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BY . PROTSENKO .", V.O. GORDIiENKoa, F.I. DANILOVaAND S.c. KWONb

aDepartment of Physical Chemistry,Ukrainian State University of Chemical Technology,Gagarin Av. 8, Dnepropetrovsk 49005, UkrainebSurface Engineering Department,Korea Institute of Machinery and Materials,66 Sangnam-dong, Changwon, Gyeongnam 641-010, Republic of Korea

Table 1Deposition time 1S min; 1.5; temperature 35°C; current density 35 A dm,2

An investigation into current efficiency, electrodepositionrate and surface morphology.

Thick Chromium Electrodepositionfrom Trivalent Chromium

Bath Containing Carbamideand Formic Acid

20.1

20.1

36.5

21.3

19.7

trolysis no mass gain was observ­able16. The principal problem seemsto be formation of Cr(IIl) hydroxo­complexes in the near-electrode layerduring Cr-deposition. The electrodesurface may be blocked by poorly sol­uble adsorbed hydroxide com­pounds ofCr(III)17. 18. Therefore, therate of chromium plating diminish­es. In addition, the particles of theCr(III) hydroxide solution are incor­porated into coating structure,which causes cracking and darken­ing ofdeposits.

In order to avoid these undesir­able phenomena, some special com­plexing agents and buffers shouldbe used1. In recent years, the triva­lent Cr-baths which contain car­bamide and formate (or formicacid) as such complexing agentsand buffers were investigated in anumber of papers4-7.

However, the effect of bath compo­sition and electrodeposition condi­tions on the current densiry, electro­plating rate and surface morphology

29.0

29.0

29.0

33.5

36.5

0.2

0.15

0.1

0.05

o

coatings are currently producedfrom chromic acid solutions con­taining highly toxic compounds ofhexavalent chromium.Environmental considerations areresponsible for the increased interestin less toxic trivalent chromiumbaths as an eco-friendly alternative tohexavalent chromium bathsl-15•

However, it is not easy to obtainthick deposits from trivalent chromi­um baths. The rate of chromiumdeposition was stated to bedecreased rapidly with depositiontime, and after 10 minutes of elec-

1. INTRODUCTIONHard chromium electroplating playsan imponant role in modem indus­try due to unique properties ofchromium deposits. Hard chromium

ABSTRACTEffect of bath composition andelectrolysis conditions on the cur­rent efficiency, electroplating rateand surface morphology ofCr-coat­ings was studied using a trivalentchromium bath containing chromi­um sulfate, sodium sulfate, alu­minium sulfate, boric acid, formicacid, carbamide and surfactant. Theoptimal concentrations of Cr(II1)­ions, carbamide and formic acidwere stated to be 1.0, 0.5 and 0.5 M,respectively. At optimal bath com­position and electrolysis condi­tions, the deposition rate does notappreciably diminish during elec­trolysis time and reaches -1-1.5 J.l.mmin'l. The value ofcurrent efficien­cy of chromium electrodepositionprocess is close to -30-40%. Thethick chromium coatings depositedfrom the bath under considerationare bright and smooth; they exhibitnodular rype of surface structure.

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Table 2. Optimal Basic Bath Composition and Electrodeposition Conditions.

Crz(S04h 6HzO

I0.5

HCOOH 0.5--

CO(NHzlz 0.5 pH 1.5

Alz(S04h18HzO 0.15 Temperature 35°C

NaZS04 0.3 Current density30 - 35 A dm-z

H3B03 0.5

Sodium dodecyl sulfate 0.05 - 0.1 gL-1

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of Cr-coatings obtained from triva­lent chromium bath which containscarbamide and formic acid (formate)has been studied insufficiently.

2. EXPERIMENTALChromium electrodeposition wascarried out in a usual thermostatedglass cell (V = 1 L). Chromium wasdeposited at a steady value ofcurrentdensity on the disc electrode of cop­per foil (5 = 1.77 cm2) ftxed in a plas­tic holder.

The electrolysis was carried outusing titanium-manganese dioxideanodes (TMDA). On the TMDAthe electrooxidation of Cr(III) ionsoccurs with a rather small rate19.

Therefore, chromium electroplat­ing can be performed without sep­aration of anodic and cathodiccompartments.

The current efficiency was calculat­ed by comparing the weight gain ofthe cathodes placed in the chrome­plating bath with that of a coppercoulometer connected in series.

The surface morphology ofdeposits was investigated by scan­ning electron microscopy (EVO40XVP). The samples used in SEM­study were electroplated on the elec­tropolished Cu-substrate, the thick­ness ofdeposits being about 20llm.

