Trivalent Chromium Electrolytes Research Articles

Damage Mechanism of Trivalent Chromium Coatings under Tensile Stress

Due to new environmental regulations, hexavalent chromium electrolytes can no longer be used for thick, hard chromium plating. In response to this industrial and environmental challenge, trivalent chromium electrolyte plating has been developed. In this paper, we propose a study of the adhesion of CrIII coatings based on the implementation of numerical models in comparison with an identified experimental scenario. The aim is to dissociate the influence of coating and substrate behaviours from the adhesion work by describing the intrinsic damage of the chromium layer and the coating–substrate interface. Two types of cracking were studied: transverse cracking and delamination. For the former, the crack density was higher for CrIII than for CrVI and increased with deformation and coating thickness. Microtensile tests with scanning electron microscopy (SEM) observations allowed us to highlight the cracking process in the coating (transverse cracking) and at the coating–substrate interface (delamination). The numerical simulation of the test allowed us to estimate a damage-initiation threshold normal stress of 1900 MPa, which occurred at an average applied strain of 2.5%. Delamination of the coating was complete at an average strain of 13.6% and an interfacial normal stress of 2600 MPa.

Characterization and corrosion behavior of Ni-Cr coatings by using pulse current electrodeposition

PurposeThe purpose of this paper is to study the effect of chromium content on the microstructure and corrosion resistance of Ni-Cr coating.Design/methodology/approachNi-Cr coating was prepared by pulse current electrodeposition with trivalent chromium. On the basis of studying effect of electroplating parameters on composition and morphology, Ni-Cr alloy coatings with various chromium contents were obtained. The microstructure was characterized by scanning electron microscopy, X-ray diffractometer and transmission electron microscopy. Corrosion behavior was studied by potentiodynamic polarization and electrochemical impedance spectroscopy techniques.FindingsElectrodeposited chromium was solidly dissolved in nickel and refined the grain of the coating. With the increase of Cr content, the corrosion resistance of Ni-Cr coating was enhanced, which is due to the formation of continuous nickel hydroxide and compact chromium oxide passive films.Originality/valueNi-Cr alloy coating without penetration crack was prepared in trivalent chromium electrolyte, and the mechanism of its excellent corrosion resistance was proposed.

Microstructure and Tribological Performance of Electrodeposited Cr Coating with Trivalent–Chromium Electrolyte

A chromium (Cr) coating was fabricated on 45 steel by electrodeposition with a trivalent–chromium (Cr3+) electrolyte. The morphologies and element distributions of the obtained coating were analyzed using a scanning electronic microscope, energy dispersive spectrometer, atomic force microscope, x-ray diffraction, and x-ray photoelectron spectroscopy. The effects of normal load and sliding speed on the tribological performance of the Cr coating were analyzed using a friction tester. The results show that the Cr coating with the roughness of 85.5 nm is composed of the Cr phase. The average coefficients of friction (COFs) of the Cr coating under the normal loads of 1 N, 3 N, and 5 N are decreased from 1.08 to 0.69, exhibiting excellent friction reduction; while its wear rates increase with the increase of the normal load, and the wear resistance becomes poor. Moreover, the average COFs of Cr coating at the sliding speeds of 600 times/min, 700 times/min, and 800 times/min are in the range of 0.71–0.76, and the wear rate is the lowest at the sliding speed of 700 times/min, showing that the friction reduction under the normal load of 3 N and wear resistance at the sliding speed of 700 times/min are the best.

(Digital Presentation) Fundamental Insights into Electrodeposition of Mixed Chromium Metal-Carbide-Oxides from Trivalent Chromium – Formate Electrolytes

