Lead-calcium alloys corrosion rate

As the grain boundaries move through the casting, they are trapped on defects and impurities. This can result in boundaries that are more prone to corrosion. It has been suggested [25] that heat treatment of the alloy can redissolve the PbsCa particles near the boundary and, thereby, reduce the rate of corrosion of the alloy. Certainly, the mechanical properties, creep rate, and corrosion rates of lead-calcium alloys are... 

Table 2.4. Corrosion behaviour of cast lead-calcium alloys with various amounts of tin. (Corrosion rate mils per year at 0.84mAcm . )...

Following the studies on pure lead [57,64], a great deal of work has been undertaken to determine the effects of tin on the corrosion layer of lead-calcium-tin alloys, which are the major alloys for VRLA batteries. One study [82] showed that the corrosion rate of lead-calcium alloys was significantly reduced by the addition of tin and the thickness of the a-PbO layer was substantially reduced. It was further found that tin enrichment at grain boundaries in cast alloys induced a high level of tin in the corrosion layer that was able to suppress passivation. Finally, it was suggested... 

The introduction of lead-calcium alloys started in the United States in the 1930s (37) and was specified for standby batteries in the Bell telephone service in 1951 (38). Corrosion of the lead-calcium alloy does not affect the electrochemistry of the battery, because calcium is not precipitated at the negative electrode but remains as Ca ion in the electrolyte. As a consequence, the hydrogen evolution rate is low and remains practically unaltered during the whole service life of the battery. 

Wrought lead—calcium—tin anodes have replaced many cast lead—calcium anodes (14). Superior mechanical properties, uniform grain stmcture, low corrosion rates, and lack of casting defects result in increased life for wrought lead—calcium—tin anodes compared to other lead alloy anodes. 

Even higher tin contents (up to 2wt.%) have been reported [89] to provide reduction in the rate of corrosion and growth of positive lead-calcium grids in VRLA batteries employed in standby service at elevated temperatures. The beneficial effects of high tin on positive-grid corrosion in VRLA batteries have recently been confirmed [90]. It is proposed that the improved corrosion resistance is due to the large number of fine precipitate particles and better accommodation of the stresses of corrosion by the high mechanical properties of the alloys. 

---