Loto, C. A.

English

Covenant University Repository

. ,, The effect of additives on the c~ lect rodeposition of zinc on rnild steel in acid chloride solution by Dr C.A.Loto Department of Mechanical Engineering. University of Lagos, Nigeria a~d Prof I.Oiefjord Department of Engineering Metals, Chalmers University of Technology, Gothenburg, Sweden 'f/Jf: EFFECT of the addirion of thiourea. dexrrin and glycin. as organic addiril·es. nn rhe swface characrerisrics of _ zinc elecrrodeposirion on mild steel in acid chloride solution at differem pH lel'els and varying combinations of rhe additive. 11·as studied in this invesTigation by SEA1. The swface state. and hence The stt'l-15 sur.face:film composition. was anal_vsed by ESCA instnanenrarion before ::inc electrodeposition. which was pe!fonned using a DC supplier at defined operaring conditions. Tl1e unique crystal srmcrures obtained are characteristic of the d!(ferent pH levels and different combinations of addirives. The overall resulrs show good zinc e!ecrrodeposilioil on mild .\'feel, excepr for the porous effect of combined thiourea and glycin addirion. Introduction The surface characterization of the effects of o rganic additives on the electrodeposition of zinc on mild steel and the influence of organic a·dditives on the surface characteristics of zinc electrodeposition on mild steel in acid-chloride solution under different conditions have been demonstrated in recent works[l-2] . This article extends these investigations by further looking into the effect of thiourea. dextrin and glycin additions in different combi nations and at two different pH levels for a constant plating time of 12.5min. This work has been undertaken in an attempt to develop further other effective addition agents from organic additives for the acid chloride bath apart !'rom the commercial proprietary additives [3-6] for bright zinc electrodeposition. Recent investigation performed on this subject has indicated the effectiveness of these additives. However, in order 10 obtain the optimum utilization of these additives, more work has to be done, and this is the object of the present investigation. The additives used in this work have been previously used in acid ~ ; ulphate baths(7-ll] Results obtained from an investigation such as this could be beneficial in establishing other areas lor the use of zinc-electroplated steel materials and/ or products. It is expected that the zinc-dectroplated products made from this acid bath using the additives under investigatiori will be commercially useful in corrosion protection of ·:nild steel products: it \vill also be aesthetically eneficial. This work is expected to point the way to rhe use of other effective additives in acid chloride baths. particularly in fundame ntal research. since the composition of the additives are known. !58 Experimental procedure Preparation of test specimens and solutions The experimental procedures used here. are described in the previous work[ 1.2]. A Swedish-made flat mild steel, SIS 141147. O. lcm thick, with a nominal composition of0.038%C. O.l9%Mn and the rest Fe, was cut into several test specimens of lO.Ocm length and l.Ocm width. A portion l.Ocm in length was marked orr at one end for the zinc electrodeposition. Henkel VR 6362-l. a commercial alkaline degreasing-chemical. was used to degrease the test specimens. 30g of the chemical was dissolved in IIi of distilled water and heated to a temperature of 65°C. The test specimens were immersed in · the Fig.1. Schematic diagram of the experirr.ental set up . .=::f--1.=-=4-- (',;.: .. ·: 3 •f -:1 -:; : ["': i• ~I:~· ':tl cl i-~=======-~---·--- ·---......J Cloolpgl~ I'RE\T;\ TIO' & CO:--TROL Dt·cemher !9<10 solution in a glass container and ultrasonically ckaned for 5min. then removed from the solution. rinsed in distilled water. immersed in methanol and then immediately removed and air dried . The specimens were in turns etched for 50sec in 3Jv1 HCI. rinsed in distilled water. immersed in rr,ethanol. air dried. and stored in a desiccator. The acid-chloride solution for the electro-dt:position consisted of ZnCl (7lg/li). KCI (207g/li) and HJBOJ ( 135g/li). Thiourea (0.75gili). dextrin (3gili). and glycin (2.5gili) were used as the additives. Five f1asks were prepared. four of the solution with additives and one without. The solutions from the group of four tlasks were each t en separated into two, \Vith one portion having its pH adjusted with zinc carbonate hydroxide to pHS from the original pH of 3.9 just before use. ESC.4 analysis of the steel surface Before immersion in acid-chloride solution. the surface-film composition. and hence the steel's surface. was determined. as this could affect the zinc-electrodeposition characteristics. A 5x8mm sample of the steel specimen (after degreasing and erching) was cut. This