till now. Cr(III>-based baths for hard, thick Cr plating are not used. They are used only for obtaining thin decorative coatings. The main reason of this is that only ...
Electroplating of Thick, Hard Chromium Coatings from Cr(1II) Baths
V.N.Kudryavtsev*,E.G.Vinokurov*,S.R.Schach~eyer~’*,O.E.Azarko~ .
* Mendeleyev University of Chemical Technology of Russia, ** EATON Corporation, USA.
Introduction For a long period of time, it was commonly accepted that hexavalent chromium baths have no alternatives. Thick hard wear resistant chromium coatings are usually deposited from these baths till now. Cr(III>-based baths for hard, thick Cr plating are not used. They are used only for obtaining thin decorative coatings. The main reason of this is that only thin coatings could be deposited from Cr(II1) baths. After deposition of some microns of coating the deposition rate and current density quickly fall and then deposition stops. There is no clear explanation of this phenomenon up till now. Anodic process is considered to be the problem also, Cr(II1) is oxidized on the anode by the reaction: cr3+ - 3e -> cr6+. Cr6+ ions upset the cathode process, decrease C.E. and deteriorates the surface appearance of the coating. These and some other disadvantages of Cr(II1) baths are known. At the same time, the advantages of Cr(1II) baths are well known. The most important of them is presence of chromium in a bath in the trivalent state, which is environmentally safer and more acceptable. In recent years, campaign against the use of toxic Cr(V1) compounds have been sharply increased in USA as well as in Europe. This causes significant increased interest in the study of hard chromium electroplating from Cr(II1) baths. Mendeleyev University of Chemical Technology ( R u s s i a ) j o i n t l y with EATON Corporation (USA) since 1993 also have been carrying out researches in this direction. The main aim of these researches is to study mechanism of chromium electrodeposition and develop process of thick, hard and wear resistant coatings from Cr(II1) baths. Results of our
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ealier studies have been published in [l-41. As in the previous studies, in this work we used baths without chlorine and ammonium ions. The main purpose was to develop Such composition of bath that could enhance deposition of thick chromium coating. We selected special organic compounds, which, as it has been established, allow to obtain good quality coatings to a thickness of 100 pm and more with current efficiency up to 30%. Coatings with a thickness at least up to 50 pm were bright. Organic compounds we used are widely applied in the electroplating industry. First results obtained in the new bath are presented in this report. This is effect of electrolysis conditions (pH of the bath, temperature and current density) on current efficiency and microhardness of chromium coatings. Experimental results Our bath contains the following components as inorganic part: Cr~(S04)~.H~0 - 250 g/l; A12(S04)3.12Hz0 - 120 g/l; NaZSO4 - 100 g / l . Apart from these components, the composition of the bath contained specially selected organic compounds. Platinised titanium anode was used. Coatings were deposited at the bath pH 2 1.2-1.8, temperature 25-40°C and current density 25-40 A/dm . By changing one of the parameters, others were kept constant. Microhardness was determied on chromium coatings with a thickness of 30-70 pm under load on indentor of 50 g by the Vicker’s method. Dependence of Cr current efficiency on pH of the bath is presented in Fig.1. As it is shown in Fig.1, this dependence has extremial form with a maximum. The highest current efficiency (30%) was obtained at pH 1.4-1.6. Current efficiency decreases at lower and higher pH. 2 current With increase in current density from 15 to 25 A/m efficiency increases (Fig.2). At current density from 25 to 40 2 A/m current efficiency has enough stable value and is 28-32%. The highest current efficiency (38%) was obtained at bath 2
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temperature 3OoC (Fig.3). The higher the temperature the lower the current efficiency and at temperature of 4OoC CE is 25%. It is known that microhardness of chromium coatings, depostted from Cr(VI> baths depending on CD has maximum [5]. It has been mentioned that microhardness of chromium plated obtained from Cr(II1) baths, decreases with increase in cathodic CD [61. In our work [41 inverse dependence was established for Cr-P alloy plated from Cr(II1) baths. Dependences of micronardness of chromium coatings, obtained from the newly developed bath, on current density at different temperatures of the bath are presented in Fig.4. As shown in Fig.4, microhardness of the coatings as a dependence on current density at all temperature of the bath has maximum. The position of the peak essentially depends on temperature. With rise in temperature the peak of microhardness shifts to the side of high current densities. Under this condition the magnitude of microhardness in the peak changes insignificantly and is 1100-1200 kg/mm2. Acidity of the bath has effect on nicrohardness of the coatings only within the interval of pH 1.2-1.4. Coatings obtained at C.D. 30 A/dmz, t 35OC bath pH 1.2 have low hardness of 700 HV. When pH of the bath is raised to 1.4 hardness of the coatings increases to 1000 HV and remains ccnstant till pH 1.8. This dependence correlates well with dependence of C.E. on pH (Fig.1). Within pH range of 1.4-1.8 coatings are deposited with the same high current efficency of 30%. Visual study of the quality of the coatings showed that it strongly depends on electrolysis conditions. Coatings with maximum hardness (C.D. 20 A/dm2, t 3OoC; C.D. 35 A/dmz, t 35OC; C.D. 40 2 ~ / d i, t 40'0 have no satisfactory quality: t h e coatings ioose brightness at thickness of 10-20pm, nudels are formed on the surface, the number and size of which increase with increase in thickness of the coating. Coatings of best q-lality correspond to hardness which is by 100-200 units lower than the maximum. 3
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Conc1us i on Electroplating conditions and coating hardness from a ricwly developed Cr(III> bath were studied in this work. It was shown that good quality coatings with thickness of 100 microns and higher were deposited from the bath. The coatings are bright, at least, to a thickness of 50 microns. The coatings exhibits very high hardness, comparable with the hard chromium coatings obtained from Cr(VI> baths. References 1. Kudryavtsev V.N., Vinokurov E.G., Bondar V.V., Borsh J . , Orovets D. Proceedings of 11th 1nt.Corrosion Congress, Florence, Italy, Italy, 2-6,IV, 1990,V.1, p.1191. 2. Kudryavtsev V.N., Gradova N.V., Vinokurov E.G.. Proceedings of the 80th AESF Annual Techncal Conference, SUR/FIN’93, June 21-24, Anaheim, USA, p.169. 3. Kudryavtsev V.N., Vinokurov E.G., Schachameyer S.R. Proceedings of the 81st AESF Annual Technical Conference, SURDIN’94, June 20-23, Ind., USA, p.579. 4. Kudryavtsev V.N., Maksimenko S.A,Vinokurov E.G., Schachameyer S.R. AESF Chromium Collogium, Jun.27-28, 1994, Hyatt Orlando, Kissimmee, Florida, USA. 5. b r e p M.A., TOR J.1. HoBbIe 3JeKTpOJIHTbI JJIR llOKpbITHfi XPOMOM M ero c m a B a m // IKypHaa Bceco~os~oro XmwecKoro o6rJlecTsa MM.&.M. MeHaeneesa, -1988.- v.33, N.3.- p.297. 6. Benaben P. Plating and Surface Finishing.- 1989.-V.76,N.11.p.60.
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Fig.1. Dependence of chromium current efficiency on the bath 2 ~ .nb.u. n - 30 ~ i d2m temperature - 3 5 ' ~
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Fig.3. Dependence of chromium current efficiency on the bath temperature. Bath pH-1.6, C.D.-30 A/dm2
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