Surface Properties Of Double Reduced Tinplate And The Corrosion Resistance Of Tinplate

Double Reduced Tinplate, called as DR materials, is thinner than common tinplate but its mechanical strength is higher than common tinplate. By second cold rolling, the thickness of sheet reduced, a certain roughness was obtained and the surface quality was improved to satisfy the client needs. The double reduced tinplate is used to manufacture the compression resistant pot body, bottom and cover in food product or chemical industry for its good corrosion resistance, lustrous appearance, innocuity and stabilization in organic solution.The steel sheets which were used to produce common tinplate, called as MR materials, are manufactured by cold rolling, annealing and skin rolling. In production of DR materials, a secondary cold rolling is used to replace the skin rolling. The surface appearance and property of DR materials are most different from those MR materials.In present study, the effect of conditioning of DR and MR materials to the quality of tinning coating was examined. The surface state, especially microscopic surface state, electrochemical performance between tin coating and substrate before and after electrolytic tinning were also investigated, the flaws in tinning coating were tracked. The mechanism and reasons were discussed in order to improve the technology of conditioning DR materials. The factors that effect to the quality of tinning coating were found out through comparison of the difference in surface properties between DR and MR materials to improve the plating quality.Surface state (roughness) and electroplating parameters have a large influence on the plating quality. The properties of tinning coating have an effect on corrosion resistance in turn. Primary sheets of DR materials are smoother than those MR materials. Therefore, the luster of DR materials after tinning is better than that of MR materials. Although tinning acts as flatting, the roughness of MR materials is still higher after tinning. The orientation of crystal plate in primary sheet of tinplate focused in<111> crystal direction parallel to rolling direction, which has an effect on the crystalline structure of tinning coating, that is, the grain orientation in tinning coating also focused in<111> direction. As a result, the grain orientation in primary sheet decides the grain orientation in tinning coating of tinplate and hence the corrosion resistance of tinplate. According to the electrochemical test, the corrosion resistance of DR primary sheet is better than that of MR materials. However, their corrosion resistance after tinning has no evident difference because they have the same tinning coating. The effect factors on corrosion resistance of DR tinplate including the content of retained oil on the surface, the current density during tinning, the concentration of Fe2+ in solution and the thickness of tinning coating were discussed.