Tin-silver-calcium alloys for low corroding VRLA positive plates

In this work, the authors present the life test procedures and results of their search for a lead alloy grid having a substantially lower corrosion rate than that they presently employ. Their interest was in investigating the alloying elements of tin and silver. The factorial design matrix employed required nine alloy compositions to be tested. The alloy element concentrations varied within the following limits, (i) calcium 0.075%; (ii) silver 0.005 to 0.033%; (iii) tin 0.30 to 2.0%. The life tests were performed at 60°C and at a positive plate polarization at this temperature was 130 mV. The results of the tests showed a decreases in the plate growth rate of 3 to 4 times for alloys containing 2% tin compared to that for 0.30% tin. The alloying of silver with tin and calcium had little if any effect on the plate growth rate. Optical microscopic examination of the various gravity cast alloys found them to be similar in lead crystal size and orientation. Also, the PbO₂ corrosion layer at the higher tin concentrations showed less intergranular penetration and a thinner corrosion layer for the same time on test. Tafel measurements were made to determine the relative alloy electrochemical characteristics. In addition to the plate growth tests each cell was discharged and the correlation of capacity to plate growth established. As expected, the capacity loss rate was greatest for those faster corroding grid alloys.