In depth analysis of the corrosion failure process and countermeasures of tinplate three piece cans

In depth analysis of the corrosion failure process and countermeasures of tinplate three piece cans

（1） Corrosion phenomenon of tin cans

The cause of corrosion in metal packaging products is the electrochemical instability of the material in corrosive contents. The main corrosion-resistant materials for tinplate three piece cans are the coating of the can body, the tin plating layer and iron layer of the tinplate, as well as the top and bottom covers containing the coating. Due to the certain shelf life of metal packaging products, when the corrosion life of the tinplate empty can design is greater than the shelf life, it can meet the production requirements and ensure the quality and safety of food and beverages during the shelf life. Excessive corrosion allowance is excess quality and increases the economic cost of the product. In order to balance the requirements of qualified design life and economic savings, the production of tinplate three piece cans has precise requirements for raw materials and processes to maintain stable product quality.

Experimental research has shown that the inner coating, tin plating layer, and iron layer of the tinplate three piece can are the main corrosion protection barriers for the can body. Stable raw materials and reasonable processes can meet the corrosion resistance requirements of the vast majority of solid can products. Related studies have also found that corrosion in some products occurs earlier in the tank. Due to the different types and locations of corrosion, the development speed varies greatly. Some actual tanks have rust spots within a few weeks, and in severe cases, corrosion perforation may occur several months later. However, some actual tanks can continue to corrode until the shelf life without corrosion perforation. During the production and storage of tin cans, it is often found that corrosion occurs before the expiration date of the actual can. The main forms of corrosion are uniform corrosion and localized corrosion. The phenomenon of localized corrosion poses a significant threat to the quality and safety of the tank, and in severe cases can lead to corrosion, perforation, and leakage of the tank during its shelf life.

1. Uniform corrosion

Uniform corrosion, also known as total corrosion, is a corrosion phenomenon that occurs on the entire surface of a metal. The corrosion rate of various parts of the metal surface is roughly the same, and the surface of the metal is relatively uniformly thinned without obvious differences in corrosion morphology. This type of corrosion is easy to detect and control because it occurs on all surfaces. The most common corrosion phenomenon in the corrosion of tin cans is uniform corrosion, which mostly occurs in the necking area at the top of the can body, the deformation area at the bottom of the can body, and the welding seam repair area.

2. Localized corrosion

Localized corrosion, also known as uneven corrosion, occurs due to the non-uniformity of electrochemical performance, such as dissimilar metals, surface defects, concentration differences, stress concentration, or uneven environment, forming localized cells and causing corrosion. The anode and cathode of localized corrosion can be distinguished. Localized corrosion is concentrated in specific locations and occurs rapidly, causing rapid corrosion and damage to materials. Localized corrosion of tin cans can easily lead to perforation and leakage. Localized corrosion presents diverse characteristics, and according to the damage caused by localized corrosion, it can be divided into main types such as galvanic corrosion, pitting corrosion, crevice corrosion, intergranular corrosion, wear corrosion, stress corrosion, fatigue corrosion, or selective corrosion.

The local corrosion of tinplate iron cans is mostly concentrated in the weld area or the expansion ring of the can bottom cover. The bottom corrosion is the main area of corrosion perforation, as shown in the figure below. Corrosion holes appear in the center of the black uniform corrosion zone. Compared with the uniform corrosion zone, the area of the corrosion hole zone is very small, which is a typical local corrosion phenomenon. The continued development of corrosion will lead to corrosion perforation of the can body.