The Electrochemical Behavior of β-Phase in the Cu30Ni-H System in Sodium Hydroxide Solutions

Abstract

During cathodically hydrogenizing a copper-nickel alloy, hydrogen is incorporated into the solid. It was long ago shown by X-ray diffraction analysis [1] that such alloys containing more than 60% copper and hydrogenized in a 0.2 N H 2 SO 4 + 5 × 10 -3 g/l As 2 O 5 solution ( i h = 30 mA/cm 2 , t h = 30 min) form with hydrogen only α -phase, that is, an interstitial solid solution. In alloys not so rich in copper, two phases, that is, α -and β -(a hydride one), are formed. It was also shown that it is the copresence of the α -and β -phases, rather than the total hydrogen content that makes the alloy embrittle and destruct under mechanical load [1].It is known that intermetallic compounds of LaNi 5 type form hydrides under cathodic polarization [2,3]. Partial substitution of copper for nickel makes the hydride phase more stable and significantly enhances the hydrogen absorption by the alloy. Therefore, we aimed at studying the electrochemical behavior of Cu30Ni alloy hydride in alkaline solutions and interpreting it taking into account our former data on the electrochemical behavior of hydrogenated copper [4] and nickel [5,6] and nickel hydride as well [7,8].The experiments were carried out in 1 N NaOH solution in N 2 (of PNG grade) atmosphere at 20 ° C. Prior to the experiments, the alloy was cleaned with emery paper, degreased, and rinsed with twice-distilled water. The hydride ( β -phase) was synthesized in 1 N H 2 SO 4 + 0.2 g/l thiourea solution under cathodic polarization ( i h = 500 A/m 2 , t h = 1 h) [9]. On the completion of the synthesis, samples were rinsed with twice-distilled water and placed into working cell filled with a 1 N NaOH degassed solution. Potentials were measured against silver-silver chloride reference electrode and converted to the SHE scale. The electrochemical measurements were performed by transient electrochemical methods and potentiostatic voltammetric curves (VACs).In Fig. 1 we show cathodic and anodic VACs of the hydrogenated alloy ( α -phase) (curve 1 ) [10] and of the alloy hydride ( β -phase) (curve 2 ) recorded by shifting the potential in the positive direction by steps of 0.01 V. The onset of hydrogen evolution at the hydride is observed at a potential by 0.2 V more negative than at the α -phase; oxygen evolution does not depend on the hydrogen content in the alloy. Cathodic voltammetric curves of the alloy hydride contain a cathodic limiting current plateau at -0.9 to -0.35 V like that for nickel hydride [7,8]. The initial segment slope of the anodic curve equals 0.06 V/dec. At a potential of -0.2 V, anodic current peak is observed, whereas from 0 to 0.7 V, we see a limiting anodic current, like at a hydrogenated alloy (curve 1 ).The cathodic limiting current in the hydride curve (curve 2 ) can be associated, like in the case of nickel hydride [7,8], with preceding chemical reaction of the hydride phase decomposition in the alloy. From the shape of the curve 2 it follows that the alloy hydride decomposition kinetics can be studied only in a potential range more negative tha...