use of solid cans, most tin cans meet the quality requirements for shelf life, and only a small amount of solid cans will undergo corrosion perforation. The corrosion of tinplate cans is closely related to the contents of the packaging. When the contents of the tin cans are highly acidic, high in salt content, and rich in sulfur-containing proteins, the content is corrosive. Although the organic coating can prevent the content liquid from coming into contact with the metal, it is usually due to the existence of pores. After being left for a period of time, the liquid will infiltrate with the metal and the tin layer will dissolve. The tin layer itself is relatively stable, but the dissolution rate is higher in the presence of highly corrosive contents and in the presence of nitrates or sulfide ions. After a period of time, metallic iron will be exposed, forming a tin-iron couple, which will accelerate the dissolution of iron. The corrosion resistance of tinplate cans depends mostly on the porosity and defects of the coating, the bluntness and thickness of the tin layer, and the thickness of the iron.
2.2 coating failure
The coating performance depends not only on the type of coating and its own properties, especially It depends on the construction process of the coating. The degree of pretreatment on the surface of the metal, the coating thickness, the degree of curing of the coating, and the adhesion of the coating are factors that affect the protective performance of the coating. major factor. For different content products, suitable coatings and reasonable hygiene Production process plays an important role in ensuring the quality of solid can products.
2.3 tin layer failure
Tin on the inner wall of tin can and oxygen remaining in the can during filling The effect, reducing the chance of the contents ingredients being oxidized. The reduction effect of tin has a good preservation effect on the flavor and color of some light-colored fruits. Therefore, the use of iron cans can better maintain the flavor and color of the contents, and the change of brown is slight, and the acceptability of flavor quality is better. Storage The time limit is thus extended. Natural However, the uniform acid dissolved tin, nitrate and nitrite caused abnormally fast De-tinning, tin sulfide corrosion will lead to gradual thinning of the tin layer. In order to enhance the corrosion resistance of the tin layer, usually the tin layer needs to be passivated in order to reduce the dissolution rate of the tin layer. Therefore, enhancing the tin layer passivation film is an important measure to improve the corrosion resistance of the tin cans.
2.4 can making process
Found in production, most of the time, the corrosion of tin cans The number of effects is small, and the percentage that appears is very low, indicating that the corrosion failure is likely to be a random phenomenon due to individual factors in the production process. Absolutely The corrosion resistance of most products meets the quality requirements indicating that the production process is Rational. The main cause of corrosion should be caused by random defects in the coating properties. Experimental studies have shown that the areas where corrosion occurs are mainly concentrated on the top of the can body, the bottom of the can, the groove at the bottom of the can, and the area of ​​the weld seam. The black corrosion area of ​​the tin layer in the bottom shrinkage area of ​​the can body is also very significant. The resulting coating thinning and defects are the main cause of perforation of the bottom of the tank. Due to the small amount of anti-corrosion coating thickness, the random thinning defects will increase, the greater the probability of corrosion. In addition, the weld repair process in the canmaking process has always been one of the weak links in the anti-corrosion processing of tinplate cans. Many of the corrosions attack the weld seam of the can body first and then expand. Due to the short drying tunnel and fast processing speed, the make-up coating process must require fast drying and solidification, and has good acid and sulfur resistance. For highly corrosive contents, a full-spray process is used on the basis of the recoating in the production of empty cans. While improving the quality of recoating, it also made up for the production of printing iron cans Mechanical damage, burns and other defects greatly improve the corrosion resistance of the tank.
2.5 Tin-Ferrocouple Corrosion
Since the electrochemical activity of iron is higher than tin, once the tin layer dissolves to gold If it is iron, it will constitute tin-iron galvanic corrosion, and it will spontaneously accelerate once it occurs. Corrosion of tin and iron layers and electrochemical anodic dissolution produce Sn2+ and Fe2+, the larger the corrosion area, the greater the amount of Sn2+ and Fe2+, although a small amount Sn2+ and Fe2+ are beneficial in supplementing the body's trace elements, but to a certain extent will change the taste of the beverage and need to be controlled. In the process of corrosion, iron works As the anode will accelerate dissolution, tin as the cathode stops dissolving and accelerates the iron Extremely soluble, causing iron corrosion to develop in depth to form localized corrosion holes, corrosion The hole is in the center of the corrosion spot, and the surrounding area is no longer corroded, eventually leading to real Cans corrode perforation leaks.
The corrosion process of tinplate cans is divided into three stages vertically. Effect, tin layer dissolution and iron layer perforation. The coating failure takes a long time, followed by the tin dissolution time, and the last iron layer is perforated. Spontaneously accelerated, the shortest time. Tinplate cans occur throughout the corrosion process The composition of the period and development period, the occurrence of the corrosion induction period, corrosion during this period is reversible, will continue to erode, will stop the corrosion. The development period is irreversible. As long as it enters the development stage, the corrosion process cannot be reversed, and it gradually accelerates until the perforation is corroded. The failure of the coating is the occurrence period and the entry into the tin The dissolution of the layer enters the development phase and the corrosion is accelerated until the can body is perforated. However, controlling the corrosion process during the occurrence of corrosion is most effective. The corrosion process of tinplate cans still exists In the lateral distribution pattern, the occurrence and development speed of each corrosion area is different. At some point, some areas are in the corrosion area, and some may enter the corrosion development area. Most of the tank body should be located in the corrosion area and will not enter the development area. Only a very small amount of weak areas will enter the corrosion development. Area.
