The study characterizes the acid corrosion resistance of six iron alloys containing additions from 0.007 to 1.03 wt. % of tin. The alloys were obtained by way of remelting in a Tamman furnace made of pure iron (99.98% Fe) with additions of spectrally pure tin. The obtained melts underwent studies of acid corrosion resistance by means of an atmospheric corrosion test, electrochemical tests and corrosion rate measurements (determination from the analysis of the corrosion solutions in respect of the Fe2+ content). The performed electrochemical measurements show that both in the environment of sulfuric acid and acidified sodium sulfate, the presence of 0.007−1.03% Sn in the iron does not exhibit a significant effect on the corrosion rate of the examined alloys. The obtained values of icor (determined by way of extrapolation to Ecor and based on Rp), within their measurement error, can be treated as independent of the tin content in the alloy. However, one can observe the effect of the presence of tin on the course of atmospheric corrosion in the acid salt spray – this rate is reduced with an increase of the Sn content in the analyzed samples.

Surface of powdered LaNi 5 intermetallic compound has been modified by active particle coverage with electroless nickel (Ni-P). The electrode degradation process in 6 M KOH solution has been tested across 70 charge/discharge cycles at −0.5 C/+0.5 C rates. It has been established that after approx. 25–35 initial cycles, the electrode degradation process fulfills first order chemical reaction kinetics law: logarithm of discharge capacity linearly decreases with cycle number. The rate constant for the Ni-P protected material is over 20 % lower than that of as received one. The surface modification also improves the alloy hydrogenation kinetics: exchange current densities of H 2 O/H 2 system are generally greater for modified material and, contrary to uncovered material, do not practically decrease with long-lasting cycling.

Effect of small addition of tin (1.7 at.%) into LaNi 4.5 Co 0.5 alloy on gas phase and cathodically charged hydrogen absorption ability as well as its corrosion resistance in 6M KOH solution is discussed. To reveal the effect of Sn doping three alloys have been selected: LaNi 4.5 Co 0.5 (precursor), LaNi 4.5 Co 0.5 Sn 0.1 and LaNi 5 - as a parent compound. The room temperature p-C isotherms indicate to beneficial effect of Sn addition which causes decrease of H 2 equilibrium pressure and does not limit atomic hydrogen solubility. Discharge capacities ( Q disch ), exchange current densities of H 2 O/H 2 system () as well as corrosion rates () have been determined for the tested alloys on the basis of cyclic galvanostatic measurements (at –0.5 C /+0.5 C rates). It has been shown that for N > 3 cycle the discharge capacity of LaNi 4.5 Co 0.5 is ca twofold greater than that for LaNi 5 reference. Addition of 1.7 at.% Sn into Co-containing alloy expands the discharge capacity by 30-40%. The Co containing alloys reveal twice as great exchange current densities of H 2 O/H 2 system compared to LaNi 5 , however, Sn addition slightly decreases the , especially for latest cycles. The partial cobalt substitution for Ni accelerates alloy corrosion in alkaline solution, however, tin addition fully eliminates this effect.

The effect of the fabrication technology of La2MgNi8Co-based alloys doped with 1.7 at.% of Al or In on their electrochemical hydrogenation properties has been examined. Three different procedures have been applied to manufacture test materials. The electrochemical characteristics of the alloys including the discharge capacity, the H2O/H-2 exchange current density, and the hydrogen diffusivity have been studied and presented vs charge/discharge cycling. It is demonstrated that the metallurgical process applied in the alloy manufacturing has a significant effect on electrochemical hydrogenation properties of the final materials. Our investigations indicate that the most promising parameters have been obtained for electrodes synthesized by powder metallurgy route followed by powdered material pressing and sintering. Multi-cycled hydrogenation experiments at high charge/discharge rates (similar to 400/+400 mA g(-1)) reveal a very good cycle life for Al- and In substituted alloys, with discharge capacities as large as 300320 mA h g(-1). (C) 2014 Elsevier B.V. All rights reserved.

