The electrolyte composition and the electrodic conditions have a major effect on the entry of electrolytic hydrogen into metals. In the case of ferrous metals there is a large body of literature and various promoters have been identified. Only a few inhibitors have been found, such as organic nitriles. This paper reports the complete inhibition of the entry of hydrogen into iron by UPD Zn in concentrated alkali solutions. Less is known about the effect of electrolyte on the entry of hydrogen into palladium. The present work shows that many of the known promoters for ferrous metals actually inhibit the entry and egress of hydrogen from palladium. Permeation results on a Pd membrane in pure 0.1 M NaOH indicate that only 20% of the surface is used for the entry of hydrogen into the metal. In 0.1 M NaOH + 10{sup {minus}3} M NaCN it drops to 5%. The fraction of the surface used strongly depends on electrolyte purity. Impurity effects can account for the discrepant results for electrochemical hydrogen loading of Pd

McBreen, J.

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_- . BNL-46107JBNL--46107ELECTROLYTIC HYDROGEN-METAL INT2RACTION8 DE92 007600James McBreenDepartment of Applied Science :i'_:i:!iBrookhaven National Laboratory //I;()Upton, NY 11973 ' ,_'_ _ABSTRACTThe electrolyte composition and theelectrodic conditions have a major effect on the '_entry of electrolytic hydrogen into metals. Inthe case of ferrous metals there is a large bodyof literature and various promoters have beenidentified. Only a few inhibitors have beenfound, such as organic nitriles. This paperreports the complete inhibition of the entry ofhydrogen into iron by UPD Zn in concentratedalkali solutions. Less is known about the effectof electrolyte on the entry of hydrogen intopalladium. The present work shows that many of' the known promoters for ferrous metals actuallyinhibit the entry and egress of hydrogen frompalladium. Permeation results on a Pd membrane inpure 0.I M NaOH indicate that only 20% of thesurface is used for the entry of hydrogen into themetal. In 0.I M NaOH + 10 .3 M NaCN it drops to5%. The fraction of the surface used stronglydepends on electrolyte purity. Impurity effectscan account for the discrepant results forelectrochemical hydrogen loading of Pd.INTRODUCTIONThe effect of the electrolyte and electrodic conditionson the entry of hydrogen into palladium and ferrous metalsis the subject of an extensive literature and severalreviews (1-4). As early as the mid thirties, several groupsworking on the entry of hydrogen from the gas phase realizedthe relevance of adsorption and surface coverage to thekinetics of the entry of hydrog_en into metals (5-7). Alsoaround the same time, Kobosev and Montblanova (8-9) deducedthe relationship between the energetics of adsorption andthe ingress of hydrogen into the metal. The energetics ofthe entry of hydrogen into metals from the gas phase hasbeen treated, more recently, by Pick and Sonnenberg (i0).,,,,.,,,,,ov,,u,, vr T_l'",o068,J_Vic)_lla i Nki[_lltU_,.,.,, i i , iJ qIt was only in the sixties that many of these importantconcepts were developed and applied, by Bockris and his co-workers, to the entry of electrolytic hydrogen into metals(11-13). The delay in application of these concepts to theentry of hydrogen into metals was caused mainly by the slowdevelopment and acceptance of the concepts electrodekinetics. Another contributor was the two conflictingkinetic models have been proposed for the entry ofelectrolytic hydrogen into metals. One view of the entry ofelectrolytic hydrogen into metals is that hydrogen entersthe metal in the same elementary act in which it isdischarged, and the intermediate state through whichhydrogen enters the metal lattice is not the same as theadsorbed intermediate which leads to the evolution ofmolecular hydrogen (i, 14, 15). The other view is that theelectrolytic hydrogen on entering the metal must go throughan adsorbed state which is identical to the intermediatethat leads to hydrogen evolution (12). The onlyexperimental work that supports the first mechanism is thework of Palczewska en the entry of hydrogen into nickel fromgas phase in the presence of a radio-frequency hydrogenplasma (16). From the effects of applied electric field onthe plasma it was concluded that H. ions,not H atoms,entered the metal. The current and the potentialdependence of the entry of hydrogen into iron agrees withsecond mechanism. Also the analysis of Pumphrey (17) ofpermeation transients has shown that the boundary conditionat