Kinetics and Mechanism of Chromium Electroplating from Cr(III) Baths

Kinetics of step-wise discharge of Cr(III) to Cr(0) in electrolytes containing aminoacetic acid is studied. The limiting current of Cr(II) ion discharge to metal is much lower than the limiting diffusion current of Cr(III) reduction to Cr(II), which results in the accumulation of Cr(II) ions in the electrolyte even at the potentials of the limiting current of chromium electroplating. The apparent catalytic effect of OH^–ions during Cr(II) discharge is associated with the formation of electroactive hydroxocomplexes of bivalent chromium. The kinetic equation of the studied process is derived and analyzed. The probable nature of the limiting currents of chromium electroplating is discussed.     

The Kinetics of Electrode Reactions III Practical Aspects

Abstract

To study the electrodeposition of Cr–Co alloy, as a possible alternative to the deposition of pure Cr, in particular from hexavalent Cr baths, Cr(III) formate–urea electrolyte containing various concentrations of CoCl₂ was used. The deposition rate, composition, phase state and structure of Cr–Co deposits were investigated. The data obtained suggest that the electrodeposition of Cr–Co alloy occurs under acceleration of Cr(III) and inhibition of Co(II) ions electroreductions. It was found that the presence of 1–2 g L^–1 CoCl₂ in Cr(III) bath has an optimum effect on the electrodeposition of the good quality thick (30 μm) Cr–Co alloy obtained. The X-ray diffraction data reveal that the Cr–Co alloy deposit has an X-ray amorphous structure with a grain size of ～1·3 nm. The morphology of the deposit shows a significant dependence of the nodular structure on the percentage of Co. X-ray photoemission spectroscopic spectra indicate the presence of oxide and metal phases for both metals in the top layers of as deposited Cr–Co alloy.     

Features of electroplating of nanocrystalline chromium coatings from electrolytes based on Cr(III)

It is shown that, for the development of new chrome-plating technologies, it is necessary to proceed from the complicated Cr(III) chemistry. The analysis of the complexation processes of Cr(III) with hydroxycarboxylic acids in aqueous solutions is given. The results of investigations of deposition processes of chromium coatings from sulfate, oxalate–sulfate, and formate–glycine electrolytes are presented. Thin chromium films, as well as chromium films alloyed with cobalt, are obtained from formate–glycine electrolyte. With the use of atomic-force microscopy, it is shown that the formed chromium films are bright and possess a nanocrystalline structure and codeposition of chromium and cobalt leads to the formation of a more uniform structure.     

Chromium electrodeposition from Cr(III) aqueous solutions

The aim of this work is to study an alternative plating process to obtain chromium coatings through electrodeposition from baths containing trivalent chromium, as aqueous solutions of Cr (III) are significantly less dangerous, in terms of human health and environmental impact, as compared to the traditional Cr (VI) baths employed for this purpose. In order to overcome problems regarding the reduction of Cr (III) in aqueous solution, two approaches were followed: i) the possibility of co-depositing chromium along with a second metal, which could help the process of discharge of Cr³⁺ on the substrate; ii) the use of a specific ligand for the Cr³⁺ ion, which can generate easily reducible complexes at the metal-solution interphase. Both approaches led to interesting results: in particular, the co-deposition enabled us to obtain NiCr alloy with a high percentage of chromium, and the deposition using specific complexing agents allowed optimal bath compositions to be developed both for decorative and hard chromium plating.     

Electrolytic Codeposition of Nickel and Phosphorus from Methanesulfonate Electrolyte

The characteristics of Ni–P alloy electrodeposition from a methanesulfonate electrolyte have been investigated. It has been found that the phosphorus content of the alloy increases with increasing the concentration of sodium hypophosphite in the electrolyte and reducing the electrodeposition current density. A mechanism of codeposition of nickel and phosphorus has been suggested. It is shown that phosphorus was formed by electrochemical reduction of hypophosphite anions and their disproportionation at a catalytically active surface of a nickel cathode. It has been shown quantitatively that the most likely path for the formation of phosphorus through the electrochemical mechanism is the direct electrochemical reduction of a hypophosphite anion to atomic phosphorus. The rate of phosphorous formation from hypophosphite anions is dependent on the concentration of hydrogen ions in the near-electrode layer. Therefore, the phosphorus content of the coatings obtained from the methanesulfonate electrolyte is slightly decreased as compared with that from the sulfate electrolyte which exhibits higher buffering properties. It has been revealed that codeposition of nickel and phosphorus reduces the kinetic difficulties of electrochemical reduction of the nickel ions. This might be due to an increased near-electrode concentration of nickel hydroxyl complexes discharging at the cathode, which is the result of an increased near-electrode pH caused by the reactions involving hypophosphite anions and hydrogen ions.     