Recent restrictions on industrial usage of hexavalent chromium under new REACH legislation have further sparked the development of hexavalent chromium-free chromium plating processes. An industrially important development in this field is Trivalent Chromium Coating Technology (TCCT®), a chromium electroplating technology for packaging steel developed at Tata Steel. In this process, aqueous trivalent chromium electrolytes rather than hexavalent chromium electrolytes are employed for the electrodeposition of metallic chromium. However, to deposit metallic chromium from a trivalent chromium electrolyte, it is necessary to incorporate a complexing agent given the kinetic inertness of aqueous trivalent chromium complexes. Formate plays this role in the TCCT® process yielding coatings comparable to the conventional hexavalent chromium-based process [1-4].However, understanding the mechanism and kinetics of chromium electrodeposition from this system is quite limited. Fundamental knowledge of the deposition process is key for industrial process optimization. Essential to determining the reaction mechanism and kinetics is the identification of the chemical species involved in the reaction. Using a hybrid multiscale experimental and computational approach, insights into chromium complexation in the bulk electrolytes and how this speciation influences the composition of the deposit have been gained.Samples electroplated in electrolytes of varied formate concentrations were characterized using X-ray Fluorescence (XRF) spectroscopy and X-ray Photoelectron Spectroscopy (XPS). Results from these analyses show that metallic chromium is only deposited when the electrolyte contains formate ions. In the absence of formate, only oxide and carbide species are deposited. The characterization results also show a current efficiency of the TCCT process of ~ 40%. From observations from surface characterization as well as spectroscopic analysis and density functional theory (DFT) and ab initio molecular dynamics studies (AIMD) of the bulk electrolyte, the coordination and complexation of formate ion in the chromium complex responsible for metallic chromium deposition have also been identified. Voltammetric studies coupled with ex-situ XPS and scanning electron microscopy (SEM) surface characterization also lead to a clear definition of the reaction mechanism of metallic chromium deposition that the incorporation of formate in trivalent chromium electrolytes makes possible.

In situ microscopic investigation of ion migration on the surface of chromium coated steels

Cathodic spreading of electrolyte on two-layers chromium coatings electrodeposited from trivalent chromium electrolyte on steel was studied on the micro- and the macroscale. The behavior is discussed in view of results obtained on electrical conductivity as measured by current-sensing atomic force microscopy. The coatings were found to hinder electron transport. Heterogeneities observed in the electrical conductivity are correlated to heterogeneities of the electrolyte spreading behavior, studied using in situ scanning Kelvin probe force microscopy. In average, the kinetics of spreading observed at microscopic scales are similar to that observed using a scanning Kelvin probe at larger scales. The scanning Kelvin probe force microscopy is demonstrated as a robust in situ technique to follow electrolyte spreading and study microscopic defects/heterogeneities on the surface.

Mechanical Behaviour of Hard Chromium Deposited from a Trivalent Chromium Bath

In view of new environmental directives, hexavalent chromium baths can no longer be used to electroplate thick hard chromium deposits. To meet these industrial and environmental challenges, deposits are developed from trivalent chromium electrolytes. Cr(III) coatings are usually studied from the point of view of the use properties and hardness, but their intrinsic properties remain widely unknown. The novelty of this work consists in the mechanical characterisation of these coatings. Properties such as hardness, stiffness, yield strength, and toughness of trivalent chromium deposits are determined by combining instrumented hardness tests, in situ FEG–SEM observations, and finite element simulations. These are explained according to the microstructure of the deposits, which is determined by scanning electron microscopy and X-ray diffraction. Their composition was characterised by glow discharge spectrometry. The structure characterisation deposits showed a more severely fractured coating of trivalent chromium than in the case of hexavalent chromium. Non-post-treated trivalent chromium deposits have a higher hardness (13 ± 1.7 GPa) and yield strength (5 GPa) than hexavalent chromium deposits. However, their stiffness (191 ± 13 GPa) and toughness (1.37 ± 0.13 MPa√m) are lower. Its mechanical behaviour is elastofragile. These differences in mechanical properties can be explained by the amorphous structure of the deposits and their high carbon content.

Influence of carboxylic acids on the performance of trivalent chromium electrolytes for the deposition of functional coatings

As a direct consequence of the restrictions on the use of hexavalent chromium compounds, the demand for a suitable replacement has arisen. In this work the electrodeposition of thick chromium layers (>1 µm) from a trivalent electrolyte is investigated with the aim to identify an electrolyte composition for the deposition of hard functional coatings. These layers can be used to surface finish tribological components experiencing high wear rates or mechanical stress in applications such as coating printing cylinders, feed rollers or piston rods. The influence of different carboxylic acids (malonic acid, malic acid, glycolic acid) on the deposition has been studied. The effect of current density on the current efficiency was investigated using in-situ microgravimetry. For a technical application the electrolyte containing malonic acid was the most promising one and was further investigated regarding the properties of the deposits, such as surface morphology, crack formation, composition, thickness and hardness, aiming at properties as close as possible to those of hexavalent chromium. In comparison to hexavalent chromium, the layer of trivalent chromium showed the same properties in terms of crack formation, hardness and layer thickness (> 1 µm).

