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Summary of metal packaging materials and their applications (5)

VI. tinned steel sheet and chromium plated steel sheet tinned steel sheet for short, commonly known as tinplate, is a low-carbon steel sheet coated with pure tin on both sides. Tinplate is a traditional canning material, which is still the main canning material for making food cans. Tinplate has bright appearance, good corrosion resistance and can making process performance, easy to weld, suitable for coating and iron printing. However, its punching and drawing performance is not as good as that of aluminum plate, so most of them are made into a three piece tank structure formed by welding and sealing process, and can also be made into a punching and drawing tank. In addition to being widely used in the canning industry, tinplate also makes tins of candy, biscuits, tea, milk powder, etc; It can also be used in cans and boxes of some industrial products such as chemical products and cosmetics. In addition, it is also a good capping material for glass bottles

1. The production process and performance of tinplate. The surface of the steel plate is silvery white after tin plating. The thickness of this tin coating is about 0.4~4 μ m. Technically, it is generally expressed in terms of tin plating amount per unit area (g/m2). The tin plating layer of tinplate after electroplating has many pores and poor corrosion resistance. Therefore, soft solution treatment or passivation treatment must be carried out after electroplating to form tin alloy layer and tin oxide layer on its surface respectively. The corrosion resistance can be effectively improved by brightening the coating, enhancing the adhesion between the coating and the steel plate and reducing the porosity

tin is electroplated on the surface of tinplate. The main reason is that the electrode potential of tin is higher than that of iron, and its chemical properties are stable. Therefore, it can protect the steel plate from rust. However, the integrity of the tin coating must be maintained. If it is scratched, even if only a small pore is exposed to the steel base, the steel plate will be corroded quickly due to anodic corrosion

the main production process of tinplate is: pickling low carbon steel sheet → electroplate tin → soft solution treatment → passivation treatment → oiling → inspection → shearing → classification → packaging

Through metallographic analysis, the microstructure of the tinplate is as follows: there are five layers from the inside to the outside, which can greatly reduce the problems of too small sample blocks such as transparent parts that cannot be fixed in the drop hammer tube and cannot be impacted due to protection

(1) steel base plate. For general can making, its thickness is 0.2~0.3mm

(2) tin iron alloy layer. It is of tin iron alloy structure. The tin content of electroplated tin plate is < 1g/m2, and that of hot-dip tin plate is 5g/m2

(3) tin layer. For pure tin layer, the tin plating amount of electroplated tin plate is 5.6g/m2~22.4g/m2, and that of hot-dip tin plate is 22.4g/m2~44.8g/m2

(4) oxide film. Mainly stannous oxide, tin oxide, etc

(5) oil film layer. It is cottonseed oil or dioctyl sebacate

see Table 4 for the thickness, structure and performance characteristics of each layer

Table 4 structural properties of each layer of tinplate

structure name thickness composition performance characteristics

hot dip tin plate electroplating ceramic matrix composite tin plate

oil film 20mg/m2 2~5mg/m2 palm oil cottonseed oil or dioctyl sebacate lubrication rust prevention

oxide film 3~5mc/cm2

(single side) 1~3mc/cm2

(single side) stannous oxide

stannous oxide

chromium oxide etc Anti vulcanization marks

tin layer 22.4~44.8g/m2 5.6~22.4g/m2 pure tin pure tin is beautiful, easy to weld, corrosion resistant and non-toxic

tin iron alloy layer 5g/m2 < 1g/m2 tin iron alloy structure tin iron alloy structure corrosion resistant, such as over thick machinability and poor weldability

0.2~0.3mm low carbon steel for can making with 0.2~0.3mm steel has good machinability and certain strength after can making

2 Main technical standard of tinplate

(1) tin plating amount

the tin plating amount on the surface of the sheet is expressed by the tin plating weight per unit area, and the unit is g/m2. Tin plating amount is one of the indexes that determine the corrosion resistance of tinplate. According to different requirements for canning, tinplate with different tinning amount can be selected. According to whether the tin plating principles of the two surfaces are equal or not, it can be divided into equal thickness tin plating plate and differential thickness tin plating plate. The former has the same tin plating amount on both sides, while the latter is the tin plating amount on one side of the inner wall and effectively connects the tensile value with the sample gauge distance, and the outer wall side is less. For the empty tank whose outer wall still needs color printing, it is not allowed to have oil return, and the tin plate with different thickness shall be selected

detailed tin plating quantity standard and brand can be found in relevant standards. The main producer of tinplate in China is WISCO. At present, WISCO basically adopts ISO standard. See Table 5 for tin plating amount of electroplated tin plate

table 5 tin plating amount of electroplated tinplate

category standard collectively referred to as minimum average tin plating amount

China ISO JIS g/m2 g/m2

equal thickness tinplate E1 e2.8/2.8 (e25) 255.6 (2.8/2.8) 4.9 (both sides)

e2 e5.5/5.6 (E50) 5011.2 (5.6/5.6) 10.5 (both sides)

e3 e8.4/8.4 (E75) 7516.8 (8.4/8.4) 15.7 (both sides)

e4 e11.2/11.2 (E100) 10022.4 (11.2/11.2) 20.2 (both sides)

difference thickness tinplate 50/255.6/2.8 5.05/2.25

d1 d8.4/2.8 (d75/25) 75/258.4/2.8 7.85/2.25

75/50\8.4/5.6 7.85/5.05

d2 d11.2/2.8 (d100/25) 100/2511.2/2.8 10.1/2.25

100/5011.2/5.6 10.1/5.05

d3 d11.2/5.6 (d100/50) 100/7511.2/8.4 10.1/7.85

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