Effect of polyethylene glycol on electrochemically deposited trivalent chromium layers

Available online at www.sciencedirect.comTransactions ofScienceDirectNonferrous MetalsScienceSociety of ChinaEL SEVIER PressTrans. Nonferrous Met. Soc. China 19(2009) 819-823www .nmsc.cnEffect of polyethylene glycol on electrochemically deposited trivalentchromium layersJoo-Yul LEE, Man KIM, Sik Chol KWONDepartment of Surface Technology, Korea Institute of Materials Science, 53 1 Changwondaero, Changwon,Gyeongnam, 641-831, KoreaReceived 18 June 2008; accepted 10 March 2009Abstract: The structural characteristics of the trivalent chromium deposits and their interfacial behavior in the plating solution withand without polyethylene glycol molecules were observed by using various electrochemical methods such as cyclic voltammetry,open circuit potential transition, electrochemical impedance spetroscopy, scanning electron microscopy and X-ay photoelectronspectrometry. It is shown that the polyethylene glycol molecules make the reductive current density lower in the trivalent chromiumplating system and promote a hydrogen evolution reaction through their adsorption on the electrode surface. And the trivalentchromium layer formed from the polyethylene glycol-containing solution has somewhat higher density of cracks on its surface andresults in a lower film resistance, lower polarization resistance, and higher capacitance in a corrosive atmosphere. It is also revealedthat the formation of chromium carbide layer is facilitated in the presence of polyethylene glycol, which means easierelectrochemical codeposition of chromium and carbon, not single chromium deposition.Key words: electrochemical deposition; trivalent chromium; polyethylene glycoland structural compactness of electrodeposits in the area1 Introductionof metal and metal alloy electroplating[4- -7]. Particularly,polyetbylene glycol (PEG) was used in the platingHexavalent chromium has been widely used as asolution as a surfactant to lower the surface energy of asurface coating material due to its excellent physical,cathode in the zinc, chromium and zinc-chromium alloymechanical and electrochemical characteristics. But nowelectroplating[7]. Hull cell tests show that the addition oft confronts worldwide environmental restrictionsPEG molecules in the trivalent chromium solution canbecause of its hazardous effects on the environment.enhance the homogeneity of trivalent chromiumTrivalent chromium electroplating has drawn a greatelectrodeposits and the current efficiency at low currentattention for several decades as the most challengeabledensities at optimum concentration. But, most researchesprocess to altemnate the hexavalent chromiumwere concentrated on revealing the relationship betweenelectroplating owing to its low toxicity[1]. However, it iscomplexants and trivalent chromium ions in thedifficult to electro-chemically reduce the trivalentviewpoint of electrode kinetics, while just a few dealtchromium ions due to the formation of kinetically inertwith organic additives systematically to understand theirouter orbital octahedral complexes in the aqueousroles in the electrochemical reduction and themedium and their bonding nature of d'sp' hybridizationdevelopment of thin flm properties[8-19] In this work,[2]. This kinetic inertness results from the 3d' electronicwe aimed to structurally analyze the trivalent chromiumconfiguration of trivalent ion, whose orbital chargelayers to elucidate the role of the organic additives anddistribution makes ligand displacement very slow[3].PEG molecules during electrochemical deposition.Actually, ligand displacement reaction of trivalentchromium complexes was reported to have half-time in2 Experimentalthe range of several hours. Various organic additives中国煤化工have been employed to enhance thsurfacechromium platingcharacteristics such as leveling, brightness, anticorrosion,wasMYHCNMHG04)3asasourceofCorresponding author: Joo _Yul LEE; Tel: +82-55-280-3518; E- mail: lcececxms.e.rDO: 10.1016/51003-6326(08)60357-X820Joo-Yul LEE, et al/Trans. Nonferrous Met. Soc. China 19(2009) 819-823trivalent chromium ion, 0.1 mo/L HCOOH as acurrent and observed that the electrochemical reductioncomplexant, 0.07 mol/L H;BO3 as a buffering agent, (0.1was limited by the mass transfer of trivalent chromiummol/L NH4Cl+0.1 mol/L KCI) as