Trivalent Chromium Electrolytes with Organic and Inorganic Ligands: Use of Pulsed Currents

Hexavalent chromium salts are targeted by health authorities and in particular by the European regulation REACH because of their high toxicity for humans and the environment. Trivalent chromium salts are a promising alternative to hexavalent ones in the perspective of hard chromium coatings and have been extensively studied [1]. However, trivalent chromium ions bind with water molecules to form complexes resistant to electrochemical reduction, requiring ligands addition to evolve toward more favorable chemical equilibrium. This addition leads to a decrease of the coating properties, due to the incorporation of impurities such as carbon and oxygen. If carbon can be removed by the use of inorganic ligand, the presence of oxygen is more difficult to avoid [2], mostly due to hydroxide species incorporation induced by the pH increase at the interface. This phenomenon is the direct consequence of the important current part used to reduce protons during the plating process. It exists different ways to limit the pH elevation [3] such as the temperature, the electrolytes pH modification or the addition of specific chemical compounds, but an attractive way consists in current modulation i.e. using pulsed currents. The relaxation of the diffusion layer is known to reduce the competition between the diffusion of electrochemical species from the bulk and the proton discharge, which will limit the pH increase and restore the chromium concentration near the surface close to the initial one.Parameters of the pulse sequences tested in the present work were chosen from pH measurements [4] undertaken at the surface vicinity (less than 100 µm), and from the behavior of copper solutions working at high potentials where the competition between copper reduction and hydrogen takes place. Potentials were monitored at two-time scales: ms for the responses to current modulation (transient curves) and min for the average potential evolution all along the process. Finally, a comparison was made between inorganic and organic based trivalent chromium electrolytes, with a special care to cations presence, because of their influence on hydrogen discharge.[1] P. Benaben, Plating and Surface Finishing, p. 8, 2011.[2] A. Baral, R. Engelken, J. Electrochem. Soc., vol. 152, no 7, p. C504, 2005[3] J. Ji, W. C. Cooper, D. B. Dreisinger, E. Peters, J Appl Electrochem, vol. 25, no 7, p. 642-650, juill. 1995[4] M. Marcelet, Y-S. Chao, B. Vuillemin, M-P. Gigandet, C. Gleyzes, R. Oltra, J. Tardelli, J-Y. Hihn, ECS Meet. Abstr., vol. MA2019-02, no 21, p. 1046, sept. 2019

Characterization of the Reduction Mechanism of Chromium(III) By in Situ Microgravimetry and Volumetric Determination of Hydrogen

Processes for chromium electrodeposition based on hexavalent chromium compounds are more and more restricted by governmental laws1 due to the toxic and carcinogenic nature of Cr(VI) compounds. The most promising alternatives to replace hexavalent chromium solutions are trivalent chromium electrolytes considering process management, quality of the deposits and cost issues.2 However, many of these electrolytes suffer from low current efficiencies and reduced process stability compared to hexavalent chromium electrolytes. Organic additives are used to reduce the overvoltage of Cr(III) reduction and, hence, reach an acceptable current efficiency.3 In order to improve performance and reliability of the deposition process, a profound knowledge of the deposition mechanism and the effect of additives is crucial.In this work the stepwise reduction process of Cr(III) to metallic chromium is investigated by gravimetric and volumetric in-situ methods under variation of current density. Electrochemical quartz crystal microbalance (EQCM) was used to quantify the mass of deposited chromium metal.4 The amount of evolved hydrogen gas was determined with a gas burette in combination with a purpose-built setup for collecting the gas. Based on the results, the partial current densities of the two reduction steps of Cr(III) and the hydrogen evolution were calculated. A shift in the ratio of the partial current densities and an increased current efficiency of metal deposition was observed after the addition of saccharin, which is used as additive in chromium electrolytes for decorative applications.5 A mechanism on the mode of action of saccharin during chromium electrodeposition is proposed. European Chemicals Agency, Authorisation List, EC No. 215-607-8, 231-801-5, 231-889-5, 231-906-6, 232-140-5, 232-143-1, 234-190-3, 236-881-5. Z. X. Zeng, A. M. Liang, and J. Y. Zhang, Recent Pat. Mater. Sci., 2 (1), 50-57 (2009). D. Del Pianta, J. Frayret, C. Gleyzes, C. Cugnet, J. C. Dupin, and I. Le Hecho, Electrochim. Acta, 284, 234-241 (2018). M. Leimbach, C. Tschaar, U. Schmidt, and A. Bund, Electrochim. Acta, 270, 104-109 (2018). R. Katirci, Int. J. Surf. Sci. Eng., 10 (1), 73-85 (2016).