the input side is one of constant surface coverage ratherthan surface concentration. Furthez_ore he deduced that theflux entering the metal is limited by the finite rateconstants for the transfer of hydrogen between the adsorbedand absorbe_ states. There is general agreement that in thegas phase hydrogen goes through an adsorbed intermediate andthat the hydrogen coverage is a_ important parameter in thekinetics of the entry of hydrogen into the metal. Morerecently the work of Graham and his co-workers, usinganalysis potentiostatic desorption current transients,confirmed that this is true in the case of iron in pureHzSO 4 electrolyte (18). In this case about 2% of thedischarged hydrogen enters the metal. The remainder isevolved as hydrogen gas. In electrolytes containingpromoters such as H_ as much as 25% of the dischargedhydrogen can enter the metal. In this case the currenttransients indicated that the process was controlled bydiffusion in the metal and not interfacial processes. Theearlier work of Bockris and co-workers has shown that highconcentrations of hydrogen in iron can damage the metal andaffect diffusion measurements (19). So any conclusions fromcurrent transients obtained under such conditions have to betreated with care. At present it can be concluded thatsurface coverage and interfacial processes are important in, , qq , ' rl ' _1 _ i,p i%determining the ingress of electrolytic hydrogen intometals.It is well known that additions of S" to acids and CN"to alkaline electrolytes promote the entry of hydrogen intoferrous metals. The early work on the effect of anions onthe entry of hydrogen into palladium has been reviewed (i)Recent work indicates that the effect of the electrolyte isdifferent for ferrous metals and palladium (20). In thispaper these differences are reviewed and new results arepresented on the effect of the electrolyte on the entry ofhydrogen into palladium and the effect of UPD Zn on theentry of hydrogen into iron.EXPERIMENTALThe effect of electrolyte on the entry of hydrogen intopalladium was investigated by cyclic voltammetry andpermeation methods. Permeation methods were used to studyelectrolyte effects on the entry of hydrogen into iron.Details of the experimental methods are given els@where (19,20). Permeation measurements were made using the bi-electrode coulometric technique of Devanathan and Stachurski(ll). The cell is shown in Fig.l. The electrolyte on thediffusion side of the membrane was 0.i M NaOH. Thepotential on the diffusion side was held at -0.5 V vs.Hg/HgO.RESULTS AND DISCUSSIONEffect of ElectrolyteThe interaction of electrolytic hydrogen with palladiumin pure electrolytes is illustrated by the cyclicvoltammograms for 0.I M NaOH that are shown in Fig. 2. Thegeneral features of the voltammogram that extends into thehydrogen region is very similar to that reported for 0.3 MNaCIO 4 (2.1). Arresting the potential at -i.0 V for 5 sgreatly Increases the hydrogen desorption current. In allcases complete desorption of the hydrogen occurred beforethe onset of oxide formation at -0.3 V.Most of the common electrolyte additives that are knownto promote the entry of h_drogen into ferrous metalsactually inhibit the entry of hydrogen into palladium.This effect can be seen in Fig. 3 which shows cyclicvoltammograms for 0.i M NaOH + 10 .3 M NaCN. Results in theoxide region show that the oxide formation process isinhibited by CN'. There was no indication of Pddissolution. In the hydrogen region the absorption processis greatly inhibited. The major effect, however, is on thehydrogen desorption process. Most of the hydrogen appearsto be desorbed in a narrow potential region around 0.05 V.This is mo_oe clearly seen after the potential arrest at -I.0V. Similar, though less striking, effects were observed for5 x 10 .5 M additions of TeO2, As203 and Na2S to 0.i M NaOH.In the case of ferrous metals ali of these additlves promotethe entry of hydrogen into the metal.A major problem in the control of hydrogenembrittlement of steel during pickling or electroplatingoperations is that there is the lack of inhibitors forhydrogen ingress to the metal. Till now the only reportedinhibitors were organic nitriles (12). Dra_ie and Vorkapi_have reported that the addition of zinc ions to theelectrolyte inhibits the hydrogen evolution reaction on iron(22). _Chey speculated that the effect may be due tounderpotential deposition of zinc on the metal. In thepresent study the UPD process and the entry of hydrogen intoiron was investigated in pure KOH and zincate electrolyte.In