3. RESULTS AND DISCUSSION3.1. Selection ofsome bath constituentsIn present work, chromium sulfateand chrome alum were proofed as asource of Cr(III)-ions. Several bufferagents (boric acid and aluminiumsulfate) as well as conducting salts(sodium sulfate, ammonium sulfateand potassium sulfate) were used forbath preparation.

The results obtained in prelimi­nary investigations allow us to drawthe conclusions as follows:

i) The additive of carbamidefavorably affected the chromiumelectroplating process if solutioriis acid enough-the coatingsbecome visually bright when pHtends to a value of-1.5 and

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lower;ii) An increase in the Cr(II1)-ionscontent improves surface appear­ance and results in an increase inthe current efficiency andchromium deposition rate;iii) When the current density israther high, the pitting appearson the coating surface. In orderto remove pitting, it is necessaryto use some wetting agents, forexample, sodium dodecyl sulfate(sodium lauryl sulfate);iv) In electrolytes with a ratherhigh chromium (III) content (0.9M and greater), the restrictedmutual solubility ofconstituentsmay be reached even at roomtemperature and salts precipitatepartly at the bottom of the bath.

The last phenomenon is especiallyexhibited when ammonium ions arepresent in the solution (in the formof (NHJ2S0J or the electrolyte isprepared on the base ofchrome alum(KCr(S04hx12H20). The chemicalanalysis showed that the sedimentwas either potassium alum or ammo­nium alum; it did not containchromium (III) ions.

The phenomenon concerned maybe related with a sufficiently smallersolubility of the alums containingpotassium or ammonium in com­parison with those containing sodi­um. For example, reference data onthe solubility of waterless saltsNaAI(S04)2 x 12H 2 0,KAI(S04)2x12H20 andNH4Al(S04hx12H20 are 39.72, 5.9and 7.17, respectively (in g in 100 g

H20 at 20°C)20.Formation of sediment at the bot­

tom of the bath containing chromealum can create obvious complica­tions in commercial operation. Thus,it is reasonable to prepare chromiumelectrolytes on the base ofchromiumsulfate. The Cr(III)-ions content inthe bath containing Crz<S04h maybe brought up to 1 M.

Concerning conducting salts, sodi­um sulfate, but not potassium orammonium compounds, seemsrationally to utilize because ofa larg­er solubility of sodium salts as com­pared with potassium and ammoni­um compounds.

The current efficiency of chromi­um electrodeposition reaction andthe quality of deposits proved todepend signiftcantly on the presenceofbuffer agents in the bath.

When the bath does not containboric acid, the current efficiency israther high but dull chromiumdeposits are obtained.

If aluminum sulfate is absent inthe bath, the current efficiency growsappreciably (up to 40-50%). However,gray and extremely rough coatingsdeposit in this case.

Thus, the high-quality deposits canbe obtained only when both boricacid and aluminum sulfate are pres­ent in the trivalent chromium bath.The optimal contents of these addi­tives are close to 0.15 and 0.5 M forAl2(SOJ3 and H3B03, respectively.

As stated above, using some wet­ting agents proved to be necessary toremove pitting formation on thedeposits' surface. A whole number of

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ElectrodopositJon time I min

decreases appreciably; whileHCOOH concentration is 0.7 M andover, the current efficiency slightlygrows. It should be noted that thedeposits readily crack and exfoliatefrom the substrate at the HCOOHconcentration of 0.3 M. At a greatercontent of formic acid (~0.5 M), thecoatings are smooth, bright;' theyadhere firmly to the substrate.

The dependencies of the currentefficiency on carbamide concentra­tion are similar to the foregoingones (Figure 2). Namely, the currentefficiency has a minimum at a "mid­dle" carbamide concentration (0.5M). When the CO(NHzh contentincreases or decreases, the currentefficiency grows at any cathodic cur­rent density. However, while the car­bamide concentration is relativelysmall (0.3 M), the deposits are brightbut somewhat strained and there is alarge number ofwide cracks on theirsurface. Note that the depositsbecome rather dull if CO(NHzhconcentration is equal to 0.7 M.