In Situ Investigation of Migration of Cations during Electrolyte Spreading on Chromium Coated Steels Electrodeposited from Trivalent-Chromium Electrolyte

Resistance to corrosion and inhibition of cathodic delamination of polymer topcoat makes chromium coated steels suitable for application in the packaging industry. These coatings were traditionally produced from hexavalent chromium electrolytes. These coatings are now being phased out due to the carcinogenic nature of hexavalent chromium used in the production process (by the REACH regulation in the European Union). Electrodeposition from trivalent chromium containing electrolyte is being considered as an alternative to replace these traditional hexavalent-chromium coated steels. The cathodic delamination behavior of polymer topcoat on these trivalent-chromium coated steels were studied in detail previously [1-3]. However, the factors that result in inhibition of cathodic delamination are still not well understood.The two important factors influencing the rate of cathodic delamination are the electron transport and cation migration on the surface. The aim of this study is to understand the influence of complex composition of these trivalent-chromium coatings on the electrical conductivity and ion migration on the surface of the coating at the microscopic scale. The electronic conductivity of the coatings was studied using current-sensing atomic force microscopy to understand electron transfer occurring at the cathodic delamination front. Cation migration during oxygen-reduction induced spreading of electrolyte on these trivalent-chromium coated steel was studied using in situ scanning Kelvin probe force microscopy. The coatings are found to highly resist electron transport. Heterogeneities observed in the microscopic scale using the current-sensing atomic force microscope are correlated to the behavior observed during electrolyte spreading. The kinetics of spreading observed at microscopic scales are similar to that observed using a scanning Kelvin probe at larger scales. The results demonstrate in situ scanning Kelvin probe force microscopy as an effective tool to study the migration of cations during electrolyte spreading and the local heterogeneities at the microscopic scale. Warren, David & N. McMurray, H & de Vooys, Arnoud. (2013). Localised SKP Studies of Cathodic Disbondment on Chromium/Chromium Oxide Coated Steel. ECS Transactions. 50. 67-78.Wint, N & de Vooys, Arnoud & Mcmurray, Hamilton. (2016). The corrosion of chromium based coatings for packaging steel. Electrochimica Acta. 203. 326-336.James E. Edy, Hamilton N. McMurray, Koen R. Lammers, Arnoud C.A. deVooys (2019). Kinetics of corrosion-driven cathodic disbondment on organic coated trivalent chromium metal-oxide-carbide coatings on steel. Corrosion Science. 157. 51-61.

Hydrogen Limitation By Background Salts in the Case of Trivalent Chromium Electrolytes

