NICKEL PREPLATED TINPLATE - A TINPLATE WITH INHIBITION OF THE FE-SN ALLOY LAYER GROWTH

In the last few years, a tendency to reduce the tin layer thickness in tinplate has been observed and, currently, low-tin steels are a reali ty in the can market. The reduction of the amount of electrodeposited tin can be a problem if, after the melting/brightening operation, no f ree tin is left as a result of Sn-Fe alloy growth. In this paper, a me thod of inhibition of Sn-Fe alloy growth is proposed by deposition of a nickel layer between the steel and tin layers. Layers with 10 to 100 mg/m2, followed by tin layers of 0.3 to 0.8 g/m2 were deposited on st eel samples. After melting of the coating, the samples were analyzed b y potentiodynamic and galvanostatic dissolution. In a series of experi ments, conventional and nickel-preplated tinplate were compared during potentiodynamic dissolution in a chloride medium. The last of these s howed possible passivation of the alloy layer, formed mainly of the Ni 3Sn4 intermetallic compound, according to X-ray test results. Calculat ions of the electric charge under galvanostatic conditions indicated t hat the amount of unalloyed tin is improved in nickel-preplated tinpla te. Results showed that for nickel layers of 10 to 40 mg/m2, the mass of free tin after melting is always larger, compared to equivalent con ventional tinplate. A maximum of free tin is observed with about 25 mg /m2 of nickel prelayer. Beyond 40 mg/m2, drastic reduction of free tin is observed. Cathodic polarization of the alloy layers was also carri ed out in a citric medium (pH 4). Free tin was removed from convention al and nickel-preplated tinplate so that only the Sn-Fe and Sn-Ni allo ys were left on the surfaces. Tests showed lower corrosion currents fo r the Sn-Ni.