The electrodeposition of tin on Al-killed steel from aqueous solutions containing ~ n + ... Tin electrodeposition was made on 0.28 cm2 Al-killed steel rotating disc ...
THE INFLUENCE OF OPERATING CONDITIONS ON THE MORPHOLOGY OF TIN ELECTRODEPOSITS AND TINPLATE QUALITY by 3.L Zubimendi, and C . Baieli, Centro de Investigacibn Industrial, Fudetec, Argentma, W. Egli and M.R. Chara, Siderar Saic, Argentina, G. Andreasen, P. Schilardi, R. Salvarezza and S. Marchiano, Inst. de Inv. Fiacoquimicas Tebrica y Aplicada, Argentina.
Summary The znfluence of the operating conditions and organic additives (brighteners) on tin electrodeposition provides data for understanding the electrocrystallizationmechanism. This knowledge is essentialfor controlling the growth modes of the deposit and improving the industrial performance of electrolytes for developing the deposition of bright tin on tinplate. Hull cell tests, cylindrical and disc rotating cathode systems were used to obtain tin deposits under different operating conditions. The root mean square roughness and particle size distribution of these deposits were determined by SEM and AFM techniques to evaluate the influence of operating conditions on the tinplate brightness and its relation to the kinetics of the electrodepositzon reaction. The efSect of iron and chloride ions, electrodeposition current density, electrolyte temperature and brightener concentration on tin deposit morphology and particle size distribution was studied. "On field runs in SIDERAR eEecholytic tinplate production line allowed us to optimise the plating process under specific critical conditions.
1. Introduction Surface defects constitute a critlcal topic in the tinplate industry which has received special attention. This problem is directly related to the production of hlgh quality films which are required for a number of technological applicabons. Many technical and scientific papers have been published in this field although only a few of them correlate laboratory and "on field" experimental results to understand and control the origin of those surface defects related with tinplate brightness. In this paper we will attempt to determine the origin of common surface defects on tinplate through the interpretaaon of these electrochemical, SEM and AFM data. Results show a correlation between the current density, brightener concentration, temperature and line speed and the appearance of different Sn deposit morphologies. It was found that only one of the different morphology types usually found in Sn electrodeposition leads to the production of high quality tinplate.
2. Experimental The electrodeposition of tin on Al-killed steel from aqueous solutions containing ~ n Ion + ~ concentration in the range 28 g/l < CS, < 38 gA; 20 gA PSA (as H2SO4) and an organic brightener concentration in the range 0 gA < CBngh < 40 gA was studied using conventional electrochemical techniques combined with scanning electron microscopy and ex-sitn atomic force microscopy (AFM). Occasionally, expenments were run with the electrolyte used m the ETL. Slmilar results to those found with the electrolyte solutions prepared in the laboratory were obtained.
Tin electrodeposition was made on 0.28 cm2 Al-killed steel rotating disc electrode (RDE) using a conventional glass-made electrochemical cell provided with a large tin counterelectrode, and a saturated calomel electrode (SCE) as reference electrode. Potentials in the text are referred to the SCE scale. The RDE rotation speed (0) was varied between 0 and 6000 rpm. Experiments were performed in the temperature range 30' C < T < 60' C. /
"
~athodic~ol&izationcurves were run at the scan rate v = 2x10 Vls between -0.4 V and -1.2 V. ~ i & e l e ~ o d e ~ o s i t was i o n also run m a conventional Hull cell employing Al-killed steel plates taken directly from the ETL. /'
/
AFM imaging of the substrate and tin deposits was performed using a Nanoscope III AFM (Digital Instruments) with Au coated Si3N4 cantilevers with integral tips. The AFM calibration was made using the mica surface. SEM observations were made using a Phillips instnunent.
3. Results 3.1. Electrochemical data The cathodic polarization curve for the Al-killed steel run at v = 2x10.~ V Is between -0.4 V and -1.2 V in the working solution containing CS, = 28 g ~ lCBrigh , = 8 g/l and PSA = 20 g/l (Fig.la) at T = 40 "C and 0 = 0 rpm, exhibits a cathodic current density (j) which increases up to -0.60 V, defining a broad peak (region I). Then, a limiting current density (jl ) (region 11) extends from -0.60 V up to a certain critical value Ec at which a marked increase in j is observed. For E < Ec the current increases as E moves in the negative direction (region 111) Vig.la). In this potential region on,reversing the potential sweep-a loop related to nucleation and growth of branched crystal process can be seen (1) (Fig.lb).
