Effect of surface contamination on the hydriding behaviors of LaNi4.5Al0.5, LaNi2.5Co2.5 and LaNi4.5Mn0.5

The aim of this study was to elucidate the role of each substituent element such as Al, Co and Mn in the hydridings of LaNi4.5Al0.5, LaNi2.5Co2.5 and LaNi4.5Mn0.5 alloys exposed to contaminative conditions. The addition of Al was found effective to yield the H-2-dissociation-controlled kinetics even in extended surface contaminations. The effect of surface oxide containing Co seem to remain even in the hydriding reaction, indicating the formation of stable oxide layers for the Go-containing alloys in spite of the occurrence of a small volume expansion due to hydriding. The addition of Co yields the inhibitive pulverization, which indicates that the Co addition acts to form coherent interfaces between surface oxide layers and the alloy. The addition of Mn increases the sensitivity to contamination and markedly decreases the reaction rate in the formation of H solid solution where no pulverization takes place. However, because the pulverization is accelerated by the presence of various nonstoichiometric Mn oxides, the hydriding rate recovers to those of LaNi5 and LaNi4.5Al0.5 with increasing number of cyclic hydriding-dehydriding reactions. The limited applicability of the Johnson-Mehl-Avrami-type equation to the measured hydriding curves is discussed from the measured kinetic results.