Study On The Degradation Behaviours Of RE-Mg-Ni-based Alloys

The key problem during the development of RE–Mg–Ni–based hydrogen storagealloys is to improve the cycling stability. To define the degradation characteristics of thisseries alloy, structural stability, pulverization and corrosion behavior of the commercial AB5alloy, commercial (LaPrMg)2(NiAl)7alloy and unalloyed La0.88Mg0.12Ni3.45alloy werestudied using crystal structure characterization, phase analysis, morphology and granularitymeasurement and corrosion electrochemical test.The commercial AB5alloy is single–phase simply material with CaCu5–type structure,while (LaPrMg)2(NiAl)7alloy and La0.88Mg0.12Ni3.45alloy show complex multiphasemicrostructure. Performance of both the gas–solid and electrochemical hydrogen storagecapacity of these three alloys is(LaPrMg)2(NiAl)7＞(LaCePrNd)(NiCoAl)5＞La0.88Mg0.12Ni3.45, but the electrochemicalcycling stability is (LaCePrNd)(NiCoAl)5＞(LaPrMg)2(NiAl)7＞La0.88Mg0.12Ni3.45. Impactof100times gas–solid cycling is weak on (LaCePrNd)(NiCoAl)5and (LaPrMg)2(NiAl)7alloy. But hydrogen storage capacity and crystal structure of La0.88Mg0.12Ni3.45alloy arechanged remarkably. Hydrogen–induced amorphization (HIA) of La0.88Mg0.12Ni3.45alloytakes place during hydrogen absorption/desorption cycles. HIA increases the stability of theamorphous La–Mg–Ni based hydrides and makes the plateau tilting severely. Among thethree kinds of alloy, the pulverization of (LaCePrNd)(NiCoAl)5alloy was the most serious,while the anti–pulverization of La0.88Mg0.12Ni3.45alloy was the best, however thepulverization was not uniform, little (LaMg)Ni2phase in the alloy was easily flaked duringhydrogen absorption, which lead to the frequency of alloy particles about10μm increaseobviously. The key reason why the two RE–Mg–Ni alloy invalided was the corrosion in theelectrochemical solution, especially the corrosion of La0.88Mg0.12Ni3.45alloy was so serious,the oxygen content in the alloy after the cycle was almost three times of La0.88Mg0.12Ni3.45alloy. Electrochemical corrosion showed that the corrosion thermodynamics and dynamicstendency of La0.88Mg0.12Ni3.45alloy were higher, which was the inner reason of the serious corrosion.To explore the relative content effect discipline to the stability of multiphase systemcycle, the stability characteristic of the different proportion of(LaPrMg)2(NiAl)7/(LaCePrNd)(NiCoAl)5alloy composite system cycle was also studied inthis paper. The effect of relative content of two kinds of alloy to the hydrogen storagecapability of the alloy presented simple accumulate relationship, the biggest capacity bothreduced with the amount addition of (LaCePrNd)(NiCoAl)5alloy. However the effect to thestability of the composite system electrochemical cycle was great, when the proportion ofthe two original alloy was equal, the stability was the best, the reason was that the corrosionof that proportion was the most weak.