Fig 1 (a) Cathod~cpolmatcon curve at 2x10 V s-' for Al-lulled steel m solntlon contamg C,, = 28 g/l, = 8 gll and PSA = 20 g/l at 40 C run between -0 4 and -1 1 V (b) First current potentla1profile at 2x10.~V S-
Region I corresponds to the nucleation and growth of three dmensional tmy tm crystals on the substrate under diffusion control (1) When the overlap of the diffusion zones around the growing fin crystals occurs, the growth process approaches the con&fions of a linear dffuslon to a plane plate under free convection so that a limitlng current (region 11) is defined Fmally, when the apphed potential exceeds Ec, the dendntic growth is tnggered at comers of 3d crystals leading to a large Increase m the electrode area, as revealed by the above mentioned current loop on reversing the potenfial sweep (region 111).
The value of jl increases linearly with Cs, (Fig.1~)and a'' (Fig.ld), whereas the value of Ec remains nearly constant. On the other hand, at constant C,, and W, the value of Ec moves in the positive direction as the brightener concentration is decreased, so that when CBrigh+ 0, the region I1 tends to disappears (Fig. 1e).
Fig l(c-d-e): (c) Relahonship between Csn and JL. (d) Levich plot for electrodeposits of tin. (e) Cathod~c polarization curve at 2 x 1 0 ~v.s-~ ~ for Al-lalled steel m soluhon containing Csn = 28 g/l, PSA = 20 g/l at 40 C at dfferent CBngh.The amow Indicates the value of Ec for each CBngb
3.2. Tin electrodeposition in the Hull cell. Tin electrodeposition on the substrate was also performed in a conventional Hull cell using a solution containing either the ETL electrolyte or an a ueous solution containing CS, = 28 gfl, CBngh= 8 g/l '3 and PSA = 20 gA, in the range 30' C c T i60 C. After tin electrodeposition, the steel plate exhibited three well-defined regions correlating closely to those described in the cathodic polarization curve. Macroscopic observations in the low current density region (region I) revealed a low brightness which increased markedly in region 11. Finally, a dark and loosely adherent deposit was formed in the high current region (region III). Occasionally, at low CBngh and high T the unusual sequence of
tllt
~ v ~ t i c m ~ m d e v E ~ 2 0 ~ d m ~ ~ ( f l o ~ e r o ~ 1 l . v d u s ) . . s i n t h e ~ = 40 T,d either En.clsodytc ar m qwur solulion wnuining Cb = 28 CBrilb m 8 @ a d P S A = 2 0 B ( I . T h c v d u c o l E i ~ w c s r h v p l yu p t o - 1 . 4 V l u d l h c n i t r m u i n s c ~ t m d c b t o 1 V (Fig.5a). Ocndrilic tin dqmts a m formed uadcr thcs eondiliau .s E l i a in redoion 111. Similar wprirnmts were made applying n qetitiw quuc wave currcllt u the h p m c y . f = L Hz (Fig.5bjb). In this nre the vdut of E dm rcwhsi a d u e l y i q in region UI. although 3 covuing dcpmit with thc churtcrirlicr of lhoseg-c to region I1 is formed. Th* nan$that r 1 Hz lhere is no lime f a the dggmhg ddendritie p w h r i*. hc elccDodcporition timc is sbancr than thc induction time far &&tic pwh,
a.
B x p i m m l mults, wncaning tin d w b x k p i h in l d m a t q ekEbochemical cells. Hull cell
d m l k ~ ~ ~ ~ a u i n ~ d ~ i t ~ l @ c a n h ~ t ~ ~ qm&g d t i m ~ u c ha~ cwrmt ihsity, m m m ~ d c m ,tanpmtm and lint spacd. Chmmcm e k t m l y l e in@hsush is Q- .ad Fe" k m dotldt significnlly & p i t mqMqip(intkmjprtudied), T o ~ ~ q u a l i t y h n p l a t e i n t ~ o f ~ g h t m s s p m i t y , W qmating d t k m must k~ d j d to p a w w c-l with h e chmmcctiatics f d i n r e g i c m U . hhdeposits-hdasmooth,bri@and-slayerWore
melting and the quality and appearance of tinplate in this case is substantially improved after the fusion tower treatment. Deposits with the characteristics of region I and 111 lead to porous and dull tinplate after the fusion tower treatment. The fine adjustments of operating conditions made in ETL at Siderar based on results presented in this paper allowed us to reduce the number of rejections caused by surface defects on tinplate, controlling operating conditions between the studied relative limits and relationships.
References ( I ) E.J. Calvo and C. Morna, "Studies on the Electrocryslalbsatcon process in Tznplate manufacture from sulphonzc baths", Proc. 3 Th Tinplate Conf., London, 1992, p. 96 (2) Ph Aubrun Interpretation of some phenomena occurrcng during electrotinning m a stirred Solutron, Proc 1st Tinplate Conf, London, 1976, p 388 and Tin electrodeposcts produced under conditions of electrochemical mstabzliry, Proc 5 Th Tznplate Conf, London, 1992, p 276.
(3) George A Fedennan, A comparative study of the effects of metallzc cmpurities on MSA-& PSA-bases tm electrolytes,Proc. 5 Th. Tcnplate Conf, London, 1992, p. 276.
