Electrodeposition of Tin from Acid Baths
Electrodeposition of tin from sulfate and methansulfonic baths has been studied. In sulfate solutions Sn(II) was oxidized into Sn(IV) and then hy-drolyzed into metastannic acid. Oxidation rate of Sn(II) was found to be lower in the methansulf onic solution (Figs 1,2) due to lower concentration of dissolved oxygen. This leads to an increase in the acidity of the solution: one mole of acid for one mole of oxidized Sn(II) (Fig.4). Acidity of the methansulf onic solution is gradually reducing both during the storage and operation (Fig.4) due to the oxidation of Sn(II), however, the hydrolysis rate is negligible and the solution is being clear. An additive belonging to the class of aromatic amines has been proposed to prevent the oxidation of Sn(II) (Fig. 1,2,3). This additive is electrochemically inactive. A new brightener - sulfoalkyl-polyalkoxilated naph-tol has been found which allowed to eliminate formaldehyde from the additives combination. This additive in a combination with benzylidene acetone enables to enlarge the current density range where bright coatings are obtained. Cathodic reduction of benzylidene acetone proceeds at limiting current density (Fig.V). These data were used in order to calculate the consumption of brightening composition as a whole and to work out the instructions for bath replenishment in the course of industrial operation. Temperature of the bath should not be above 30°C provided bright coatings should be obtained especially for shaped parts and lower current densities. Therefore tanks should be equipped with cooling systems. In order to remove fine fraction of metastanic acid from the solution addition of polyacrylamide was used as a flocculant (Fig.9) and maximum degree of purification was reached at its concentration equal to 5 mg per 1 g of metastannic acid (as metal tin).