The first chromium metal was obtained by electrodeposition in France. This first result was realized by electrolysis of an aqueous trivalent chromium solution, published and patented in 1848 by Junot de Bussy.1,2 In 1854, Bunsen3 studied the influence of the cathodic current density on chromium deposition using a hot chromium chloride solution, with separation (porous pot) of anodic and cathodic compartments. The importance of the separation between anodic and cathodic compartment was considered a major discovery for a long time and was further developed by others researchers, including Placet and Bonnet4 in 1901, Voisin5 in 1910 and Recoura6 in 1913.
Currently, decorative trivalent chromium processes are available worldwide and deposits are currently “quasi” indistinguishable in color from hexavalent deposits. The solutions used are aqueous solutions with primarily chloride and/or sulfate trivalent chromium salts with various additives acting as complexants (formates, thiocyanates, hypophosphites, etc.) and different types of wetting agents. A separator may or may not be used between anodic and cathodic compartments.
Until now, no aqueous trivalent chromium process has been available to obtain a hard thick chromium deposit on an industrial scale. Why it is so difficult to deposit hard chromium from trivalent salts? The many reasons to explain the lack of success have been enumerated in literature. Among numerous publications, the reasons which seem the most important are:
What follows is a description of several processes which have been considered in overcoming the difficulties described above.
Before describing the various electrolytic processes, it is clear that electroless deposition of chromium would be a very suitable method for understandable reasons.
In 1955, published work showed that it was possible to deposit chromium using an electroless method. Two types of aqueous solution were disclosed using hypophosphite and citrate ions, chromium chloride and chromium fluoride with or without glacial acetic acid. Solutions were operated at 71-88°C (160-190°F). The deposition of electroless chromium was found to be better on copper or brass substrates. For steel plating, it was recommended that a copper flash be deposited before electroless chromium plating. Also, the solution, and so the process, was very sensitive to foreign ion contamination.
Currently, two main process classes have been developed to overcome the difficulties in obtaining thick hard trivalent chromium deposits:
It is interesting to note that using reduction processes allows one to have trivalent chromium solutions with high salt concentration at a low pH.
These processes are used in decorative trivalent chromium applications, but they have been extended to obtain hard chromium layers on a laboratory scale. These baths use complexing agents such as formate, thiocyanate or hypophosphite, in some cases with additives such as glycolic acid, citrates or sulfamates. For each of these compositions, the conditions of deposition, including deposition rate, throwing and covering power, microhardness and texture, vary.
For solution using formates, several formulations have been proposed. In general, the chromium salt used is chromium chloride with certain additives. A listing of the various formulations follows. This is not an exhaustive list but rather a general listing of the general complexant formulations is given. In a later section, the characteristics of trivalent chromium processes using pulsed and/or cyclic current or using brush plating are presented.
General formate process:
CrCl3•6H2O 100 - 150 g/L
HCOOH 50 - 80 ml/L
NH4Cl + NaCl 70 - 100 g/L
H3BO3 10 - 40 g/L
pH 0.1 - 1.0
Cathodic current density 20 - 100 A/dm2
Temperature 20 - 30°C
Glycolic acid or citrate acid:8
CrCl3•6H2O 100 g/L
HCOOH 30 mL/L
NH4Cl 80 g/L
Glycolic acid 35 g/L or Sodium citrate 95 g/L
H3BO3 40 g/L
Cathodic current density 30 - 50 A/dm2
Temperature 20 - 50°C
Solution composition for a formate solution with sulfamate additives:
CrCl3•6H2O 125 g/L
KCr(SO4)2 25 g/L
HCOOH 60 mL/L
NH4Cl 80 g/L
NH4NH2SO3 180 g/L
Cathodic current density 10 - 20 A/dm2
Temperature 20 - 25°C
CrCl3•6 H2O 210 g/L
NaCl 30 g/L
NH4Cl 30 g/L
H3BO3 35 g/L
AlCl3 50 g/L
pH 0.1 - 1.0
Cathodic current density 20 - 70 A/dm2
Temperature 30 - 50°C
CrCl3•6 H2O 30 - 65 g/L
NaH2PO2 200 g/L
NaF 4 g/L
NH4Cl 320 g/L
H3BO3 15 g/L
pH 2.0 - 5.5
Cathodic current density 2.0 - 50 A/dm2
Temperature 25 - 35°C
Cr2(SO4)3•6H2O 30 - 90 g/L
NaSCN 25 - 80 g/L
Na2SO4 200 g/L
H3BO3 20 - 40 g/L
pH 2.0 - 3.0
Cathodic current density 2 - 10 A/dm2
Temperature 20 - 40°C
In general, these deposits have a microhardness of about 700 HV100, a cathodic efficiency lower than for hexavalent chromium processes and thus a very slow rate of deposition (0.2 to 0.4 μm/min). However the covering power and the cathodic current density zone in which fair metallic chromium deposition takes place, is more important for those processes than for hexavalent ones.
