The tin plating unit consists of three main process sections: pre-treatment, electroplating and post-treatment.
The pretreatment of electroplating tin consists of pickling, caustic washing, water washing and brushing in order to make the plating quality good and the surface of the strip steel plate must be clean.
(1) Alkali washing
The purpose of caustic washing: The strip steel passes through the hot caustic solution in the caustic tank, electrolytic cleaning takes place under the action of the energized electrode, the oil and dust of the strip surface pass through the chemical action of the caustic washing solution and electrolysis generates hydrogen and oxygen machinery. Scouring is removed.
The purpose of pickling is to remove the oxide film and oil stains on the surface of the original plate and obtain a clean surface suitable for tin plating.
The pickling operation has a great influence on the quality of the tin-plated surface:
A. If the pickling is bad: Oxide and oil are still left on the surface of the original plate, which will cause tin defects;
B. Excessive pickling: roughens the surface of the original board, loses gloss, and increases the amount of tin plating.
Electroplating tin process: high-speed continuous production process, a wide range of tin plating thickness, tin layer control is simple and accurate, and the quality of the final tin layer is stable.
According to the use of different electrolytes can be divided into the following
A. Acid sulphate type---Floster method; B. Aqueous solution of stannous halide and alkali metal fluoride---halogen method; C. borane type; D.Alkaline stannate type
Bath: A solution of divalent tin in phenol sulphonic acid (PSA). Additives are added to the bath to prevent oxidation of the tin divalent to tetravalent tin. The last bath in the electroplating section is the rinse bath and the bath is recovered to reduce the bath loss, pass through several pairs of squeeze rollers, and then dried in a hot air drying oven.
The tin on the strip after electroplating is observed under the microscope as fine-grained tin particles with poor adhesion and no gloss. Usually the strip is heated to a temperature above the melting point of tin to melt the tin layer and then immediately quenched into water. It becomes a shiny tin plate.
During the reflow process, the tin-plated layer is instantaneously melted to form a bright surface on the tinplate. During the reflow process, a thin layer of iron-tin alloy is formed at the interface between the tin-plated layer and the iron substrate, which improves tin solderability and corrosion resistance of the tin-plated sheet. The reflow operation takes only a few seconds and slightly exceeds the melting temperature of tin.
Heating method: There are two methods of induction reflow heating and resistance reflow heating, 00 uses resistance reflow heating method, this method is to make large alternating current through the conductive roller, strip, the heat generated to make the tin coating melt.
The reflowed tinplate has a thin layer of tin oxide film on the surface. This oxide film will continue to grow during the long-term storage of the tinplate or during baking after painting. Tin plate discoloration. The purpose of the passivation treatment is to stabilize the oxide film and improve the resistance to color change of the tinplate. Passivation methods: H (chemical passivation), Y (negative electrochemical passivation), passivation solution: sodium dichromate solution. The naturally occurring tin oxide film is dissolved by chemical action or electrochemical action, and a very thin but very dense tin oxide film is formed, which isolates the tin layer from the outside air and plays a good protective role.
After the passivation treatment of the steel strip, the treatment solution is completely washed away in the washing tank and then dried.