THE EFFECT OF SELENIUM ON THE ELECTROCHEMICAL-BEHAVIOR AND CORROSION OF PB-SN ALLOYS USED IN LEAD-ACID-BATTERIES

In valve regulated batteries of the adsorbed glass mat type the connec ting strap and lugs of the plates (top lead) are covered by a thin liq uid film of H2SO4 solution. The H2SO4 is consumed for the formation of a PbSO4 corrosion layer. The liquid film has high ohmic resistance an d hence the strap and lugs of the plates are cathodically unprotected. As a result of this, often the life of the battery is limited by corr osion of the negative semiblock top lead. In battery manufacturing pra ctice, Pb-Sn alloys are often used for the straps in the above battery type. The present paper examines creeping of the thin liquid film up a strap electrode, the structure and phase composition of the corrosio n layer formed on the electrode surface, as well as the rate of corros ion of Pb-Sn and Pb-Sn-Se electrodes partly immersed in an absorbing g lass mat soaked in H2SO4 or H2SO4 + Na2SO4 solutions. It has been esta blished that the potential of that part of the electrode immersed in t he solution controls the potential of the electrode up to a height of 1 cm above the solution level. The corrosion layer formed on the elect rode surface above the solution (in the air) features zones of differe nt phase compositions. The introduction of Na2SO4 to the H2SO4 solutio n leads to a sevenfold increase in corrosion rate of Pb-Sn alloys, whi le the addition of 0.03% Se to the alloy suppresses completely this ef fect of Na2SO4. When the Sn content in the alloy is below 1%, the rate of corrosion increases. Selenium suppresses this effect, too, in allo ys containing 0.6% Sn. Selenium has an anticorrosion effect and acts a s a grain refiner in Pb-Sn alloys decreasing also the size of PbSO4 cr ystals and facilitating their nucleation.