Processes using trivalent salts obtained by the reduction of hexavalent chromium ions.
Reduction by SO2:13
The first application was done in 1946 by the U.S. Bureau of Mines. This process allows the preparation of chromium metal by electrolytic deposition with anodic and cathodic compartment separation.
Cr2(SO4)3 250 g/L (obtained by the reduction of Na2CrO4 by SO2)
(NH4)2SO4 40 g/L
Na2SO4 27 g/L
(NH4)2S2O8 100 - 200 mg/L (constant addition, function of electrolysis time)
pH 1.8 - 2.2
Cathodic current density 6.0 A/dm²
Temperature 27 - 42°C
Reduction by alcohol:14
CrCl3 130 - 180 g/L
NH4Cl 50 - 100 g/L
H3BO3 30 - 50 g/L
Cathodic current density 60 - 120 A/dm2
The process allows deposition with high efficiency (up to 30%). Deposits have a microhardness of about 1000 HV100, but the throwing and covering power are lower than other processes. However these processes are interesting because of their high rate of deposition (2.0 - 3.0 µm/min). The bath obtained by the methanol reduction method was developed in our laboratory. However, it is very important to note that this process is dangerous and hazardous during the stage of preparation of trivalent chromium solution, due to the high exothermic reaction of hexavalent chromium reduction by alcohol.
Other process technologies:
Some interesting studies on the influence of the current waveform shape on the deposition of trivalent chromium have been reported. The results depend on the shape of the cathodic current waveform. The influence of the first deposited layer and the solution-cathode interface has been clearly demonstrated.15,16
In general the electrolyte in these works was a formate system complexed with different additives. Typical operating conditions were: Current density, 30 A/dm2; Tc, 80%; Ta, 20%, Frequency, 100 Hz. The current efficiency was 30 - 35% and the microhardness was 850 HV100.
Brush plating applications have also been disclosed which allow the deposition of a chromium layer on substrates using mobile systems.17,18 This application could be of major interest in the future for in situ repair of in chromium layers after wear or abrasion, or after deposition.
The operating conditions for the proprietary formulation** are as follows:
Cathode current density 93 - 232 A/dm2 (864 - 2160 A/ft2)
Voltage 4 - 15 V
Temperature 71 - 77°C (160 - 170°F)
(for decorative applications, room temperature)
Anode-to-cathode movement 40 - 60 ft2/min
Organic solvents: DMF and ionic liquids
During the early 1970s, processes using organic solvents (or mixed with water) were proposed, in particular one consisting of dimethyl formamide (DMF) as a polar aprotic solvent which decreases the formation and stability of the aquo-complex with Cr+3 ions and H2 gas evolution. The results are very interesting but due to the toxicity of using large quantities of DMF, industrial development has not been pursued to any significant extent.
More recently, following the development of the use of ionic liquids for electroplating during the IONMET European research program (ended in 2009), research was done on hard chromium plating using trivalent chromium salts.19 Using a mixed solution of CrCl3•6(H2O) and choline chloride,*** hard chromium layers were deposited on steel. Operating conditions were as follows:
CrCl3 / choline chloride ratio 1 / 2.5
Cathodic current density 15 - 20 A/dm2
Anode Platinized or Ir2O3 titanium grid
The main results were:
Metallic and bright uniform appearance (darker than hexavalent processes)
Cathode efficiency ~ 30 - 40 %
Deposition rate ~ 0.7 to 1.0 µm/min (at 15 A/dm2)
Hardness ~ 600 to 700 HV100
Cross-sections of the deposit are shown in Fig. 1. The process seems to be robust and a replenishment method has been defined following intensive use in the laboratory.
Figure 1 - Cross-section of chromium deposits obtained by electrodeposition from ionic liquids.
Influence of thermal treatment
Some information has been published on the increase of hardness in trivalent chromium by heat treatment.20,21 In particular, in a research grant sponsored and published by AESF22 involved work in this area.