was surface analysed using X-ray photoelectron spectroscopy (XPS). The su rface film was further sputtered by Ar ion etching to a depth ofO.Jnm for an in-depth study of the steel's film composi tion. An unclean surface could have some adverse plating effects. Elecrrodeposi1ion of ::inc on sreel The electrodeposition of zinc on the steel used in this work has been previo usly described[ 1.2] . Here. it was performed by partially immersing the steel specimen and the zinc electrodes in the 20-mm deep plating solution th rough the rectangular hole made on a prepared perspex cover for the 250ml-beaker used as the plating bath (Fig.!). The steel specimen was connected to the negative side of a DC supply. while the zinc was connected to the positive side: the two zinc electrodes were also connected through a wire. The plating solutions were put. in turn. into the beaker (bath). and their respective pH was obtained by adjusting the original solution with zinc carbonate hydroxide. The operating conditio ns were : pH of the solutions ··3.9 and 5 temperature current plating time gentle stirring 27-30°C 30mA/c m! 12.5min After each zi nc electrodeposition. the specimen was taken out. rinsed in distilled water. immersed in methanol and quickly air dried before characterizing the surface with the scanning electron microscope (SEM). CORROSION PREVENTION & CONTROL December 1990 ~ z I 0 -. lerK:i~-KK I f \ I --KK-K~~ ... ~IKK: ':-. ··-. . i 535 525 i Sir.dbg Energy 1 ev; I Ql-SJS 525 6in!Mg .Energy leVI I I.!!. rc 'I (;01 I;{) LC·H ' ~ c-0 l ! Ci- I ~r ___ . : ~--------~ I 295 285 •1 210 21)0 190 Binding Energy leVI Binding Energy leVI Af'-etched C-H.-c-c Ar-etched ((-~~ 29 5 265 210 zoo Bindi Binding Energy leVI Fig.2 . ESCA spectra recorded for the surface of the degreased and etched mild steel specimen. SEi'vf characterization A small portion of each of the plated specimens was cut, mounted on a stub and exami11ed in tum in the SEM. Electron micrographs were made of the representativ~ areas of the surface at xlOO and x 1000 magnifications. Results and discussion ESCA spectra The ultrasonically-cleaned steel surface film was analysed by ESCA instrumentation before · the electrodeposition process. Fig.2. On the surface film oxygen, iron. chloride and carbon elements were detected. De-convolution of the signals for these elements was performed. giving two different peaks for oxygen and iron. The oxygen's two peaks represented oxide (01-) and hydroxide (OH-) formed on the si.trface. The two peaks from the Fe signal represented Feh and Fe2•. Chloride.' which comes mostly from the HCI used to etch the metal specimen. shows no change. The traces of carbon are b~lieved to have miginated from t~ methanol used to rinse the specimen before air drying. That OH- was onl)':- present on the tilm's surface was indicated by further sputtering of the surface film by Ar to an etching depth ofO.Jnm. There was a chemical shift_ in which OH- signal almOS! disappeared. The peaks representing ·carbon a~d chloride were obtained at very low intensity levels. The plating medium was very acidic and could · easily destroy the unstable surface film of metal 159 .,.---Fig.3. SEM micrograph of the etched surface of the mild steel test specimen before zinc electrodeposition. 1(>() specimen. There was no significant impurity obtained in the film. The steel's surface was therefore expected to serve as a good substrate for zinc electrodeposition. providing zinc plating v.ith a minimum adverse effect. if any. Electrodeposition of zinc in acid svlurion rrithour additive Fig.3 shows the SEM micrograph of the mild steel (test sample) surface before immersion in acid solution prior to zinc electrodeposition. No significant impurity was observed. At a pH3.9 and plating time of 12.5min. the zinc-electrodeposited surface of the mild steel specimen (Fig.4{i-ii)) shows crystal structures that could be considered good. At a magnification of x 100. · Fig.4{i). no definite crystal structure could be discerned. although a wry close observation indicates the presence of some pores. At a higher magnification of x!OOO. Fig.4(ii). a slightly· coarse crystal structure is distinct. and very few tiny pores could be observed. The viability of this type of zinc plating will definitely depend on the conditions Fig.4 . SEM micrographs of the steel surface after zinc electrodeposition in acid chloride solution without additive : plating time 1 2.5min. CORROSIOi'\ PREVEi'\TIO:\ & CO:\TROL December 1990 cigK~K SEM micrographs showing the effect of thiourea (0.75g/ li) and dextrin (3 g / li) on the eleC1.rodeposition of zinc on mild steel in acid chloride Jolution; plating time 12.5min. under \vhich it !s applied. The poor throwing power of the acid solution. though not very apparent here. mu st have been responsible for the pores. The slightly coarse structure is due to the absence of ( ~ lewlling agents in the acid solu tion. Fig..:t(iii-iv) shows the surface structure of the zi n.:-plated :i teel at pHS .. Fig.