The toxicity of some chemical compounds for humans and the environment encouraged health authorities to adopt new legislations. This is the case for hexavalent chromium salts, CMR known, which are targeted by the REACH European regulation. These salts are used in various surface treatments processes and their use is subjected to authorization. This situation has severely impacted the 'hard' chromium plating industry. Several projects all over the world deal with alternative solution and most of them are based on non toxic trivalent chromium salts, but more difficult to implement. The present work is undertaken within CRONOS 2024 project supported by a consortium of 20 companies and led by the IRT M2P (Institute for Technological Research Metallurgy Materials Processes in Metz). Trivalent chromium ions form very stable hexa-aqueous complexes in solution, difficult to reduce. It is therefore necessary to introduce complexing agents in order to facilitate their reduction [1]–[3]. These complexing agents, mostly organic, add a difficulty in the realization of trivalent chromium electrolytes since various phenomena must be considered. In addition, the chromium based electrolytes have a low faradic yield which induces permanent competition between its own reduction and the one of protons to dihydrogen. Excessive consumption of protons near the cathode leads to a pH rise which can cause precipitation and incorporation of chromium hydroxides.In order to improve the performance of trivalent chromium electrolytes, it is possible to adjust different parameters such as the nature and concentration of the complexing agents, the pH, the temperature, the addition of additives, etc. In this study, we focused mainly on the nature and concentration of the support salt. Previous works demonstrated that over a given concentration of Na2SO4 or K2SO4 support salts, a diffusion plateau appears, which can be attributed to the reduction of protons to dihydrogen [4]. Irrespective of the substrate nature, the composition and concentration of the support salts have an impact. It has been shown that a modulation of this plateau towards the most negative potential values would be able to help in decreasing the part of current attributed to the generation of dihydrogen. Moreover, as demonstrated by Phuong et al. [5], a comparison is possible with faradaic yields. Finally, the influence of organic and inorganic complexing agents has also been studied.[1] S. Imer et T. Varnali, Applied Organometallic Chemistry, 14(10) p. 660-669, (2000)[2] Z. Zeng, Y. Sun, et J. Zhang, Electrochemistry Communications, 11(2), p. 331-334 (2009)[3] Y. Song et D.-T. Chin, Electrochimica Acta 48(4) p. 349-356 (2002)[4] Y. Mukouyama, M. Kikuchi, et H. Okamoto Journal of Electroanalytical Chemistry 617(2) p. 179-184 (2008)[5] S. De, J. White, T. Brusuelas, C. Patton, A. Koh, et Q. Huang Electrochimica Acta 338 p. 135852 (2020)

A Study of the Effect of Ligand-Metal Interactions on the Electrodeposition of Chromium from Trivalent Chromium Electrolytes

With electrolytic chromium coated steel (ECCS) technology, a thin layer of chromium and an outer layer of chromium oxide are deposited on packaging steel to provide corrosion protection and a suitable layer for adhesion of subsequent polymer coatings. In an ECCS plating line, the coating is deposited via a hexavalent chromium electrolyte. Hexavalent chromium has been found to be an environmental and health hazard and is therefore subject to restrictions under REACH legislation. A new coating process based on trivalent chromium chemistry has therefore been developed (TCCT).Coatings produced via trivalent chromium electrolytes have been found to provide comparable corrosion resistance to those achieved with the traditional Cr(VI)-based process [1]–[4]. However, there is limited understanding of the mechanism and kinetics of electrodeposition from trivalent chromium based electrolytes. This knowledge is key to optimizing process design of industrial plating lines. Formate ions are incorporated in the Cr3+ electrolyte as complexing agents to make Cr0 deposition possible. It has been shown that coating composition is dependent on the relative quantity of HCOO- ions present. The present study is comprised of two main research goals - to determine the complexes that form in solution as a function of the relative complexing agent (HCOO-) concentration and to determine the effect of this complex speciation on the deposition mechanism.Results of a multitechnique approach addressing the first of our research goals - complex speciation - will be presented. Density functional calculations have been used to establish the energetic feasibility of the reaction mechanisms suggested by results from UV Vis and FTIR spectroscopy and rotating cylinder cyclic voltammetry experiments. UV Vis and FTIR results show that a clear chemical transition occurs once the formate ion is introduced in the electrolyte – starting ratio 1:6 HCOO-:Cr3+. Beyond this point, no significant change in the spectrum apart from increasing absorbance up to a ratio of 12 HCOO-:Cr3+ is observed. These results suggest the formation of a single new complex in the solution upon introduction of the formate ion in the electrolyte solution which is not modified by further increase of the formate ion concentration.[1] M. Inman et al., ‘Green Process for Fuctional Trialent Crhromium Electroplating’, in 223rd ECS Meeting, 2012.[2] J. H. O. J. Wijenberg, M. Steegh, M. P. Aarnts, K. R. Lammers, and J. M. C. Mol, ‘Electrodeposition of mixed chromium metal-carbide-oxide coatings from a trivalent chromium-formate electrolyte without a buffering agent’, Electrochimica Acta, vol. 173, pp. 819–826, Aug. 2015.[3] J. E. Edy, H. N. McMurray, K. R. Lammers, and A. C. A. deVooys, ‘Kinetics of corrosion-driven cathodic disbondment on organic coated trivalent chromium metal-oxide-carbide coatings on steel’, Corrosion Science, vol. 157, pp. 51–61, Aug. 2019.[4] S. Hesamedini and A. Bund, ‘Trivalent chromium conversion coatings’, J Coat Technol Res, vol. 16, no. 3, pp. 623–641, May 2019.