Taking into account all thesefacts, we assume that the optimalcontent of both formic acid and

Fig. 6 - Photo of steel cylinder for air com­pressor in air-conditioning with Cr-depositsfrom the bath under considerationThickness of Cr layer is about 30 11m

Fig. 5 - Effect of eleetrodeposition time oncurrent efficiency and chromium electrodepo­sition rateBath composition (M): 1.0 Cr(III), 0.5 HCOOH.0.5 CO(NHzh. 0.5 H3B03• 0.15

Alz(SOJ3xlSHzO, 0.3 Na2S004. and 0.05 g L-1

sodium dodecyl sulfateTemperature 35 ·C; pH 1.5; current density 35Adm-z

1.8

43% 40 40

(2.26 Jim mm") E~ 35

:l.

g30 30 1" "~ 25

'0

20 '0

" ::~ 20 "c" 10 isu 15 :E

~

10 00 5 10 15 20 25

50

tf!. 40

~c: 30Ol'uIEOl

20C~" 10u

01.2 1.5

pH

25 30 35

Current density I A dm-2

Fig. 4 - Effect of current density and tempera­ture on current efficiency of chromium elec­trodepositionBath composition (M): 1.0 Cr(III). 0.5 HCOOH.0.5 CO(NHzh. 0.5 H3B03•0.15

Alz(SOJ3x18HzO. 0.3 NazSO•• and 0.05 g L-1sodium dodecyl sulfateDeposition time 15 min; pH 1.5

50

~ 40 2:::~c.~ 30!i:Q)

~ 20?/40'C

t::OJ

U

because a decrease in the current effi­ciency is not very appreciable in thepresence of this substance. The opti­mal concentration of the sodiumdodecyl sulfate lies in the range from0.05 to 0.1 g L-1. There is an intensivefoam formation when the content ofsodium dodecyl sulfate is more than-0.1 g L-1.

3.2. Effect of formic add and car·bamide content on current eJfldencyand surf«e appearance ofCr-coatingsAs shown in Figure 1, when formicacid concentration changes from 0.3to 0.5 M, the current efficiency ofchromium electrodeposition

10

Fig. 3 - Dependencies of current efficiencyand electrodeposition rate upon pHBath composition (M): 1.0 Cr(III). 0.5 HCOOH.0.5 CO(NHzlz. 0.5 H3B03. 0.15