The Randles–Sevčik relationship has been applied to evaluate atomic hydrogen diffusivity in massive LaNi 5 intermetallic compound. The electrode was cathodically hydrogenated in 6 M KOH solution (22 °C), and then voltammetry measurements were carried out at various, very slow potential scan rates ( υ = 0.01–0.1 mV · s −1 ). At potentials more noble than the equilibrium potential of the H 2 O/H 2 system, the anodic peaks were registered as a consequence of oxidation of hydrogen absorbed in cathodic range. The peak potentials linearly increase with the logarithm of the scan rate with a slope of 0.059 V. The slope testifies to a symmetric charge transfer process with symmetry factor α = ½. The peak currents linearly increase with the square root of the potential scan rate, and the straight line runs through the origin of the coordinate system. The slope of the I a (peak) = f( υ 1/2 ) straight line is a measure of the atomic hydrogen diffusion coefficient. Assuming the hydrogen concentration in the LaNi 5 material after cathodic exposure to be C 0,H = 0.071 mol · cm −3 (63 % of theoretical value), the hydrogen diffusion coefficient equals D H = 2.0 · 10 −9 cm 2 s −1 . Extrapolation of rectilinear segments of potentiodynamic polarization curves with Tafel slopes of 0.12 V and linear polarization dependencies from voltammetry tests allowed the exchange current densities of the H 2 O/H 2 system on the tested material to be determined. The exchange current densities on initially hydrogenated LaNi 5 alloy are close to 1 mA · cm −2 , irrespective of the electrode potential scan rate.

The effects of cycling on LaNi4.5Co0.5 hydrogen storage material oxidation (during electrode discharge) and reduction of its corrosion products (during charge process) are discussed. The time periods of the Ni and Co oxidation considerably decrease, whereas corresponding oxide phases reduction somewhat increase with electrode cycling. As a result, material corrosion rate increases with cycling with apparent tendency to settle down for final cycles.

The effect of M = In or Al on the hydrogenation behavior of the La-2(Ni,Co,Mg,M)(10) alloys at room temperature is presented. The ceramic like samples have been prepared by powder metallurgy route using pure Mg- and the La2Ni9-xMx alloy powder precursors. XRD analysis revealed predominantly the CaCu5-type structure for all final alloys. Partial substitution of Co by In in La2Ni8MgCo causes a slight decrease of hydrogen concentration whereas Al addition increases this parameter. The highest hydrogen concentration of 1.87 wt.% has been reached for La-2(Ni8Co0.8Al0.2)Mg composition at hydrogen pressure of 10 bar. Indium addition dramatically decreases the middle-plateau hydrogen equilibrium pressure from p(eq) = 0.37 bar (In-free alloy) to p(eq) = 0.06 bar (1.7 at.% In). The electrochemical performance of the studied materials has been characterized using chronoamperometric and chronopotentiometric techniques. The galvanostatic hydrogenation experiments at 185 mA/g discharge rate revealed the largest discharge current capacity of 355 mAh/g for La-2(Ni8Co0.8Al0.2)Mg alloy. The relative diffusivity factor of hydrogen ((D) over bar (H)/a(2)) varies for the tested materials in the range of (2.0-5.4).10(-5) s(-1). Copyright (C) 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

The accuracy of determination of major parameters characterizing the composite, LaNi4.5Co0.5-based, hydride electrode (containing 0.03 g of active material) has been evaluated on the basis of 6 repetitions of 10-cycle charge/discharge curves. Relative error of discharge capacity decreases a little with electrode cycling and attains values on the level of 6% after 10 cycles. The exchange current density of the H2O/H-2 system is charged with an error of 12-15%, irrespectively of cycle number. Material corrosion rate determined in 6 M KOH solution increases with cycling and is charged with ca 15% experimental error.

Derivative galvanostatic charge curves allow for determining of both oxide phase reduction and LaNi5 based composite electrode hydrogenation times with unique accuracy. We present a series of charge/discharge curves for powder composite LaNi5 (50-100 mu m) electrode in 6M KOH solution (22 degrees C) and at vertical bar i(c)vertical bar = i(a) = 0.5C. Effective hydrogen diffusion coefficient, based on Crank's diffusion model, have been determined for the material and presented as a function of cycling. The effective diffusion coefficients of atomic hydrogen increase with cycling from 5 to 7 . 10(-10) cm(2).s(-1) as a result of active material surface development and increase of its porosity. The effects of electrode material corrosion