Chromium coatings from trivalent chromium plating baths: Characterization and cathodic delamination behaviour

Novel two-layer chromium-based coatings comprised of a first layer containing chromium, oxygen and carbon (Cr-O-C) and an oxygen rich (Cr-O) topcoat were electrodeposited from trivalent chromium electrolyte. The complex structure and composition of the coatings were studied using complementary characterization techniques. The electrodeposited oxide was found to be amorphous and oxygen-deficient. In operando ambient pressure X-ray photoelectron spectroscopy when heating the sample from room temperature to 450 °C and Raman spectroscopy after the heating ascertained the metastable nature of the oxide. The cathodic delamination of a weak model polymer on these samples was studied using in situ scanning Kelvin probe.

Investigation of The Reaction Kinetics of Chromium(III) Ions with Carboxylic Acids In Aqueous Solutions and The Associated Effects on Chromium Deposition

Electroplated chromium is an important industrial coating. Both in the decorative and functional field, research is being carried out to develop trivalent chromium electrolytes. In this work, the interaction between CrIII ions and carboxylic acids (malonic acid, malic acid, oxalic acid and glycolic acid) is investigated. The use of carboxylic acids to improve the current efficiency in trivalent chromium plating baths has been known for a long time. But the thermodynamics and kinetics of the associated complexing reactions are not well understood. It is shown that the complexes form at different rates depending on the acid and its concentration and have a varying influence on chromium deposition. For the development of a technical bath, care must be taken to ensure that the molar ratios between chromium and carboxylic acids can be controlled. This is possible with the HPLC method presented here.

Hydrogen Limitation By Background Salts in the Case of Trivalent Chromium Electrolytes

The toxicity of some chemical compounds for humans and the environment encouraged health authorities to adopt new legislations. This is the case for hexavalent chromium salts, CMR known, which are targeted by the REACH European regulation. These salts are used in various surface treatments processes and their use is subjected to authorization. This situation has severely impacted the 'hard' chromium plating industry. Several projects all over the world deal with alternative solution and most of them are based on non toxic trivalent chromium salts, but more difficult to implement. The present work is undertaken within CRONOS 2024 project supported by a consortium of 20 companies and led by the IRT M2P (Institute for Technological Research Metallurgy Materials Processes in Metz).Trivalent chromium ions form very stable hexa-aqueous complexes in solution, difficult to reduce. It is therefore necessary to introduce complexing agents in order to facilitate their reduction [1]–[3]. These complexing agents, mostly organic, add a difficulty in the realization of trivalent chromium electrolytes since various phenomena must be considered. In addition, the chromium based electrolytes have a low faradic yield which induces permanent competition between its own reduction and the one of protons to dihydrogen. Excessive consumption of protons near the cathode leads to a pH rise which can cause precipitation and incorporation of chromium hydroxides.In order to improve the performance of trivalent chromium electrolytes, it is possible to adjust different parameters such as the nature and concentration of the complexing agents, the pH, the temperature, the addition of additives, etc. In this study, we focused mainly on the nature and concentration of the support salt. Previous works demonstrated that over a given concentration of Na2SO4 or K2SO4 support salts, a diffusion plateau appears, which can be attributed to the reduction of protons to dihydrogen [4]. Irrespective of the substrate nature, the composition and concentration of the support salts have an impact. It has been shown that a modulation of this plateau towards the most negative potential values would be able to help in decreasing the part of current attributed to the generation of dihydrogen. Moreover, as demonstrated by Phuong et al. [5], a comparison is possible with faradaic yields. Finally, the influence of organic and inorganic complexing agents has also been studied.[1] S. Imer et T. Varnali, « Modeling chromium sulfate complexes in relation to chromium tannage in leather technology: a computational study », Applied Organometallic Chemistry, vol. 14, no 10, p. 660-669, 2000, doi: 10.1002/1099-0739(200010)14:10<660::AID-AOC55>3.0.CO;2-P.[2] Z. Zeng, Y. Sun, et J. Zhang, « The electrochemical reduction mechanism of trivalent chromium in the presence of formic acid », Electrochemistry Communications, vol. 11, no 2, p. 331-334, févr. 2009, doi: 10.1016/j.elecom.2008.11.055.[3] Y. . Song et D.-T. Chin, « Current efficiency and polarization behavior of trivalent chromium electrodeposition process », Electrochimica Acta, vol. 48, no 4, p. 349-356, déc. 2002, doi: 10.1016/S0013-4686(02)00678-3.[4] Y. Mukouyama, M. Kikuchi, et H. Okamoto, « Appearance of new potential oscillation during hydrogen evolution reaction by addition of Na2SO4 and K2SO4 », Journal of Electroanalytical Chemistry, vol. 617, no 2, p. 179-184, juin 2008, doi: 10.1016/j.jelechem.2008.02.006.[5] S. De, J. White, T. Brusuelas, C. Patton, A. Koh, et Q. Huang, « Electrochemical behavior of protons and cupric ions in water in salt electrolytes with alkaline metal chloride », Electrochimica Acta, vol. 338, p. 135852, avr. 2020, doi: 10.1016/j.electacta.2020.135852.