AI2(SOJ3xlSHzO. 0.3 NazSO•• and 0.1 g L-1sodium dodecyl sulfateDeposition time 15 min; temperature 35°C;current density 35 A dm-z

~~~ ... 3,,~~::::__----_. 2

,," ,,'"w ....",,'"yo'"

...~~~~~~~.. 1

,~

",~..'

25 30 35Current density I A dm-2

different wetting agents may be usedthereto. Introduction of the surfac­tant into the chromium bath resultsin a decrease in current efficiencyand deposition rate (Table 1). Wechose sodium dodecyl sulfate as asurfactant for further investigations

o I I

25 30 35Current density I A dm-2

Fig. 2 - Dependencies of current efficiency oncurrent density at different concentration ofCO{NHz>z(1) 0.3 M CO(NHz>z; (2) 0.5 M CO(NHz>z; (3)

0.7 M CO(NHz)zBath composition (M): 1.0 Cr(lll). 0.5 HCOOH.0.5 H3B03, 0.15 Alz(SOJ3x 18HzO. 0.3

NazSO•• and 0.1 g L-1 sodium dodecyl sulfate

Deposition time 15 min; pH 1.5; temperature35°C

Fig. 1 - Dependencies of current efficiency oncurrent density at different concentration offormic acid.(1) 0.3 M HCOOH; (2) 0.5 M HCOOH; (3) 0.7MHCOOHBath composition (M): 1.0 Cr(III). 0.5CO(NHzh, 0.5 H3B03• 0.15 Alz(SOJ3xlSHzO.

0.3 NazSO., and 0.1 g L-1 sodium dodecyl sul­

fateDeposition time 15 min; pH 1.5; temperature35°C

50

eft. 40-->-ulii 30TjlE~ 20c

~<3 10

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eft. 40~~ .. 1

--~~~

e">- "u "c 30 "

.!!1..' .~.... 3

ulE

' e~~~~---" 2Q)

.;' ",""

C 20 "~

",'".. '":lU 10

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7a 7b 7c

7d 7e

.'.

2O)IFIl ..... ,••w hKl III ]~ wo· ..._ _

7f

7g

eo..~ ..,·.uow ...." , """H WO-IIO_ ",- *1i _

Fig. 7 - SEM image of coating from the opti­mal basic chromium bath without (a) andwith surfactant ((b)-(h»(a) and (b) - at current density 35 A dm-2;

temperature 35·C and pH 1.5;(c) pH 1.3. (d) pH 1.7 - at current density 3SA dm-2 and temperature 35 ·C;(e) 30 A dm-2• (f) 40 A dm-2 - at temperature35·C and pH 1.5;(g) 30 ·C. (h) 40 ·C - at current density 35 Adm" and pH 1.5

7h

~~,,' ,:J;,.,.' <,g1.;:t~ :",';'" i_ ,

~~·-r",·f':f ~~ .Y.~~;~· .--ir-~" '.'.. "K .'

f&t~~'~i~'" -:carbamide is 0.5 M.

It is noticeable that the currentefficiency of chromium depositiondramatically decreases and depositsreadily exfoliate from the substrate ifthe electrolyte does not containeither formic acid or carbamide.

3.3. Effect ofelectrolysis conditions oncurrent effICiency, electrodepositionrate and surface appearance of Cr­coatingsThe current efficiency and the rate ofchromium electrodepositionincrease with an increase in bath pH(Figure 3). However, the dark andstriped deposits with poor adhesionto the substrate are formed when pH

value is equal to -1.8 and more. ThepH value being decreased, the sur­face appearance of chromium coat­ings improves but the values of cur­rent efficiency and deposition ratediminish essentially. Therefore, wethink that the optimal value of pHis about 1.5.

There is an increase in the currentefficiency when increasing cathodiccurrent density and decreasing elec­trolyte temperature (Figure 4).

It should be stressed that growingthe bath temperature results in dete­rioration of deposits-the depositsare not bright enough for the tem­perature of 30°C and lower.Therefore, the optimal value of bath

temperature is about 35°C.When the current density of

chromium deposition is equal to -40A dm-2 and higher, the deposits sur­face becomes rough and not brightenough. Additionally, the "burnings"occur on the deposits surface with anincrease in the current density. Thus,the most favorable value of currentdensity is close to 30-35 A dm-z.

The current efficiency and thevelocity of chromium electroplat­ing process from the bath underconsideration diminish slightlywith deposition time (Figure 5). Itis very important and favorable thateven after 10 minutes ofelectrolysisthe plating rate remains high and

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TECHNlqLLYspeal\lng

large and thick chromium layerscan be obtained during relativelyshort time.

The optimal basic bath composi­tion and electrodeposition condi­tions are summarized in Table 2.

Figure 6 shows a sample with Crcoating deposited from the trivalentchromium bath under consideration.

3.4. SEM characteriZAtion ofcoatingssurfaceAs shown in Figure 7(a), when thebath does not contain surfactant, thesurface ofcoating is rough and irreg­ular with a great number of sphe­roids ofdifferent size. The surface ofcoatings deposited from electrolytescontaining sodium dodecyl sulfate ismore uniformly and smoother(Figure 7(b»; the number of sphe­roids diminishes substantially.

The surface morphology changesby variation of pH. For pH 1.3,there are no cracks on the surface ofCr-layers (Figure 7(c). The numberof nodules is not large; these sphe­roids seem to have a small size. Anincrease in bath pH results in dete­rioration of the coating surface(Figure 7(d). The cracks appear andthe number of nodules rises. Thesurface morphology becomes notuniform and defective.

The current density of chromiumelectrodeposition affects the surfacemorphology (Figs. 7(e) and (t). For30 A dm·2, the surface is rathersmooth and uniform; some sphe­roids of small size are observed. Thecracks are practically absent at thisvalue of cathodic current density.The current density being increasedup to 40 A dm'2, the cracks appear onthe chromium surface.

By decreasing the temperature to30°C, a great number of smallspherical nodules occur and some

defects and cracks are observed onthe deposit surface (Figure 7(g).When the bath temperatureincreases up to 40°C, the numberof nodules formed on the surfacediminishes, but simultaneously thequantity of microcracks seems tobecome larger (Figure 7(h).

CONCLUSIONSTrivalent chromium electrodeposi­tion from a sulfate trivalent chromi­um bath which contains both formicacid and carbamide as the complex­ing agents was considered. Usingchromium sulfate but not chromealum is reasonable for preparation ofhigh-concentrated trivalent chromi­um bath. Some wetting agentsshould be used in order to removepitting formation.

The value of current efficiency ofchromium electrodeposition processreaches -30-40%. The chromiumelectroplating rate does not decreasedramatically during electrolysis time;it is close to -1-1.5 mm min,1.

The thick Cr-coatings with a nodu­lar type of surface structure depositfrom the trivalent chromium bathunder consideration.

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* Corresponding author. Tel. (fax):+380-5~2-474586.E-mail address:Vprotsenko@smtp.ru

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