dilute solutions of KOH and zincate no UPD process couldbe observed. This may be due to the high hydrogen evolutioncurrents in these electrolytes. However in concentratedelectrolyte (8.4 M KOH + 0.74 M ZnO) a clear UPD patterncould be seen in the voltammogram shown in Fig. 4. Figure5 shows the effect of zincate on the hydrogen evolutionreaction and on the permeation rate through a 1.0 mm ironmembrane. In pure KOH the Tafel slope is 2RT/F and thecorresponding variation of the permeation current is 4RT/F.In the zincate containing electrolyte the inhibition of thehydrogen evolution reaction can be clearly seen. There issome hysteresis, depending on whether the measurements aremade with increasing or decreasing overvoltage. The moststriking feature, though, is the complete inhibition ofhydrogen permeation by the UPD zinc. The fact that the UPD[process can only be observed in concentrated electrolytesmay be due to a chemisorption process, prior to UPD, thatdoes not involve a net Faradaic current. Recentelectrochemical quartz balance studies of UPD of Pb indicatethat such processes can indeed occur (23). There is alwaysthe trivial explanation that the effect is due to someimpurity in the zinc oxide. However, it is too much of acoincidence that the observed underpotential shift is aboutthat expected for zinc on iron.Uniformity of the Surface ProcessFigure 9 shows plots of the cathodic current and thepermeation current for a 0.025 mm Pd membrane in pure NaOH,and in alkaline cyanide electrolyte. Over most of thehydrogen adsorption region the currents balance. In otherwords, all discharged hydrogen permeates through themembrane and arrives at the other side. There aredeviations from current balance at extreme negative and_ positive potentials. The deviations can be caused by theoccurrence of a parasitic cathodic process (e.g. oxygeniii:! / <reduction), hydrogen evolution, hydride formation or/ihydrogen trapping in the metal. The deviation from current,_ i balance at low currents can be ascribed to either the.... occurrence of another cathodic process such as oxygen, reduction or to hydrogen trapping in the metal. Trapping is.... the more plausible explanation The calculated C0f(C 0 = Hconcentration on input side) for the convergence o thecathodic and permeation current indicate that the traps canhold about 5 x 10 .6 g-atom of H/cm 3 of Pd. The divergence inthe cathodic current at higher potentials is due to acombination of hydrogen evolution and formation of the B-phase. The formation of the B-phase is associated with slow/ permeation decay transients. These slow decay transients,obtained at negative potentials, indicate a process governedby decomposition of the B-phase rather than diffusion.These are characterized by an initial very slow decay_ i_ followed by a faster decay process. The time for completionof the slow process depends on the cathodic potential andthe time of polarization The slow process corresponds tohydride decomposition and the fast process is completion ofthe diffusion process. The calculated C0 at which the slowdecay transients were observed in pure NaOH was about fivetimes lower than the terminal solubility of the s-phase. In0.I M NaOH + 10 .3 M NaCN it was about twenty times lower. Apossible explanation is that certain regions of the s-phasetransform to the hydride more readily than others (24).This also strongly suggests that only a fraction of thesurface is used for the entry of hydrogen into the metal.If this is true then only 20% of the surface is used in pureNaOH and a mere 5% is used in the cyanide containingelectrolytes. The recent electron microscopy results ofRolison support this hypothesis (25). These resultsindicate that the fraction of the surface used for theingress of hydrogen into palladium depends on the purity ofthe electrolyte. Apart from electrolyte effectsmetallurgical effects may also be involved, which couldaccount for the low surface utilization in the pureelectrolyte. All these effects could be a contributingfactors in the lack of reproducibility of the Pons-Fleischman effect.rACKNOWLEDGMENTThe author gratefully acknowledges support of the U.S.Department of Energy under Contract Number DE-AC02-76CH00016._REFERENCESi. A.N. Frumkin, in P. Delahay and C. W. Tobias(Eds.),Advances in Electrochemistry andElectrochemical Engineering, Vol. 3, Wiley New York,1963, p.287.2. T. Zakroczymski, in R. A. Oriani, J. P. Hirth and M.Smialowski (Eds.), Hydrogen Degradation of FerrousAlloys, Noyes Publications, Park Ridge, NJ, 1985,p.215.3. J. McBreen