Influence of Pulse-Current Parameters on pH Measured By Local Method

The electrodeposition process often comes with the reduction of protons at the cathode interface. In addition to the creation of dihydrogen bubbles, this reaction leads to an increasing of the pH at the interface vicinity, which can induce problems such as metallic hydroxides generation and precipitation. Several electrolytes are concerned by this phenomenon, but this is particularly crucial for trivalent chromium electrolytes for which faradic efficiency remains very low. Moreover, trivalent chromium ions must be complexed to be reduced and the pH value influences complexation equilibrium. To limit the local pH variations, pulse current sequences with scare waves can be used. In fact, periodically reversing the polarity of the current and/or applying off times lead to the double layer discharge and counterbalance the effect of pH increase. To have a better comprehension of the current parameters influence on the surface pH variations, it is necessary to measure this value. Local pH increasing have already been measured in several electrolytes with direct current using a rotating pH electrode and metallic meshes as cathode [1]. The method used in this study is totally different, based on micro-capillary pH electrodes (10 µm inner diameter) [2]. The micro-capillaries composing electrodes, are firstly silanized, then filled with a buffer solution, except in its sharpened end, which is filled with an ionophore resin synthesized for this purpose and acting as a selective proton membrane. Electrode position is managed by a three-dimensional micromanipulator and by two cameras. Its calibration indicates a Nernstian response on a given pH range, systematically tested before and after each measurement. The local pH measurements are realized during the relaxing phases, just after turning off the current, to avoid perturbations of the electrode potential induced by the electromagnetic field existing during the polarization. The local pH is measured until the bulk pH value is reached. Measurements have been done on simple copper sulfate and on trivalent chromium electrolytes. The results show that turning on the current, pH close to the interface grown of several units. It has been also shown that using current modulation, compare to direct current, have an impact on the pH increase. To support the results found using micro-capillary pH electrodes, a micro-picking system has been developed. The process consists on picking samples at the cathode vicinity surface and to measure its pH thanks to pH paper observed with a microscope. [1] H. Deligianni et L. T. Romankiw, « In situ surface pH measurement during electrolysis using a rotating pH electrode », IBM Journal of Research and Development, vol. 37, no 2, p. 85-95, mars 1993. [2] A. Q. Vu, B. Vuillemin, R. Oltra, et C. Allély, « Cut-edge corrosion of a Zn–55Al-coated steel: A comparison between sulphate and chloride solutions », Corrosion Science, vol. 53, no 9, p. 3016-3025, sept. 2011. Acknowledgments : the authors would like to thanks IRT M2P, project CRONOS 2024 for its financial support Figure 1