Kinetics of Pit Initiation at the Alloy Fe5Cr

Pit initiation at the passive alloy Fe5 Cr was studied and compared to the results obtained with pure iron. At constant electrode potential the addition of chloride to the solution increases the dissolution rate of Fe(III). Above the critical pitting potential the first pits develop during a time of incubation after addition of chloride. The total current density and the dissolution rate of Fe(II) rise simultaneously and suddenly. The logarithm of the times of incubation increase linearly with the reciprocal of the difference between the actual electrode potential and the pitting potential. Galvanostatic experiments show that, with Fe 5 Cr as with iron, chloride catalyzes the transfer reaction of oxygen ions at the interface between oxide and electrolyte more strongly than the dissolution of Fe(III). Addition of chloride. The total current density and the dissolution rate of Fe(II) rise simultaneously and suddenly. The logarithm of the times of incubation increase linearly with the actual electrode potential and the pitting potential. Galvanostatic experiments show that, with Fe5Cr as with iron, chloride catalyzers the transfer reaction of oxygen ions at the interface between oxide and electrolyte more strongly than the dissolution of Fe(III). Addition of Chromium lowers the current efficiency of Fe(III) dissolution and favors oxide growth. The ionic conductivity of the passivating oxide is not significantly changed by chloride but is lowered by chromium. Fluctuations connected to the onset of pitting are slower with Fe 5 Cr than with iron.     

Hardness variation and corrosion behavior of as-plated and annealed Cr-Ni alloy deposits electroplated in a trivalent chromium-based bath

Cr-Ni alloy deposits with various concentrations of Cr, Ni were obtained with different electroplating current densities in a plating bath containing trivalent chromium and divalent nickel ions. The Cr-rich alloy deposit with a very high hardness of 1150 Hv can be obtained by annealing at 700 °C for 30 min. These alloys displayed poor corrosion resistance due to through-deposit cracking during annealing. For optimal hardness and corrosion resistance, we recommend a sequential two-step electrodeposition with Ni-rich alloy electroplating followed by Cr-rich alloy electroplating, or Cr/Ni alloy electrodeposition, to create a protective coating.     

Estimation of the protective ability of chromium coatings deposited from sulfate and methanesulfonate electrolytes based on Cr(III)

Voltammetric and impedance techniques are used to determine the protective ability of chromium coatings deposited on a steel substrate from sulfate and methanesulfonate electrolytes based on trivalent chromium. It is found that chromium deposits obtained from methanesulfonate electrolyte have lower porosity and higher protective ability as compared with deposits from sulfate electrolyte. It is shown that, when the thickness of chromium coatings reaches 15 μm, there are no through pores in deposits for both electrolytes and, thus, the maximum degree of protection of the steel support is achieved in an aggressive medium. An equivalent circuit correctly modeling the corrosion-electrochemical behavior of the studied system is proposed on the basis of data of electrochemical impedance spectroscopy, and the values of its parameters are calculated.     

Electroplating of wear-resistant nanocrystalline coatings from a bath containing basic chromium(III) sulfate (chrome tanning agent)

Regularities of the electroplating of chromium-carbon alloy coatings from a bath containing basic chromium(III) sulfate, carbamide, formic acid, sodium sulfate, aluminum sulfate, orthoboric acid, and sodium dodecyl sulfate are studied. Replacement of chromium sulfate as a source of trivalent chromium ions in the solution with basic chromium sulfate (chrome tanning agent) results in a decrease in the current density when metal deposition begins. As a result, the covering power of the bath increases. The effects discovered are determined by changes in the composition of the discharged chromium complexes. A certain excess of OH^? groups in the inner sphere of electroactive chromium complexes results in acceleration of electroplating. The studied electrolyte based on chrome tanning agent enables one to produce thick high-quality nanocrystalline Cr-C alloy coatings with improved tribological characteristics.     

Electroplating with Chromium–Cobalt Alloy

Electroplating of Cr–Co alloy from sulfate electrolyte based on Cr(III) and Co(II) is studied. The effect of electrolyte composition, pH of solution, and the electrolysis conditions on the alloy composition and the coating quality is shown. The peculiarities of the plating kinetics are studied by a method of partial polarization curves. It is shown that the protective properties of Cr–Co alloy coating are higher than those of chromium and Cr–Fe alloy coatings.     

