The Effect of Suspended Particles on the Corrosion Resistance of The Effect of Suspended Particles on the Corrosion Resistance of Electrodeposited Zinc Powders Electrodeposited Zinc Powders

Zinc powders were electrodeposited from ZnCl 2 baths with containing sus


Introduction
Zinc powder is a popular anode material used in many rechargeable and primary batteries.
Since t he corrosion behavior of zinc powder is important to battery performance , many attempt s in recent years have been made to prepare Hg2f ree zinc wit h better corrosion resistance , such as adding some effective inorganic and/ or organic corrosion inhibitors into t he elect rolyte , cont rolling t he particle sizes and morp hology of zinc , and preparing zinc alloys [ 1～3 ] .Based upon t he fact t hat zinc powder can be elect rodeposited f rom ZnCl 2 bat h or alkaline bat h [ 4 ] , t he properties of zinc powder were likely to be cont rolled under certain elect rolytic conditions such as elect rolysis ways , current density , solution concent ration and t he type of additive etc. .Suspended elect rolysis is a met hod in which elect rolysis is carried out wit h suspended species under mechanical stirring or ult rasonic dispersion.Compared wit h conventional elect rolysis , it has t he following advantages : 1) to hinder t he passivation of anode even at high current density [ 5 ] , 2) to prevent dendritic growt h [ 6 ] ; 3) to modify t he properties of deposit s such as microst ruct ure , particle size etc. [ 7 ] .Therefore t his met hod has gained much interest in recent years.As we have known t hat elect rodeposition of zinc often leads to four kinds of deposit s : spongy , compact , boul2 ① Received 25 Nov. ,1997 , accepted 13 Mar.,1998 der and dendritic.Being harmf ul to t he reversibility of batteries , dendritic deposit s of zinc should be avoided during t he course of elect rodeposition.Thus use of t he technology of suspended elec2 t rolysis could not only prevent t he formation of dendritic zinc , but also int roduce additives into zinc powders to modify t heir corrosion behaviors at t he same time.So in t his paper , we attempt to employ t his met hod to prepare some zinc powders , and discuss t he effect s of suspended particles on t he morp hology and corrosion behavior of zinc.

Experimental
Zinc deposit s were obtained under galvanostatic conditions wit h or wit hout mechanical st r2 ring.The bat h solutions were made of 30 g/ L ZnCl 2 and 100 g/ L N H 4 Cl , suspended species which included grap hite , In 2 O 3 , TiO 2 , SiO 2 , SnO 2 , PbO 2 , Ge 2 O 3 , MoO 3 etc.were added to t he bat h in amount s between 0 and 1. 0 g/ L .All t he reagent s used were of analytical grade.
The elect rodeposition of zinc was carried out at 25 ℃, t he anode was zinc plate wit h an ex2 posed area of 10 cm 2 , t he cat hode material was Pb , and current density was selected at 20 -100 mA/ cm 2 .After 2 hours deposition , t he deposit s were removed and washed wit h et hanol , t hen dried and kept in t he vacuum drier.The deposit p hases were analyzed by X2ray diff raction and t he morp hology was observed by scanning elect ron microscopy ( SEM) .The corrosion behavior in 5 mol/ L KO H was analyzed wit h EIS spect ra.The AC impedance experiment was performed using PAR.m 378 system wit h ac signal of 5 mV and f requency range between 100 KHz and 0. 01 Hz , where working elect rode was made by pushing t he mixt ure of zinc powder wit h a little P TFE onto a stainless steel web , Pt wire was served as counter elect rode and Hg/ HgO , KO H (5 mol/ L) was as reference elect rode.

1 The morphology and microstructure of the deposits
The zinc deposit s were electrodeposited f rom conventional bat h (i.e. wit hout stirring) and suspension bat hs.The morp hology of various zinc powders was examined by SEM , f rom Fig. 1 and Fig. 2 , it could be observed t hat elect rodeposition of zinc f rom conventional bat h led to t he dendritic zinc , but dendritic growt h of zinc was limited f rom suspention bat hs.In addition , t he morp hology of zinc powder pre2 pared wit h suspended met hod varied according to t he kinds of suspended particles.The result s indicated t hat t he existence of stirring which caused t he flowing of solution did not help t he growt h of dendritic , and also t he existence of suspended species might act as crystal nuclei to Fig. 3 showed t he X2ray diff raction pat2 terns of zinc powders deposited f rom suspension bat hs of grap hite and PbO 2 .From Fig. 3 , we found t he characteristic diff raction peak of C at d = 3. 34 , an increase of amount of suspended grap hite gave rise to t he increase of t he height of diff raction peak at d = 3. 34.However , sus2 pension of ot her species only caused some changes in t he height of certain peaks and t he appearance of new peak out not t he characteris2 tic diff raction peak of suspended species.There2 fore , could draw a conclusion t hat t he sus2 particles were codeposited wit h zinc powders , and we called t hese powders t he com2 posite zinc powders.