3. Strengthen the corrosion resistance of tinplate cans
The corrosion of the inner wall of tin cans is an inevitable phenomenon, whether it is high Tin-iron, also a coating and metal protective system due to the contents of the package Corrosion, Processing Deformation, and Transport Collision Caused by Coating Damage Corrosion Failure It happens from time to time. Therefore, the corrosion resistance of tinplate cans is enhanced and the cans are slowed down. Corrosion of the inner wall to minimize it requires raw materials and raw materials from the tank Various aspects of the production process, etc., according to the characteristics of the content to choose the right Empty cans allow a good match between the two. Corrosion resistance with other metals The higher the value and the longer the better, the tinplate can have shelf life. Life restriction, excessive corrosion life leads to excess quality and wastes economic efficiency beneficial. In the existing production process, taking into account the quality and safety and the most economic benefits Dahua, improve the protective performance of the coating and the performance of the tin layer of tin to improve the horse The corrosion resistance of iron cans is of great feasibility.
3.1 Development of New Coatings
The main method to improve the corrosion resistance of coatings is to increase the coating thickness and coating Layer adhesion and degree of cure to reduce the porosity of the coating and enhance coating adhesion Force and coating flexibility. In the tin can production, increase the coating thickness and Hardness and reduced porosity do not significantly enhance the resistance to shrinkage Force does not produce obvious results. Enhances coating adhesion and flexibility Has an important role, through this measure can significantly enhance the coating resistance to shrinkage processing The ability to reduce defects produced during processing increases corrosion life. The main way to enhance coating adhesion and flexibility is to improve coating Learn recipes. Taking into account the changes in the coating formulation is a systematic project, but also take into account Coatings for coating, food safety and other important properties should be as far as possible System commissioning based on proven coating formulations. Coating Formula Improver The case will significantly affect the empty can production process, but in the long run, this measure The application may completely solve the corrosion resistance of the tank, which is beneficial to the rise of the product. Level and long-term development.
3.2 Enhancement of Corrosion Resistance of Tin and Iron Layers
One of the effective ways to improve the corrosion resistance of the tin layer of tinplate is to carry out Tin layer passivation treatment. The passivation of the tin layer involves downstream steel suppliers The production capacity and technical conditions for capable tinplate suppliers, Can explore the relationship between the passivation performance and corrosion resistance of tinplate, passivation tin Layer-to-coating binding force matching properties and other issues to produce high-performance horses Iron products.
The iron layer of tinplate bears the role of structural skeleton compared to the coating As for the tin layer, the room and effect of enhancing the corrosion resistance are small and will increase. Add raw material costs. Especially to enhance the corrosion resistance of the coating and tin layer After that, it is not necessary to enhance the corrosion resistance of the iron layer, but also to meet the corrosion resistance of the can body. The request of energy.
3.3 Process for Enhancing Corrosion Resistance of Empty Cans
Due to the limited shelf life of products and product cost factors of tinplate cans, The thickness and strength of the tank material are limited, the toughness of the inner and outer coatings is limited, and The need for bottom shrink production process, the ability of the tank to withstand external forces is weak and easy Deformation and coating defects occur at all stages of the production process, increasing full-spray The process is a good remedy. Tank bottom shrinkage process is mature automation Production process, the process conditions are critical to the strength and sealing performance of the tank To reduce the processing power is very small. If you improve the overall spray process, Making it possible to perfect and repair the damage caused by the coating of the bottom shrink zone can still be achieved Enhanced coating protection and corrosion life.
The study found that the full-spray nozzle is far from the bottom of the tank, and the eruption force is weak. It will lead to imperfect full-spray repair of tank bottom groove area, and there is an omission Area. So you can reduce the spray distance by moving the nozzle into the tank, or Increase nozzle jet power, or reduce jetting angle, or reduce paint stickiness Degree, so that the full-spray coating can reach the bottom shrink groove area of ​​the tank to ensure the bottom shrinkage groove Groove area defects can be fully repaired, enhance the defense of the shrink coating Protective capacity, which will effectively reduce the corrosion of the bottom shrink zone of the tank Rate to extend the corrosion life of tin cans.
In addition, due to the different types of full-spray and primer coatings, it is possible that Coating compatibility issues can occur, reducing the overall spray coating and primer coating Together, resulting in imperfect repair of the bottom shrinkage coating, should be chosen with the tank Primer coating with good compatibility, enhance the binding of two coatings, confirm Preserve the integrity and strength of spray paint.
3.4 Strengthen the Protection of Empty Tanks
Tin cans factory packaging and transportation are empty cans and solid cans production must not In a few links, the study found that the impact of transport collisions on tinplate cans There is a greater impact, slower production and transportation of tanks and filling of solid tanks should be slowed down. Tinplate cans collide impact measures to prevent cans from collapsing and damaging And other issues. Use cushioned collision packing material to increase the capacity of tinplate cans Plus buffer material and so on. Increase the regular sampling of empty cans during production of solid tanks Inspection, to avoid damage to the coating resulting from packaging collision Production, screening qualified empty cans for solid can filling production.