Application of Chrome-plated Steel Sheet in Can Packaging

The chrome-plated steel sheet is referred to as chromium-plated steel sheet and is a tin-free steel sheet. It is a new type of can-making material developed under the condition that the metal tin resources are small and the cost is high. The United States, Europe and Japan have more and more applications. It can replace some tinplate materials and is a promising new material.

Chrome plate development and characteristics

Chromium is a hard, silvery white shiny metal with good hardness, wear resistance, and matrix binding. It also has high heat resistance, and its potential is lower than that of iron. However, after the passivation, the potential is higher than that of iron. As early as in the 1920s, chromium plating process has been used for industrial production. The United States began to study chromium plated plates in 1940. It was treated with chromic acid chemical impregnation and electrolytic chromic acid, and transferred to mass production. After living in Japan from 1955 onwards, the chromate passivation method was studied and many results were obtained.

For example, the Hitop chrome plate of Toyo Steel Corporation entered industrial production in 1961; Fuji Steel also produced Con Cupper chrome plates in succession; in 1972, Japanese steel pipes produced Bright Coat chrome plates. board. European countries also successively produced chrome plates in the 1970s.

The original plate used for the chrome plate is the same as the tin plate. They are low-carbon steel plates, but the chrome plating layer is very thin (<1.3 μm). The process is the same as that of tin plate. Therefore, in general, the original tin plating production line needs only a slight modification, and the addition of a chrome plating bath can achieve tin or chrome plating.

The chrome plated structure has four layers: a steel substrate, a metallic chromium layer, a hydrated chromium oxide layer, and an oil film. Since the substrates are the same, the mechanical properties are the same as tinplates, but there are some unique features in their application:

1, the cost is about 10% lower than the tin plate, the appearance of the luster is not as good as the tin plate;

2. Corrosion resistance is lower than that of tin plate. The coating is thin and the pinhole rate is high. Therefore, the inner and outer surfaces must be coated when used.

3, chrome plate adhesion, adhesion to organic coatings than tin plate strong 3-6 times. Sulfide corrosion resistance is also stronger than tin plate;

4, chrome plate can not be soldered, can only use lap resistance welding or bonding;

5, thin coating and poor toughness, cans easy to rupture, and therefore should not be redeemed cans, can be used for deep-drawn cans;

6, chrome plate high temperature resistance, generally no change in color and hardness at 500 °C, to 700 °C only began to soften.

Main issues in food can making applications

There are two main technical problems that restrict the use of chrome plating in the food can making industry:

One is the problem of corrosion resistance. This problem can be studied from the improvement of the chrome plating technology and the improvement of the post-plating paint. It should be noted that any technical improvements must consider the cost in order to maintain the advantages of the chrome plate. For the moment, the corrosion resistance of chrome plate is not as good as that of tin plate, but it also has certain corrosion resistance. It is suitable for weak acidic foods and packaging. This type of food has a large market, such as meat, fish, poultry, dairy products, vegetables, carrots, wrist beans, beets, potatoes and various beverages, soups and so on. If the technical indicators of the coating are strictly guaranteed, some acidic foods with a pH of 3.7-4.5 can also be packaged with chrome plates, such as fruits and kimchi. It is worth mentioning that the chrome plate is a cathode coating and can not play an electrochemical protection role. Under the condition of high porosity, the alloy effect between the steel substrate and the chromium layer is worth studying. Some people test the resistance of chrome steel in seawater. Erosion, found that short-term (2 years) soaking, chromium on the corrosion resistance of carbon steel improved; but long-term (more than 2 years later) not only useless soak, but also accelerate the corrosion.

The second is the can manufacturing process. The chrome plating has a thin coating and poor toughness, and lacks the same lubricant as the tin, so the flushing process needs careful study. At present, it is feasible to use deep-drawing process for two-piece cans. Since it is impossible to solder, three-piece cans must be welded or bonded. If a tinplate chrome plate is used in combination, a tin plate is used for the can body and a chrome plate is used for the bottom lid of the can. In addition, the double seaming process should not be used for sealing the can body and lid.

At present, China's food canning processing enterprises mainly use tinplates, and their food processing, can sales, and corresponding equipment and mechanisms are all based on canned tinplates. Any technological innovations and innovations are related to the benefits of the company. It is related to the credibility of the market. Therefore, the application of chrome plate is a risk to them, must have a scientific and economic analysis, and reliable conditions can only be fully accepted. However, the application of chrome plated in food packaging has its limitations, but it also has its own characteristics and advantages. It has a broader market, so it is worthy of promotion and development.