an mixed electrolytecomplexes both in the presence and in the absence ofsystem, and 1.3 mmo/L polyethylene glycol (PEG, M=PEG molecules (not shown here). This also indicates that1 500) as an organic additive. The plating solution wasPEG molecules do not specifically interact with trivalentadjusted at pH 2.3 and maintained at 30 C duringchromium cormplex on the electrode surface.experiment. Trivalent chromium layers were depositedstationaryworking electrode b0.04potentiodynamic or potentiostatic polarization in the二&$ tPEG moleculesstock solution in the presence and in the absence of PEG0.03-molecules, followed by rinsing with purified water anddrying with N2 gas. Cu plate (area 1 cm2 ), platinum plate0.02-(area 4 cm'), and Ag/AgCl (in saturated KCI) were usedworking, counter, and reference electrodes,respectively, for the electrochemical experiments. The0.01electrochemical preparation and characterization oftrivalent chromium layer were made by usingPGSTAT30 (Autolab, Netherland). For the impedancemeasurements, the instrument was controlled in the-0.4 -0.6 -0.8 -1.0 2 -1.4 -1.6frequency range of 100 kHz - 100 mHz with an AC wave∞(vs SCE)/Vof 5 mV peak-to-peak overlaid on a DC bias potential,Fig.1 Cyclic voltammograms at copper electrode at 50 mV/s inand the impedance data were obtained at a rate of 10solution containing 0.05 mol/L Cr2(SO4)s, 0.1 mol/L HCOOH,points per decade change in frequency. The physical0.07 mol/L H,BO, 0.1 mol/L NH,CI and 0.1 mol/L KCl withmorphology of the trivalent chromium deposits wasand without 1.3 mmol/L PEGanalyzed by scanning electron microscopy (SEM,JSM-5800, JEOL).3.2 Open circuit potential transitionFig.2 represents the transition of open circuit3 Results and discussionpotentials of copper electrode in the trivalent chromiumsolution with and without PEG molecules. Open circuit3.1 Cyclic voltammetrypotential reflects the amount of electroactive species atFig.1 shows the cyclic voltammograms of electro-the electrodelectrolyte interface and informs thechemical reduction of trivalent chromium ions in thesurface state, that is, the degree of adsorption of additivesPEG-free and PEG-containing solution. Actually, thereon the electrode which hinder the approach of metallicwas lttle difference in the onset potentials for theions. PEG-containing solution showed higher openreduction of trivalent chromium ions between twocircuit potential than PEG-free solution by 10 mV or so,systems except that PEG-containing solution representedlower current density than PEG-free solution. Lowcurrent density observed in the additive-containing-0.17solution is related with some phenomena such asadsorption of PEG molecules at the electrode/electrolyte-0.18-interface, interaction between PEG molecules andtrivalent chromium complexes in a solution, or台-0.19/2interference of the reduction of trivalent chromium dueto the competitive hydrogen evolution reaction[20-21].三-0.20We ascribe the low current density of this PEG--0.21containing system mainly to the severe hydrogenevolution reaction, judging from the fact that there was-0.22二& PEC moleuleno onset potential shift compared with PEG-free solution,-0.23which is a definite proof of 'strong’adsorption of organic500100015002000additives at the electrode. This means that PEG-中国煤化工containing system is more susceptible to the bydrogenFig.2 (JHCN M H Gcopper etode asevolution reaction than PEG-free system in the course of, Cr2(SO4)3,trivalent chromium deposition process. As well, we also0.1 mol/L HCOOH, 0.07 mol/L HBO3, 0.1 mol/L NH4Cl andmeasured the relationship between scan rate and peak0.1 mol/L KCl with and without 1.3 mmol/L PEGJoo-Yul LEE, et al/Trans. Nonferrous Met. Soc. China 19(2009) 819- -823821even though this potential deviation was not soconsistent during measurement. Considering the cyclic30(a)voltammetric analysis, we assume that PEG moleculesare weakly adsorbed on the electrode surface and cannot250-act as a practical barrier layer to induce specific surfacereaction with incoming trivalent chromium complexes.2003.3 Electrochemical impedance spectroscopy; 150: tPEG moleulesFig.3 depicts the electrochemical impedance spectraof trivalent chromium layers in the 0.5 mol/L NaCl100-solution at open circuit potential and anodic potential(0.10 V). Each chromium layer was prepared by5o- x- Simulated data for.