Influence of Microstructure and Crystallographic Texture on the Surface Brightness of Industrially Produced Tinplated St
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INTRODUCTION
TINPLATING of low-carbon steels is a commercially important process and is extensively used in packaging industries, especially in the food sectors.[1,2] Besides its good corrosion resistance and non-toxic properties, the bright tin coating gives an aesthetic appeal to the products as well. Many tinplating customers look for this property as a value addition. The initial electrodeposition process of tin on steel gives a dull finish (matte). It is followed by the flow-brightening treatment, i.e., heating the matte tinplated steels briefly above the melting temperature of tin (232C) and quenching thereafter, which gives the bright luster to the products.[3] During flow-brightening treatment, a fraction of total Sn reacts with Fe and forms a thin layer of Fe-Sn intermetallic at the interface. The quality and continuity of this interlayer dictate the protective property of the tinplating,[4] while the top re-solidified Sn gives its beautiful luster. Researchers have reported interdependency of surface roughness and brightness in many coating and plating processes. Bigerelle et al.[5] worked out one such correlation in brass specimens, while Darrort and his co-workers[6] experimentally showed that brightness of an electroplated Nickel layer increased when roughness
PAMPA GHOSH, and MAHADEV SHOME are with R&D Tata Steel, Jamshedpur, India. Contact e-mail: [email protected] DYUTI MONDAL is with the Materials and Metallurgy Department, IIEST, Shibpur, West Bengal, India. AASHUTOSHA TIWARI is with the Tata Tinplate, Jamshedpur, India. Manuscript submitted July 26, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS A
decreased and a Gaussian relationship existed between them. Well and Paquin[7] indicated similar relationship in their electroplated Ni coating on copper. In industrial tinplating practice, temper rolling prior to tin plating process is used to control strip roughness.[8] Besides, suitable additives are also added to electroplating baths to brighten the products. Here the brightness also depends on the extent to which morphological components of the surface are in the same plane, i.e., the flatness/roughness of the surface.[8] However, close control of these processes does not always guarantee similar brightness in the strips and quite often differs significantly from one sheet to another.[9] This implies that additional parameters are also impacting the luster of tinplated steels. The present study has been undertaken to find out these parameters and their influence on the tinplate brightness. Materials’ properties are often found to affect the final properties of coatings. Granzier et al.[10] showed that the color constancy of surfaces depends on the material properties like roughness and gloss. They experimentally proved that higher gloss and lower roughness resulted in better color constancy. The coating structures are often influenced by substrate conditions apart from the coating itself. Saikia et al.[11] investigated the oriented growth of TiN on bare and plasma-nitrided high-speed steels. Preferentia
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