TRANSIENT EFFECTS OF LITHIUM HYDROXIDE AND BORIC-ACID ON ZIRCALOY CORROSION

The effects of transient exposures to high concentrations of LiOH (1.0M) at 300 degrees C (573 K) were investigated. Weight gains were small (or negative) during the transient, but porosity developed rapidly throughout the oxide. A return to a lower concentration LiOH solution (0.1M) permitted the regrowth of a barrier oxide, and a weight gain transient similar in magnitude to the initial oxidation curve ensued. The oxidation rate returned to a value close to that before the LiOH transient after about 14 days, with the addition of an increment of weight gain close to that existing prior to the transient. No effect of boric acid was detected on the development of porosity by local oxide dissolution in 0.1M LiOH solution. Pore depths and frequencies were similar for LiOH solutions with and without boric acid additions. The magnitude of the weight gain transients during subsequent regrowth of barrier oxides after a 1 day exposure to 1.0M LiOH also appeared to be unaffected by the presence of boric acid during regrowth. Only when boric acid was added to 1.0M LiOH during the brief 24 h transients employed here did it reveal an ability to inhibit the growth of the deep pores that penetrated close to the oxide-metal interface. Thus, boric acid appears to reduce corrosion rates in LiOH solutions by preventing the deep penetration of pores in the oxides. It apparently has no effect on the formation of shallow pores, however. It is thought that enhanced reprecipitation of a zirconium berate complex plugs any deep pores as they form and prevents the development of the deep pore network.