Tinplate is an important packaging material for food and beverages. In the production process of tinplate, the chromate cathode purification process is generally performed on the tinplate in order to improve the oxidation resistance, corrosion resistance, blackening resistance, adhesion to the paint film, etc. of the tinplate. performance. However, the potential dangers of chromate cathode purification process has forced people to continuously develop various chromium-free surface treatment methods. The following describes the chromium-free surface treatment technology of tin-plated plates developed by Nippon Steel and JFE Steel Corporation.
Phosphate Anodizing Technology
Nippon Steel Corporation has developed a phosphate-based chrome-free surface treatment method for electroplated tin plates, namely phosphate anodizing. This chromium-free purified tin plate is mainly used to make powdered milk cans. The specific treatment conditions were: the anode was purified in a sodium phosphate solution having a pH of 2.5 and a concentration of 30 g/L at 50 degrees Celsius. The experimental results show that in the sodium phosphate solution, the anodic treatment can give tinplate good anti-yellowing and anti-pollution properties. At the same time, cathode pretreatment and then anodization can further improve the antifouling performance of the tinplate. This is mainly because the cathode treatment reduces the original tin oxide on the tinplate, and subsequent anodization can produce dense tin oxide that does not peel off easily. Anodic treatment in phosphates, by transforming iron into iron phosphate, prevents the increase in the content of the tin oxide film and also serves to close the pinhole.
Phosphate-silane treatment
JFE has developed a new phosphating method for low-tin-plated steel sheets: Phosphate-silane treatment (P-Si treatment). The low-tin-plated steel sheet is first to be flash-plated with Ni, and heated and annealed in an N2-H2 atmosphere to form an Fe-Ni alloy layer, followed by plating, electrolytic degreasing and pickling followed by electroplating of tin, followed by reflow . This tin-plated steel sheet has a Fe(Ni)-Sn alloy layer, and unalloyed Sn remains as free tin. The low-tin-plated steel sheet was subjected to P-Si treatment after cathodic treatment in an aqueous solution of sodium carbonate. Specific processing conditions: immersed for 1 to 5 seconds at 40 degrees Celsius to 60 degrees Celsius, and the immersed tinplate is dried with hot air at 50 degrees Celsius to 100 degrees Celsius. In Japan, low-tin-plated steel sheets have a tendency to replace tin-plated sheets, and the low-tin-plated steel sheets have island-like metal Sn and a purification film. Conventional chromate purification membranes are produced after plating in a chromate solution. In this P-Si treatment method, the phosphate is mainly deposited on the Fe(Ni)-Sn alloy site, and the silane film layer is uniformly formed on the metal Sn and the Fe(Ni)-Sn alloy, P-Si The treatment effectively covers the Fe(Ni)-Sn alloy layer, giving the product excellent corrosion resistance.
development trend
According to the results of the chromium-free purification surface treatment technology in the field of tinplates, it can be found that chromate substitutes have evolved from simple composition systems to a variety of compositional systems, often a combination of inorganic and organic compounds with various functions. In addition to a substance that can form a conversion film with tin or itself can form a dense and purified film on a tin plating layer, organic polymers that can form a uniform film layer, such as organosilanes, water-soluble polyacrylic resins, and the like, are also added. In addition, the conductivity can be improved by adding a supporting electrolyte, and the wettability of the surface of the tinplate can be increased by adding some organic surfactant or the like. From the processing point of view, instead of simply pursuing electrolytic purification, a simpler, lower-cost impregnation process is often used, and a roll-free purification process without rinsing has also become a research hotspot. With people's increasing concern for environmental protection and human health, a number of laws and regulations have been gradually formulated to limit the use of chromate, and the use of chromium-free purified surface treatment for tinplates is also an irresistible trend. At present, major steel companies around the world are actively exploring alternative methods for the chromate purification of tinplates. It is believed that in the near future, there will be new breakthroughs in the research on the chromium-free surface treatment of tinplates.
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