-t(iii). at a ma:~n ificati on of :<100. shows no detined stmcture. but an almost smoo th surface. At a magnification of.\ 1000. Fig.-Hiv). the crystal structure is clear. and the crys tals ;Jre more round and tine. This unique structure. which is eYidence of good zi nc-electrodeposition. arises from the addition of the zinc carbonate hydroxide that was used to adjust the pH from 3.9 to 5. The structure of Fig.4(iv) is actually due to a combination of factors - the pH res u lting from zi nc carbonate hydroxide addition and hence the va riation in the bath compositi01,1. AddiTion of thiourea and dexTrin The addition of thiou rea (0.75g!li) ;1nd dextrin (3gJ li) to the acid solution at pH3.9 produces surface characteristics or structures. Fig.S(i-ii). which are different from those of Fig.-+ in general. CORROSION PREVE!'oiTION & COi\TROL December 1990 Here the crystal structure is difficult to define. The surface looks like a unique weathered rock without a ny trace of pores. both at the low (xlOO) and at higher (xlOOO) magnifications. The addition of thiourea and dextrin has provided a levelling effect and also enhances better thro\ving power. The result indicates that good zi nc electrodeposition on mild steel is obtained at this pH level and \Vith the additives used. Fig.5( iii-iv) shows the SEM micrograph obtained for the same bath but at different pH. At pHS. the structure obtained is slightly different from that obtained at pH3.9. The crystal structure here. thoug[J. looking somewhat amorp ho us. is finer. The effect of this finer structure (if any) on ,the plating performance is theoretically difficult to say at present. It is apparent that the pH has a modifying efktt through the addition of zinc carbonate hydroxide. AddiTion of thiourea. dextrin and glycin The addition of thiourea (0.75g/li). de~trin (3g/li) and glycin (1.5g/li) to the acid solution at pH3.9 and for a plating time of 12.5min produces a unique and 161 ·-: ( Fig .6 . SEM micrographs showing the effect of thiourea (0.75glli) and dextrin (3g/1i) and glycin (2.5g/li) on the electrodeposition of zinc on mild steel in acid chloride solution; piating time 12.5min. beautiful surface structure both at the low magni-fica tion of xlOO. Fig.6(i), and at high mqgnification, xlOOO - Fig.6(ii). Fig. 6(ii) gives a clearer picture of the arrangement of the surface crystals which is difficult to describe. The surface structure indicates good zinc electrodeposition with an absence of surface pores. The surface structure obtained here looks different from that of Figs 5(ii) and 5(iv). This difference is apparently because of the difference in bath composition due to the further addition of glycin. At pHS, after adjustment of the bath composition by zinc carbonate hydroxide, the surface structure of the el ectrodeposited zinc again looks different. Fig.6(iii-iv). Though still very compact and with no trace of any pore, the surface crystals, (Fig.6(iv)). are r1ot as well defined as in Fig.6(ii). The technical effect of the difference on surface crystal arrangement or structure cannot be expressed at present. eoweIv~rI ~ t is appare~t that pifference in pH level provides· a different surface structure. As mentioned earlier, the difference in structure is not due to the pH effect alone. but also to the · consequential e,ffect of the pH-adjusting chemical -the zinc carbonat r;: hydroxide. Adc.'irion of g/ycin and dexrn"n Further variflti~n in bath compos1t10n by the addition of glycin and dextrin to the acid solution 162 Fig.] . SEM micrographs showing the effect of ~lycin (2.5g/ li) and dextnn (3g/li} on the electrodeposition of zinc on mild steel in acid chloride solution: plating time 12.5min. at pH3.9 produces a completely different surface structure from all • those previously discussed. Fig.7(i-ii). At xlOO magnification. (Fig.7(i)). the real crystal structure is not discernible. There is a complete absence of pores. and the compactness of the zinc deposition is very clear. At the higher magnification of x 1000, Fig.7(ii). the .crystal structure is uniquely clear. The surface crystals are in platelet-like form and arranged all over the surface in piles of different orientations. At this pH level it is apparent that the zinc electrodeposition is good. It is not clear, however. which of the additives has a greater ~nfluence on this unique surface structure. Their effect is probably synergistic. On re-adjusting the pH of 3.9 to 5 with zinc carbonate hydroxide. at the same bath composition and for the same plating duration. a completely different surface crystal structure from the above is obtained. Fig.7(iii-iv). The surface crystals are smaller and are in amorphous/angular shapes all over. without any particularly defined a rrangement (Fig.7(iv)). There are no pores observed at both low (Fig.