Hot-dip tinning coating is a coating technique applied to the surface of a steel plate by immersing the steel in molten tin to form a tin alloy overlay. This process was first used in the manufacture of tinplate and is widely used in food canning, electronics and radio industries, among other fields. Hot-dip tinned steel sheet has good welding performance and bright surface, and has different corrosion resistance in different environments.
Corrosion in the air – Hot-dip tinned steel plates are not easy to rust in dry air, but in humid conditions, due to the formation of local cells on the tin and steel substrates, it may cause anodic dissolution of the substrate, causing rust.
Reactions in acidic environments – In weak acids that do not contain oxygen, contact between tin and steel causes potential reversal, which dissolves the tin and protects the steel matrix from corrosion. In an oxygenated environment, this protective effect may be reversed.
Formation of oxide film - The oxide film is mainly composed of tin oxide (SnO and SnO₂), where SnO is more unstable and SnO₂ has better corrosion resistance. These oxides form gradually during long-term storage or under high-temperature baking conditions.
Oil Film Protection – Hot-dip tinned steel sheets are usually coated with an oil film of 2-10mg/m² to prevent mechanical scratches during transportation and enhance corrosion resistance.
In the hot-dip tinning process, the steel plate reacts with molten tin at high temperatures to form an alloy layer (FeSn₂) composed of iron and tin. This alloy layer adheres to the steel matrix at certain pores and provides an adhesion base for the upper layer of tin. However, due to the porosity of the alloy layer, the contact between tin and steel can still trigger local cell reactions under certain conditions, which can lead to corrosion. High-quality tinned steel sheets require as continuous alloy layers and uniform tin layers as possible.
The process mainly includes pickling, flux treatment, hot-dip tin plating, oil immersion treatment, air cooling and sorting. In the pickling process, the steel plate is usually cleaned with hydrochloric acid or sulphuric acid to remove surface oxides. The flux treatment uses a solution containing zinc chloride and ammonium chloride to help form an iron-tin alloy and enhance the tin plating. After tinning, the plate is subjected to an oil immersion process to prevent oxidation of the tin, and finally cooled and sorted into a finished product.
Hot-dip tinned steel sheets are mainly used in food packaging and the E&E industries, especially for products that require a thick tin layer or special corrosion resistance. Its good weldability and surface properties make it widely used in food cans, electrical materials and radio.
The quality inspection of hot-dip tinned steel plate mainly focuses on the determination of the amount of tin plating and the amount of alloy tin. Commonly used determination methods include electrolytic equivalence method, electrolytic stripping method, radiation method, and potassium iodate titration. With these methods, the thickness of the tin plating and the tin content can be accurately determined, and thus the quality of the tin plating can be evaluated.
Hot-dip tin plating is an important metal surface treatment technique that creates a strong and uniform tin layer by immersing a metal material in molten tin. This process not only significantly improves the corrosion resistance and weldability of the material, but also extends its service life. Hot-dip tin plating is widely used in a variety of fields, including food packaging, construction, electronics and automotive industries, to meet the needs of modern lightweight and durability, and is an important means to improve the performance of metal products.