The Behaviour of copper-tin alloys in alkaline solutions upon alternate anodic and cathodic polarization

Abstract

Galvanostatic cyclic anodic and cathodic polarization curves of seven copper-tin alloys were traced on 0-01, 0-1 and 2N NaOH solutions at 25°. The tin content of the alloys was varied between 8 and 76 wt-%. The results obtained are explained in relation to the phase diagram of the copper-tin system. Alloys corresponding to the α-phase (C 14 wt- % Sri) behave in a manner similar to that of pure copper. Arrests in the anodic curves are attributed to oxidation to Cu20 and Cu(OH)2 and to the dissolution as HCuO2 . CU2O3 forms also at oxygen-evolution potentials. Similarly, alloys of the η + ω' region (> 60 wt- % Sn) give polarization curves identical with those for pure tin (ω-phase). The tin of the electrode oxidizes to Sn(OH)2 and Sn(OH)4 before oxygen evolution.Alloys falling within the α + δ a region limits (14-32 wt- % Sn) behave mainly as copper but show, upon oxidation, a new arrest at potentials positive to any of the known redox systems of either metal. This arrest is ascribed to the oxidation of the δ-phase (or its copper component). The tin of these alloys oxidizes to a very limited extent.Alloys falling within the ε + η region (38-59 wt-Y. Sn) behave as if they were formed of pure copper and pure tin, each giving its own characteristic oxidation arrests independent of the other.All alloys require upon oxidation less quantities of electricity to passivate than do the pure metals.Critical current densities for the passivation of copper, tin and the seven alloys are determined in 0-1 M Na2SO4. Comparison is made between these quantities and the passivation indices calculated in alkaline solutions.