Hydrogenation properties of LaNi5 (-) In-x(x) alloys (x = 0.1, 0.2 and 0.5) were examined by their direct reaction with gaseous hydrogen and by cathodic charging in 6 M KOH solution. The gas phase measurements were carried out using Sievert's type apparatus in 300-400 K temperature range and at hydrogen pressures up to 40 bars. Indium substitution for Ni in LaNi5 significantly modifies the hydrogenation behavior, decreasing the equilibrium pressure of hydrogen and limiting the hydrogen capacity as compared to LaNi5. The LaNi4.9In0.1 revealed a distinct presence of two pressure plateaus on the high temperature isotherms. Apart from the a-phase (hydrogen solid solution) and beta-phase (LaNi5H6 hydride), formation of a new sigma*-hydride phase was postulated at the hydrogen content extended over the region of H/f.u. = 1.3-1.8. Thermodynamic functions: enthalpy and entropy of the hydrogen absorption process were calculated from the H-2-pressure/composition (p-c) isotherms at several temperatures, applying the Van't Hoff s (Inp - 1/T) dependence. Electrochemical galvanostatic hydrogenation experiments at 185 mA/g charge/discharge rate revealed the greatest discharge current capacity of 319 rnAh/g for LaNi4.0In0.1 alloy after 4-5 cycles. The hydrogen discharge capacities decrease with further increase of indium content in the alloy. Copyright (C) 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

In this paper the effect of indium on the hydrogenation behaviour of the LaNi4,3(Co, Al)0.7-x Inx (x = 0, 0.1, 0.2, 0.3) alloys at room temperature is presented. The pressure–composition (p–c) measurements show that the highest hydrogen concentration of 1.68 wt.% is reached for indium-free (x = 0) alloy. The partial substitution of Al or Co by In causes a slight diminishing of maximum hydrogen concentration. Indium decreases also the plateau hydrogen equilibrium pressure from peq = 0.37 bar (In-free alloy) to peq = 0.06 bar (for LaNi4.3Co0.2In0.2Al0.3Hy) hydride. The electrochemical performance of the studied alloys were characterized using dc. polarization techniques. Electrochemical galvanostatic hydrogenation experiments at 60 mA/g discharge rate revealed the largest discharge current capacity of 305 mAh/g for LaNi4.3Co0.4Al0.2In0.1 alloy. The hydrogen discharge capacity evidently drops with total substitution of aluminium by indium. There is no significant difference in the discharge potentials for the tested electrodes. The relative diffusion coefficient of hydrogen (D/a 2 ) varies in the range of (1.2 to 3.5) ·10 -5 s -1 .

On the basis of potentiostatic discharge method, the diffusion rate of atomic hydrogen as well as its solubility in LaNi5 crystal lattice have been evaluated for three LaNi5 powder - paraffin composite electrodes, with different LaNi5 powder particle diameters: 0 - 20 μm, 20 - 50 μm and 50 - 100 μm. The chronoamperommetric tests have been carried out in strong alkaline (6 M KOH), deaerated solution, at 25°C. Apparent hydrogen diffusion coefficients have been determined using Crank’s spherical diffusion model. It has been shown, that increase of particle size is prone to increase of hydrogen apparent diffusion coefficient and to decrease of hydrogen concentration in the solid phase. To explain the granulation effect on hydrogenation ability parameters, the inhibition of hydrogen transport by surfacial corrosion products present on powder particles has been assumed.

In this paper the effect of indium on the hydrogenation behaviour of the LaNi4,3(Co, Al)0.7-x Inx (x = 0, 0.1, 0.2, 0.3) alloys at room temperature is presented. The pressure–composition (p–c) measurements show that the highest hydrogen concentration of 1.68 wt.% is reached for indium-free (x = 0) alloy. The partial substitution of Al or Co by In causes a slight diminishing of maximum hydrogen concentration. Indium decreases also the plateau hydrogen equilibrium pressure from peq = 0.37 bar (In-free alloy) to peq = 0.06 bar (for LaNi4.3Co0.2In0.2Al0.3Hy) hydride. The electrochemical performance of the studied alloys were characterized using dc. polarization techniques. Electrochemical galvanostatic hydrogenation experiments at 60 mA/g discharge rate revealed the largest discharge current capacity of 305 mAh/g for LaNi4.3Co0.4Al0.2In0.1 alloy. The hydrogen discharge capacity evidently drops with total substitution of aluminium by indium. There is no significant difference in the discharge potentials for the tested electrodes. The relative diffusion coefficient of hydrogen (D/a2) varies in the range of (1.2 to 3.5) ·10-5 s-1.