2 Effect of the amount of suspended on the corrosion properties of zinc po wder
It has been reported t hat mixing certain amount of grap hite wit h zinc powder was bene2 ficial to decrease t he corrosion rate of zinc elec2 t rode in alkaline media [ 8 ] .So grap hite was cho2 sen as t he first suspended substance.0～1.6 g/ L amount s of C were suspended in t he bat hs and t he effect of t he amount of C on t he corrosion behavior of zinc deposit s elect rodeposited at 100 mA/ cm 2 was examined by EIS measurement s.Fig. 4 showed t he Nyquist plot s for composite zinc powders wit h different amount of C.There existed two regions in all t he EIS plot s , two ca2 pacitive loops at high f requencies and inductive one occurred at low f requencies , it indicated t he existence of passive film at zinc surface in 5 mol/ L KO H.The corresponding equivalent cir2 cuit consisting of resistors , capacitors and induc2 tor was shown in Fig. 5. to simulate t he elect rochemical behavior of composite zinc elecrode in 5 mol/ L KO H solution.Element s in t he equivalent circuit respectively reporesented as : R 1 2solution resistance ; 电 化 学 t ransfer resistance ; R 4 2film resistance ; L 2inductor.If regardless of t he existence of inductive loop , t he polarization resistance R p was t he sum of R 2 and R 3 , ot herwise R p was calculated by t he equation Tab. 1 Electrochemical parametres simulated from EIS plots of composite zine wit h suspension of different amount of C(0～1.6 g/ L) amount As shown in Table 1 ,an increase in C amount led to t he decrease of charge2t ransfer resistance R 3 and also t he decrease of R p .Compared wit h t he EIS data of zinc wit hout grap hite , it indicated t hat t he existence of a little C was effective to t he corro2 sion protection of zinc elect rode.However , excess C would cause zinc elect rode to become more unsta2 ble in 5 mol/ L KO H.The reason could be att ribut2 ed to t hat better conductivity of grap hite made t he loss and winning of elect rons in zinc elect rode be2 come easier.

3 Effect of current density on the corrosion behavior of zinc
At fixed amount of C(1.0 g/ L) , zinc deposit s were prepared at current density range f rom 20 mA/ cm 2 to 200 mA/ cm 2 .The simulated EIS data were shown in Table 2. From Table 2 , we found t hat zinc anode elect rodeposited at lower current density showed higher corrosion resistance.

4 Effect of kinds of suspended species
Different kinds of particles ( 0. 1 ～ 0. 5 g/ L ) were suspended in t he elect rolytic bat h.The exper2 iment curves were showed in Fig. 6 and simulated Tab. 3 Electrochemical parametres simulated from EIS plots of zinc (powders) wit h different kinds of suspention species species

In2O3
SiO 2   The int roduction of Pb , Sn or In in zinc would greatly improve t he corro2 sion protection of zinc anode owing to t he high overpotential of hydrogen on t hem.In addition , t hey could also act as crystal nuclei [ 9 ] .In t he case of TiO 2 , SiO 2 , C , GeO 2 , MoO 3 , t heir influence may take place in t he manner of 2) and/ or 3) .

5 Fractal theory on the corrosion behavior of zinc
When zinc powder was used for anodes in many batteries , t he surface of zinc anode is porous and rough and t herefore f ractal.In recent years , f ractal t heory has been int roduced into t he field of elect rochemist ry such as t he elect rodeposition of metal , t he redox behaviors of elect rode and corrosion properties of materials etc. [ 10 ,11 ] .The f ractal dimension (D f ) of an elect rode surface can be determined by t he measurement of impedance spect roscopy [ 12 ] .
In f ractal t heory , if an elect rode is a blocking elect rode or ideally polarized elect rode , it s impedance Z can be represented as : where αis CPE exponent (0 <α< 1) , ω is t he angular f requency applied , j = -1 , and K is a

Fig. 1
Fig. 1 SEM picture of zinc powder electrodeposited from conventional ZnCl 2 bat h at 10 mA/ cm 2

Fig. 3
Fig. 3 X2Ray diffraction profiles of zinc powders obtained from a bat h wit h different suspension species

Fig. 4
Fig. 4 Nyquist plots for zinc powders wit h different amount of C in 5 mol/ L KOH

Fig. 5
Fig. 5 Equivalent circuit for simulating EIS plots

Fig. 6
Fig. 6 Nyquist plost of zinc powders wit h suspension of different kinds of species