Pulse Reverse Current Cr+3 Deposition for Wear Applications

Faraday Technology Inc. is a research, development and engineering firm developing electrochemical innovations based has pulse/pulse reverse electrolytic principles [1]. One of Faraday’s current activities is directed towards the development of an innovative functional REACH compliant chrome plating process based on a trivalent chromium electrolyte to replace hexavalent chromium plating. Hexavalent chromium plating has been used for many decades to provide hard, durable coatings with excellent wear and corrosion resistance properties. However, hexavalent chromium has come under increasing scrutiny due to the toxic nature of the bath, effects on the environment, and workers’ health. Faraday has demonstrated that the chrome coatings prepared using the FARADAYIC(R) Process have equivalent functional properties to the coatings produced with a hexavalent chromium bath. The data demonstrates equivalent or superior: 1) plating rate, 2) Knoop hardness, 3) current efficiency, 4) hydrogen embrittlement behavior, 5) adhesion, 6) corrosion resistance, 7) porosity, 8) thickness, 9) Taber Abrasion, Ball on Flat Reciprocating and Dithering wear resistance, and with no hexavalent chromium formation in the bath. Therefore, this paper will discuss the 2013 EPA Green Chemistry Challenge Award winning work on the development of a REACH compliant functional trivalent chromium plating process from a single, simple-to-control trivalent-based electrolyte to replace hexavalent chromium plating. Furthermore, it will specifically discuss electrochemical deposition approaches to improve the wear index and microstructure of the coating. Acknowledgements: This study is supported by the US Army (W911NF-11-2-0014) and private sources. [1] E. J. Taylor “Adventures in Pulse/Pulse Reverse Electrolytic Processes: Explorations and Applications in Surface Finishing” J. Applied Surface Finsihing, 3(4), 178-189 (2008).

Relation between Color and Surface Morphology of Electrodeposited Chromium for Decorative Applications

Electrolytes for decorative chromium plating based on trivalent chromium salts are known since several decades. As the use of conventional, hexavalent chromium based plating baths is more and more restricted by governmental regulations, these electrolytes gain ground in electroplating industry. However, compared to hexavalent chromium electrolytes, trivalent chromium electrolytes cannot fully meet the requirements with regard to appearance of the electrodeposited chromium, and there is little knowledge about the influencing factors on the shade of color. In this paper, chromium plated from a solution of chromium(III) sulfate was characterized by colorimetry, SEM and AFM and compared to a sample plated from a chromic acid electrolyte in order to reveal correlations between visual appearance and surface morphology. A relation between an increase of grain size and a color shift from blueish to yellowish was observed. Unlike in hexavalent based systems, grain size, roughness and color depend on layer thickness as the grain growth mechanism appears to be different. A model based on the theory of light scattering at rough surfaces is provided that links roughness and reflection behavior of the chromium surface.

Trivalent chromium electrodeposition using a deep eutectic solvent

PurposeThis paper aims to investigate the electrolytic deposition of corrosion-resistant chromium coatings from a trivalent chromium plating bath based on deep eutectic solvent, a new generation of room temperature ionic liquids.Design/methodology/approachThe electrolyte contained chromium (III) chloride, choline chloride and the additive of extra water. The surface morphology was estimated by means of SEM technique. The microstructure of as-deposited and annealed coatings was studied using X-ray diffraction method. The kinetics of the chromium electrodeposition and the corrosion electrochemical behavior of the coatings were investigated by cyclic voltammetry technique.FindingsChromium coatings with an amorphous type of microstructure are electroplated from this bath. Some carbon and oxygen are included in deposits obtained. The step-wise mechanism of the electrochemical reduction of Cr(III) ions to Cr(0) is detected. The current efficiency in this system sufficiently exceeds that typical of usual aqueous electrolytes. The coatings fabricated using plating bath based on deep eutectic solvent showed enhanced corrosion resistance in an acidic medium: there is no current peak of active dissolution in polarization curve and the corrosion potential shifts to more positive values as compared with “usual” chromium.Originality/valueThe electrodeposition of chromium coatings from an environmentally acceptable trivalent chromium electrolyte, a deep eutectic solvent containing chloride choline and extra water additive has been investigated for the first time.

Cyclic voltammetry study of trivalent basic chromium sulphate electrolytes contaminated with sulphite

The discovery of sulphite in basic chromium sulphate, typically used as the source of Cr(III) ions in trivalent chromium electrolytes, sparked an investigation into the effects of this species on the reactions at a platinum electrode in a Cr(III)-formate electrolyte. The specific adsorption and oxidation-reduction reactions of sulphite were mapped using cyclic voltammetry and linked to competitive adsorption between sulphite, formic acid and the Cr(III)-formate complex.It was found that sulphite adsorption and reduction/oxidation dominates the cyclic voltammogrammetric response. Formic acid oxidation is completely inhibited as long as sulphite is present at the surface.A feasible way of removing any sulphite from a contaminated electrolyte is by applying a prolonged reductive current.