Kinetics of the Codischarge of Nickel and Chromium Ions from Oxalate–Sulfate Solutions into an Alloy

The kinetics of individual and concurrent discharge of nickel and chromium ions from oxalate–sulfate solutions is studied. At a high cathodic potential (–1.2 V), the process of nickel deposition is sharply decelerated due to the surface passivation of the cathode with nickel oxides and the adsorption of organic products. During the coreduction of the ions with the formation of an alloy, the discharge rate of nickel ions is sharply decreased and the deposition of chromium is accelerated, compared to the electrodeposition of individual metals. The phenomenon observed is accounted for by a sharp decrease in the rate constant of the electrochemical reaction of nickel discharge on the surface of an alloy.     

Preparation and characterization of Cr-P coatings by electrodeposition from trivalent chromium electrolytes using malonic acid as complex

Cr-P coatings were prepared by electrodeposition from trivalent chromium plating bath using malonic acid as complex. The influences of bath composition on the trivalent chromium electrodeposition process and deposited coating properties were studied. The effects of plating parameters such as current density, bath pH and plating time on structure and morphology of deposited coatings were investigated in detail. XRD, SEM, EDAX and XPS techniques were used to characterize the Cr-P deposited coatings. Results show that the composition, microstructure and surface morphology of the Cr-P coatings depend on bath composition and plating conditions including bath pH, current density, plating time, etc. Results of EDAX and XPS indicate that the deposited coatings contain Cr(s), Cr(III), phosphorus, oxygen and carbon. The optimum bath composition was obtained using malonic acid as complex and the mechanism of Cr-P electrodeposition was analyzed. The optimum plating parameters for good-quality chromium deposited coating are pH 2-3, current density 3-12 dm², temperature 35 °C and Ti/IrO₂ as anode. These results may be of great practical and theoretical significance for further improvement of trivalent chromium plating process.     

The corrosion-protective traits of electroplated multilayer zinc-iron-chromium deposits

A novel kind of three-layer zinciron-cbromium electroplated coating was tested and proven to possess good corrosion protective characteristic as well as acceptable decorative properties. A citric-chloride bath containing Fe(III)-complexes is suggested for electrodeposition of iron intermediate layer on the zinc deposit surface. Due to the low acidity (pH 5) and medium operating temperature (20-30°C), the electrolyte involved has no a strong etching effect and can be used for direct iron deposition on zinc. A finish chromium layer is deposited from formate-sulfate trivalent bath. The zinc layer corrodes through the pores of Fe and Cr layers and protects the steel substrate against corrosion. The iron and chromium layers decelerate the corrosion rate of zinc. The finish chromium layer provides with good decorative properties, and the intermediate iron layer ensures adhesion of chromium to the zinc.     

Applying a theory of generalized variables to electrochemical kinetics: Interpreting the results of studying chromium deposition from Cr(III) baths

By using the theory of generalized variables (the similarity theory and dimensional analysis), universal system of dimensionless complexes (the similarity criteria) is derived, which allows describing the kinetics of multistage reaction (metal electrodeposition at rotating disc electrode). The proposed mathematical apparatus can be flexibly adapted to describing the kinetics of virtually any complicated electrochemical system. The dimensionless complexes derived are used in interpreting the results of our studies on the chromium electrodeposition from Cr(III) oxalate and formate salt solutions at a rotating disc electrode. Possible mechanism of the electrodeposition from the oxalate and formate electrolytes is offered. Original Russian Text ? V.S. Protsenko, T.E. Butyrina, F.I. Danilov, 2007, published in Zashchita Metallov, 2007, Vol. 43, No. 4, pp. 429–438.     

