Discussion on anti-corrosion technology of the hot

Discussion on the anti-corrosion technology of tinplate food cans II

III. factors affecting the anti-corrosion ability of tinplate food cans

the anti-corrosion system of tinplate food cans is based on tinplate materials, on which a layer of anti-corrosion coating is coated, and then further improved by can making, coating or spraying process. Therefore, the corrosion resistance of tinplate food cans is mainly considered in the following aspects

1. Tinplate

tinplate is composed of steel base, ferrotin alloy, tin layer, passive film and oil film, as shown in Figure 1. The steel matrix is composed of high-quality low-carbon boiling steel, tin iron alloy is the alloy layer produced during tin plating, the tin layer is pure tin body, the passivation film is chromium compound formed after passivation treatment, and the oil film is used as the antirust layer. See Table 1 for the main structure and characteristics of tinplate

in the composition of tinplate, tin layer and passive film play a key role in corrosion prevention

generally speaking, the corrosion resistance of tinned tinplate can be greatly improved. The higher the amount of tin plating, the stronger the corrosion resistance. However, at the same time, we should also observe whether the tin coating is dense and uniform (determined by the soft melting process level after tin plating) and whether there are oxide skin defects (oxide skin is caused by impurities in the environment during tin plating, which is the weak point of sheet corrosion prevention). The introduction is also very clear. It is worth noting that in addition to increasing the cost, too thick tin coating will also lead to the reduction of the coating processing suitability of tinplate. Iron printing manufacturers should consider it as a whole

in order to further enhance the sulfur resistance and corrosion resistance of tinplate, after tinning, the tinplate needs to be chemically treated in sodium dichromate solution to form a layer of chromium oxide and chromium hydroxide on the surface of tinplate, that is, passivation film. Theoretically, the thicker the passive film, the stronger its corrosion resistance, but at the same time, it will also reduce the adhesion of the anti-corrosion coating in the post-processing process. Therefore, the thickness of the passive film also needs to be paid attention to

of course, for the production process of tinplate, the iron printing manufacturers generally cannot directly control the system of "smart car public road test management specification", but we have the right to choose tinplate manufacturers with stable production quality and high product reputation as partners to ensure that the quality of the tinplate used meets the requirements

by the way, for the poor thickness tinplate processed with incoming materials, unless specified by the customer, the thick side of the tin layer is generally used as the internal coating surface, which is conducive to improving the corrosion resistance of food cans

2. Anti corrosion coating

as mentioned above, there is an anti-corrosion coating in the anti-corrosion system of tinplate food cans, and its film-forming material is mainly epoxy phenolic resin. Let's start with the epoxy phenolic resin anti-corrosion coating

in the national standard of epoxy phenolic coated tinned steel sheet (GB 8230-87), the coating amount of epoxy phenolic resin is divided into three levels, see Table 2

the anti-corrosion ability of food cans is directly proportional to the thickness of the anti-corrosion coating. For the food cans with several thicknesses listed in Table 2, we suggest that the scope of application and coating process can be referred to table 3

in order to ensure the final film-forming performance of the film, the two coating and two drying or three coating and two drying coating process requires that the drying temperature at the first drying should be 15 ~ 30 ℃ lower than the normal construction temperature. In addition, as the film thickness increases, the adhesion of the film will be reduced. Therefore, when engaging in the coating operation where the film thickness exceeds the class a standard, it is generally necessary to apply the supporting internal primer as the first coating to enhance the adhesion of the film

for tinplate food cans containing high sulfur contents (such as asparagus, shrimp and crab, steamed eel), simply coating epoxy phenolic resin is not enough to prevent the reaction between tinplate and sulfur. It is the important foundation of biotechnology and biomedical engineering, the two pillars of modern medicine, to generate sulfide. Therefore, many manufacturers use the method of adding zinc oxide to the inner coating to form a high sulfur resistant coating. The ratio of inner coating to zinc oxide is about 10:1 (the amount of zinc oxide can be appropriately increased or reduced according to the sulfur content of the content). It is required to prepare and use it now according to the amount used. Otherwise, it will thicken and deteriorate if it is placed for too long

the sulfur resistance mechanism of adding zinc oxide to the inner coating is that zinc oxide absorbs H2S released from food and reacts to produce white zinc sulfide, so as to prevent H2S and Fe from producing black iron sulfide, which plays a sulfur resistance role. Zinc oxide particles are needle shaped, which is also conducive to improving the sealing property of the film

for food cans containing lunch meat, beef and mutton and other contents, in addition to the above factors, it is also necessary to add some anti adhesion agents (such as synthetic wax) to the film to prevent the contents from adhering to the anti-corrosion coating

the epoxy phenolic resin coating process mentioned above is mainly used for the coating of tinplate food cans, and it is not recommended to be used for the coating of aluminum food cans, because the compactness of epoxy phenolic resin and aluminum is not completely reliable. Therefore, the inner coating process of aluminum cans is currently ethylene coating, which has strong compactness

3. can making link

the use and maintenance of weld fatigue testing machine in can making process has always been one of the weak links in the anti-corrosion processing of tinplate food cans. Many corrosion attacks the weld of food cans first, and then spreads. Due to the short drying path and fast processing speed of the touch up coating process, the touch up coating agent must be able to dry and solidify quickly and have good acid and sulfur resistance. In order to ensure the quality of touch up coating, we must not unilaterally emphasize cost saving when purchasing touch up coating materials

when the performance of the drying channel is poor, the can making speed can be appropriately reduced to ensure that the supplementary coating is dry. At the same time, pay attention to whether the wettability of the supplementary coating and the internal coating is good, and check the supplementary coating quality at any time. In order to solve the shortcomings of the touch up coating process, many can manufacturers also adopt the full spraying process on the basis of the touch up coating, which not only improves the quality of the touch up coating, but also makes up for the defects such as mechanical injuries and burns in the iron printing can making process, and greatly improves the anti-corrosion ability of food cans

one thing we need to pay special attention to is that tinplate cans containing alcoholic substances (such as Baijiu) are particularly prone to corrosion at the weld repair coating. If you are not sure about the repair coating quality of your own factory, you should be very careful when receiving orders. (to be continued)