Due to chromium carbide (Cr7C3) and oxide (Cr2O3) formation during heat treatment (300-350°C, 30 min), the microhardness was increased to 1700-1800 HV100. Under the same conditions, the microhardness of hexavalent chromium deposits decreased, as shown in Fig. 2.
It was found through x-ray diffraction studies23 that as soon as the temperature reaches 290-300°C, a structural modification takes place and there is a precipitation of chromium carbide. The chromium carbides formed prevent dislocation movement and thus an increase of hardness occurs (pin effect).
From Fig. 2, it appears clearly that the variation is fundamentally distinct, and in the case of trivalent chromium deposits, the variation is similar to variation of microhardness versus temperature for electroless nickel.
Figure 2 - Evolution of the micro-hardness of the deposit [Cr(VI) and Cr(III)] as a function of heat treatment.
The history of alternative processes to hexavalent hard chromium plating is a long but generally not successful story. The substitution of hexavalent chromium processes by trivalent ones seems to be the best way but currently, no processes have reached significant industrial development. Currently, the ionic liquid processes seem to be the most promising, but the adhesion of chromium layer, as for all the trivalent processes, seems to be an obstacle for a large industrial application. Where strong adhesion is not a factor, there is potential for some development.
However, the question remains as to the reason for a substitute for hexavalent chromium in hard chromium plating process. After deposition, there is no (or very little) Cr(VI) on the parts and so the only domain where the “Cr(VI) risk” is present is in the plating job shop. If regulation in the job shop is well defined and respected to avoid contamination of workers, there is no real or serious reason to banish hard chromium plating with Cr(VI) in the first place.
- J. Debussy, French Patent 13902 (1848).
- J. Debussy, French Patent 13902 (1855).
- R. Bunsen, The Chemist, 11, 685 (August, 1854).
- E. Placet & J. Bonnet, Bull. Soc. Chim. Fr., 3-25, 620 (1901).
- J. Voisin, Rev. Metall., 7, 1137 (1910).
- A. Recoura, Comptes Rendus, 157, 1525 (1913).
- D. Smart, T.E. Such & S.J. Wake, Trans. Inst. Met. Finish., 61, 105 (1983).
- D. Lashmore, I. Weisshaus & E. NamGoong, U.S. Patent 4,804,446 (1989).
- C.E. Johnson, D. Lashmore & E. Soltani, U.S. Patent 5,415,763 (1995).
- M. El Sharif, S. Ma & C.U. Chisolm, Trans. Inst. Met. Finish., 73, 19 (1995).
- J-Y. Hwang, Plating & Surface Finishing, 78 (5), 118 (1991).
- A.K. Hsiel & K.N. Chen, Metal Finishing, 92 (5), 11 (1994).
- R.R. Lloyd, W.T. Rawles & R.G. Feeney, U.S. Bureau of Mines, Trans. Electrochem. Soc., (Proc. 89th General Meeting, Birmingham, Alabama), 89 (1), 443 (1946)
- P. Benaben, Plating & Surface Finishing, 76 (11), 60 (1989).
- R.P. Renz, et al., Proc. AESF SUR-FIN 1996, Cleveland, Ohio, NASF, Washington, DC, 1996.
- R.P. Renz, et al., Proc. AESF SUR-FIN 2001, Nashville, Tennessee, NASF Washington, DC; p. 439.
- Z. Mathe, Proc. AESF SUR-FIN 1996, Cleveland, Ohio, NASF Washington, DC; p. 361.
- D. Hutchinson, Proc. AESF SUR-FIN 1996, Cleveland, Ohio, NASF Washington, DC; p. 429.
- P. Benaben, Proc. AESF SUR-FIN 2007, Cleveland, Ohio, NASF Washington, DC; p. 382.
- V.N. Kudryavtsev, et al., Proc. AESF SUR-FIN 1996, Cleveland, Ohio, NASF Washington, DC; p. 433.
- J. Dash & J. Dehaven, U.S. Patent 5,413,646 (1995).
- P. Benaben & F. Durut, Final Report, AESF Summer Research Project, December 1996, NASF Washington, DC, 1996.
- R.Y. Fillit, A. Rousseau & P. Benaben, Matériaux et techniques, 89 (11-12), 55 (2001).
**Liquid Development Company, Cleveland, Ohio.
***Choline refers to quaternary ammonium salts containing the N,N,N-trimethylethanolammonium cation.