and M. A. Genshaw, Proc. of Conf. onFundamental Aspects of Stress Corrosion Cracking, OhioState Univ., Columbus, OH, Sept. 10-15, 1967, R. W.Staehle, A. J. Forty and D. Van Rooyen, eds., Nat.Assoc. of Corrosion Engineers, Houston (1969), pp.51-63.4. H.J. Flitt and J. O'M Bockris, J. Hydrogen Energy, 6,119 (1981).5. C.J. Smithells and C. E. Ransley, Proc. Roy. Soc.,150, 172 (1935).6. H.W. Neville and E. K .Rideal, Proc. Roy. Soc., A 153,89 (1936).7. J. Wang, Proc. Cambridge Phil. Soc., 3/2, 657 (1936).8. N. Kobozev and V. Montblanova, Zhuz. Fiz. Khim., 6, 38(1935) .9. N. Kobozev and V. Montblanova, Acta Physiochem.U.R.S.S., !, 611 (1934).i0. M. Ao Pick and A. Sonnenberg, J. Nuc1. Mat., 13___11,208(1985).ii. M. A. V. Devanathan and Z. Stachurski, Proc. Roy. Soc.,A 270, 90 (1962).12. J. O'M. Bockris, J. McBreen and L. Nanis, J.Electrochem. Soc., _11/2, 1025 (1965).13. E. Gileadi, M. A. Fullenwider and J. O'M. Bockris, J.Electrochem. Soc., I__U, 926 (1966).14. I. A. Bagotskya, Zhur. Phys. Khim., 3_66,2667 (1962).15. L. D. Kovba and I. A. Bagotskya, Zhur Phys. Khim., 3_/7,161 (1963).16. W. Palczewska, Bull. Acad. Polon. Sci. chim., 12, 817(1964).17. P. H. Pumphrey, Scripta Metal1., 1___4,695 (1980).18. B. G. Pound, G. A. Wright and R. M. Sharp, Acta Metal]..35, 263 (1987).19. W. Beck, J. O'M. Bockris, J. McBreen and L. Nanis,Proc. Roy. Soc. A 290, 191 (1965).20. J. McBreen, J. Electroanal. Chem., 28__/7,279 (1990).21. J. Horkans, J. Electroanal. Chem., 209 (1986) 371.22. D. M. Dra_i_ and _.. Vorkapi_, Corrosion Science, 18,907 (1978) .23. M. Hepel and S. Bruckenstein, Electrochim. Acta, 3__44,1499 (1989).24. T. Harris and R. Latanison, J. Hydrogen Energy, 14, 683(1989) .25. D. Rollison and P. P. Trzaskoma, J. Electroanal. Chem.28..__7, 375 (1990).DISCLAIMERThis reportwas preparedas an accountof worksponsoredby an agencyof the United States: Government. Neither the United States Governmentnor any agency thereof,nor any of theirempio._ees, makes any warranty,expressor implied,or assumesany legal liabilityor responsi-= bility for the accuracy,completeness,or usefulnessof any information,apparatus,product, orprocessdisclosed,or representsthat its use would not infringe privatelyownedrights. Refer-ence herein to any s_cific commercialproduct, process,or sera'iceby trade name, trademark,manufacturer, or otherwisedoe.snot necessarilyconstituteor imply its endorsement,recom-mendation,or favoring by the United States Governmentor any agency thereof. The viewsand opinions of authors expressedherein do not necessarilystate or reflect those of the: United States Governmentor any agencythereof._,,,,*,',.'ql j _I'"' " °PFI '"Vlll'll I pr' l'llll" 'PIR!cothode onodeV'd"toP_-i_lectrolyslscellFig. i. Cell for permeation measurements (ii).p • %.\ /- 1.o -o.e -o.e -o.4 -o.i o.o o.2 o.4E vs, SCE/VFig. 2. Cyclic voltammograms on Pd in 0.i M NaOH;voltammogram in oxide region (----), afterextension to hydrogen absorption region (- - -)and anodic sweep after a potential arrest of 5 sat -i.0 V (-,-.-). Sweep rate 50 mV/s. Electrodearea 2 cm 2.! i0,2 mA -_-"-_ / tt ',,;..,.;-- - ...Ii" III 0.04 mA• I " I • I , , I • II , I , I •- _,o -o.a -o.a -o.4 -o.2 o,o 0,2 0.4E vs. SCE/VFig. 3. Cyclic voltammograms on Pd in 0.1 M NaOH + 10 .3 MNaCN; voltammogram in oxide region (----), afterextension to hydrogen absorption region (- - -),and anodic sweep after potential arrest at -i.0 Vfor 5 s (-.-o-). Sweep rate 50 mV/s. Electrodearea 2 cm _.Z ##'#' •0.4 mA- 1.5 - 1.3 - 1.1 -0.9POTENTIAL/V vs, Hg/HgOFig. 4 Cyclic voltammogram for an iron electrode in 8.4 MKOH + 0.74 M ZnO, electrode area = 2 cm z, sweeprate = 50 mV/s.=.HYDROGEN EVOLUTION CURRENT/A-5 -4 -3 -2 -10cm-I.0:_ -1.1 -_ -1.3 -I " " ,".. s" _ _I-" le"0r, -1.4 I I I-6 -5 -4 -3 -2PERMEATION CURRENT/AFig. 5 Tafel and permeation current plots for an ironmembrane. Solid lines shows data for 8.4 M KOH,broken lines show data for 8.4 M KOH + 0.74 M ZnO.Electrode area = 1.26 cm z10000 ........ , ,:::t......,o SZ .,,...,. _, .,,. _WQ:." 1000 - ,"0Z0100 ":ZI,O_" / .... .-7Q/. -o"o__-/Z 10* //mBoz(_ • I , I ....1 I-0.7 -0.9 -1.1 - 1.3 - ,5E vs, SCE/VPig. 6. A comparison of the cathodic current ( ) andthe permeation current (------)for variouspotential steps in (1) 0.1 M NaOH and (2) 0.1 MNaOH + 10 .3 M NaCN. Working electrode area was1.26 cm 2. Membrane thickness = 0.025 mm.

Electrolytic hydrogen-metal interactions

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Electrolytic hydrogen-metal interactions

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