(Cr+PEG molecules),potentiodynamic polarization from OCP to-1.6 V at 10●Simulated data for (CP+ alone)mV/s and the impedance spectra were ftted by using50100150Randles equivalent circuits. Both impedance spectraZ/k9were composed of two semicircles. A small semicircleappearing at high frequency region represents the charge..1.0(b)transfer impedance (Rer -Ca) for the active dissolutionreaction of chromium layers by chloride ions, and a large0.8-semicircle at low frequency is related with thecharacteristics (or porosity) of chromium filmsg 0.6-themselves and the diffusion behavior of anions throughthe pores of deposits (Rr -C)[22].得。0.4-●C3+PEG moleculesFrom the fited results (not shown here), thex- Simulated _data for,trivalent chromium layer deposited from PEG-free(Cr3+ PEG molecules)solution was revealed to have larger charge transfer0.2|Simulated data for(Cr' alone)resistance and film resistance than the deposit fromPEG-containing solution at both open circuit potential0.5 1.0T2.02.5and 0.10 V. Especially, when the trivalent chromiumZ/kQlayer was under the active dssolution condition,PEG-added chromium layer shows an abrupt increase inc)Rr_the flm capacitance, which describes the increase ofRswnrsurface area, that is, rough surface. Therefore, this●Ww*suggests that the PEG-free deposits should have morecompact structure than the PEG- containing one, which isCaCrcoarsely structured and of somewhat apparent crack (orpore) distributed along the thickness direction and makesFig.3 Impedance diagrams recorded at electrodepositedthe movement of chloride ions through the deposit easier,trivalent chromium layers at OCP (a) and at 0.10 V (b)as confirmed from the surface images before and after(Experimental and simulated Nyquist plots for copper electrodethe active dissolution in Fig.4.covered with trivalent chromium layer are overlaid) andequivalent circuit used for impedance data analysis (C)3.4 Microstructure analysisSEM microscopic images are obtained for thePEG-containing solution had a larger deposit grains andtrivalent chromium deposits prepared by applyingcrack width compared with those from PEG-free solution.constant potential of-1.0, -1.2, and -1.6 V for 30 min inIt is known that cracks develop when the bydrogenthe presence and absence of PEG molecules. In Fig.5, nocodeposited with chromium is released from matrix anddeposition happened in the PEG containing solutionthe internal stress constraints are decreased and(Fig.5(d)) at -1.0 V due to the more competitiveconsequently the total volume of deposit is reduced[9].hydrogen evolution reaction, as described in the cyclicTherefore, trivalent chromium deposition fromvoltammetric analysis in Fig.l. The occurrence of cracksPEG中国煤化工-sed to accompanyon the chromium surface was dependent upon themucland a bydrogenapplied potential in both solutions. As the appliedincofYHCNMHGrormofachromiumpotential became higher, the crack density and crackhydride, which coincides with the result of cyclicwidth got larger. Trivalent chromium layers formed from voltammetry in Fig.1.822Joo-Yul LEE, et al/Trans. Nonferrous Met. Soc. China 19(2009) 819-823CFig.4 Surface SEM photographs of trivalent chromium layers before (a, b) and after (C, d) anodic polarizationat -0.1 V for 400s in5% NaCl solution by potentiodynamic polarization from 0CP to -1.6 V at 10 mV/s: (a), (c) Deposited in the presence of 1.3 mmol/LPEG; (b), (d) Deposited in the absence of 1.3 mmo/L PEG20 um2 umP4 m4 pum4四4 JmFig.5 SEM photographs of trivalent chromium layers prepared by potentiostatic polarization for 30 min in solution containing 0.05mo/L Cr2(SO4)3, 0.1 mol/L HCOOH, 0.07 mol/L H,BO, 0.1 mol/L NH4Cl and 0.1 mol/L KCl with (a, b, c) and without (d,e, f) 1.3mmol/L PEG: (a), (d)-1.0 V; (b), (e)-1.2 V;(C),(1)-1.6 Vadsorbed on the electrode surface, they are not likely to4 Conclusionsact as a practical barrier layer to induce specifie surfacereaction with incoming trivalent chromium complexesIt is observed that PEG molecules added in thedue to中国煤化二-olecules make thesolution make hydrogen evolution reaction moretrivaler:d, which may beapparent during trivalent chromium deposition processascribeHC N M H Gution reaction andwithout modifying the interaction with trivalenthydrogen incorporation into the matrix as a chromiumchromium complexes. 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