(ii). x!OO) and high . (Fig.7(iv). xJOOO). magnifications. an indication th at a good plating is obtained. It could be said that the effect of pH vari ation is more one of modifying th e surface structure. the technical effect of which has not yet been determined. CORROSIO!'i PREVEl'\TIO!'i & CO:".'TROL December 1990 Addirion o/glycin and thiourea Glycin (2 . .5g/li) and thiourea (0.75g/li) addition to the acid solution at pH3.9 and for a plating time of 12.5min provides a porous result as shown in Fig.::\(i-ii). The plating porosity is clear at the low-magnification of xlOO (Fig.8(i)). At the higher magnification of x!OOO. (Fig.S(ii)). it is clearer that the electrodeposition is not good. Further repeated experimental work cou ld not prov·ide a better result c:ither. The reason for this poor plating with the use of glycin and thiourea together is not understocd. It might be that the experimental work in this particular regard is faulty. or the: additives enhance the poor throwing-pov.·er of the bath instead of improving it. Readjustment of the bath pH level from 3.9 to .5 with zinc carbonate hydroxide did not provide a better result. Fig.S(iii-iv): in fact. it produced a worse result. The bath composition here. with the use of glycin and thiourea as additives, could be described either as not suitable for zinc electrodeposition on mild steel in acid solution. or the operating parameters (in particular the pH requirement) is sensitive. and the right pH level suitable for good electrodeposition has yet to be used for this bath with these additives. Conclusion l. The addition to acid-solution. in turn. of thiourea and dextrin: thiourea. dextrin and glycin: and glycin and dextrin, gives good zinc electrodeposition on mild steel without any pores being obserYed at both pH4 and pHS for a plating time of 12.5mins. 1 The combined additives. representing different bath compositions. give different surface structures at both pH4 and pHS. 3. The plating time of l2.5mins for each of the e lectrodeposi tion operations was generally found suitable throughout this investigation. 4. The additives used and the pH levels at which this inYestigation was carried out. in addition to the plating time mentioned i:1 (3). are generally found suitable for zinc electrodeposition on mild steel in acid solution. with the exception of the addition of combined glycin and thiourea Acknowledgement This work was carried out" by one of the authors (Dr C.ALoto) with financia l assistance (fellowship J from the Swedish Institute. Stockholm. and travelling sponsorship from the Federal Ministry of Science and Technology. Lagos. Nigeria. to whom the authors are gratefuL The co-operative laboratory assistance of Messrs Goran Fritze and CORROSION PREVE:-.TION & COI'iTROl December 1990 Fig.8. SEM micrographs showing the affect of glycin {Jglli} and thiourea (0.75g/ li) on the electrodeposition of zinc on mild steel in acid chloride solution; plating time 12.5min. Urban Jelvestam from the Department of Engineering Metals. Chalmers University of Technology. Gothenburg, is highly appreciated. References I. C..-\.Loro. LOletjord and H.Mattson. Surface characterization of the elq~cts of organic additives ori the ~kctrodc:pos ition of zinc on mild steel in acid-chloride: solution. Submitted to Corrosion Prevention & Comrol. UK ~ C _-\.Lata and LO!etjord. Inlluc:nce of organic additives on thi! surface chara.::teristics of zinc ekctroJeposition on mild stal in acid chloride: solution - pH. hath .::cmposition and time: ,·ariation dTects. Submitted to Corrosion Pre>·ention & C,>ntro/. UK. _\. e KeKd~duldK 1982. ln Mc:tals Handbook ASM. 9th Ed .. 5. t:'P 250-252. ~- R.K.Preiksaitc: )nd R.R.Sarmaitis. 1981. Plating and SurfFin .. 68. ~- 60. 5. pKpchneicg~rK ri9S7. ibid.. 74. 20. 6. :\Lp.o·Angdo: 1986. ibid.. 73. 9. 23. I. S.:'vforisaki. T.:-vlori and S.Tajima. 1981. ibid..'68. 8. 55. S. :\I.Pushpayanam. 1986. ibid.. 73. 8. 47. 9. T.V.Vc:nkatc:sha. J.Balachandra. S.M.Mayanna and R.P.DambaL 1987. ibid.. 7.:1. 6. 17. 10. R.C.Hiremath and S.M.Mayanna. 19t:~K Jfikrochimica Acta i\"ienna). 2. 175. II. K.V.Uma and S.l'vf.Mayanna. 1980. Jlndian Clr~mK poc .. ·57.' 655. t63 '. 

S.M.Mayanna and R.P.DambaL

The Effect of Additives on the

Electrodeposition of Zinc on Mild Steel in

Acid Chloride Solution

http://eprints.covenantuniversity.edu.ng/1364/1/The%20effect%20of%20additives%20on%20the%20Electrodeposition%20of%20Zinc%20on%20Mild%20Steel%20in%20Acid%20Chloride%20Solution.pdf

The Effect of Additives on the Electrodeposition of Zinc on Mild Steel in Acid Chloride Solution

Abstract

Similar works

Full text

Available Versions

Covenant University Repository