Corrosion performance of a hot-dip galvanized steel treated by different kinds of conversion coatings

The anticorrosive performance of various kinds of conversion coatings including Cr(VI) and Cr(III) were studied on hot galvanized steel (HDG). The effect of Co(II) and Ni(II) ions was also examined on the Cr(III) coating performance. DC and electrochemical impedance spectroscopy (EIS) were conducted when the specimens were immersed in 3.5 wt.% sodium chloride solution. The coating morphology was observed using SEM. Micro-cracks were observed on both Cr(III) and Cr(VI) treated samples. The results indicated a decrease in the corrosion rate of HDG when the Cr(III) or Cr(VI) conversion coatings were found on the surface. The greater anticorrosion resistance of a Cr(VI) treated sample (compared with a Cr(III) one) was obtained at low immersion times. However, a significant decrease in self healing behavior and therefore corrosion resistance of the Cr(VI) treated sample was observed at longer immersion times. The anticorrosion resistance of Cr(III) treated samples was significantly improved using Co(II) or Ni(II) conversion coatings on Cr(III) treated samples. On the other hand, a lower decrease in corrosion performance of samples treated by Cr(III) and Co(II) or Ni(II) was obtained after a long immersion time in corrosive environment. It was found that, the passive and barrier layer which the Co(II) or Ni(II) conversion coating can produce on the Cr(III) coating can superiorly improve its anticorrosion resistance.     

Oxidation state and localization of chromium ions in Cr-doped cassiterite and Cr-doped malayaite

The oxidation state and the localization of the chromium ions in the ceramic matrix of Cr-doped cassiterite (SnO₂) and Cr-doped malayaite (CaSnSiO₅) pigments have been investigated through the use of X-ray absorption near-edge (XANES), extended X-ray absorption fine structure (EXAFS), optical absorption and electron spin resonance (ESR) spectroscopies as well as by measurements of the unit cell parameters and the magnetic susceptibility of the pigments. We have found that three types of chromium species are present in the Cr-doped cassiterite pigments; a majority phase consisting of Cr(III) oxide clusters, a small amount of CrO₂ nanoparticles and Cr(IV) ions dissolved in the cassiterite lattice (∼5% of the total Cr amount), which must be responsible for the violet colour of this pigment. In the case of Cr-doped malayaite, most chromium cations are tetravalent forming a solid solution with the malayaite lattice by mainly substituting for Sn(IV) cations in isolated octahedral positions, although a very small amount of Cr(IV) also substitutes for tetrahedral Si(IV).     

Corrosion behavior of Cr electrodeposited from Cr(VI) and Cr(III)-baths using direct (DCD) and pulse electrodeposition (PED) techniques

A comparison was made between the electrochemical corrosion behaviors of chromium deposited from hexavalent [Cr(VI)] and trivalent [Cr(III)] chromium baths using direct current (DCD) and pulse electro deposited (PED) techniques. Chromium coatings were deposited on mild-steel (MS) substrate. The corrosion behavior of both DCD and PED chromium from Cr(VI) and Cr(III)-baths in 3.5%NaCl solution was studied using potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). The results indicated that PED chromium from Cr(VI) and Cr(III)-baths have higher charge-transfer resistance R_ct and very low I_corr than that of DCD chromium on mild-steel substrate.     

氯化物水溶液体系三价铬电沉积机理研究

采用循环伏安、恒电流阶跃和稳态极化等电化学研究方法和紫外分光度法,研究了Cr3+电沉积机理,结果表明其沉积过程分2步进行。计算得出Cr3+2步放电的表观活化能为34.9 kJ/mol和19.77 kJ/mol,表明Cr3+还原过程受电化学反应控制;初步拟定了甘氨酸作配体氯化物水溶液体系三价铬还原机理。     

Bi-Cu film deposition in aqueous solutions

The relatively uniform bismuth-copper film was electrodeposited between −15 and −20 mV in the sulfate electrolyte containing 4 mmol/L bismuth ion and 2 mmol/L copper ion. Only copper was electrodeposited at −5 mV. The dendritic bismuth-copper film was electrodeposited under −20 mV. The cathodic current became constant between −20 and −400 mV. Therefore, bismuth-copper electrodeposition changes from charge transfer controlling to diffusion controlling at −20 mV. On the other hand, the uniform bismuth-copper film was electrodeposited between −5 and −35 mV in the methanesulfonate electrolyte containing 4 mmol/L bismuth ion and 2 mmol/L copper ion. The dendritic bismuth-copper film was electrodeposited under −35 mV. The potential region for good electrodepositon in methanesulfonate electrolyte is wider than that in sulfate electrolyte. Therefore, it is easy to control electrodeposition conditions by using methanesulfonate electrolyte.     

Electrodeposition of nanocrystalline chromium coatings from Cr(III)-based electrolyte using pulsed current

The effect of the pulse ratio on the current efficiency of the chromium electrodeposition reaction and the properties of coatings obtained from sulfate-formate electrolyte based on Cr(III) salts is studied. It is shown that the dependences of the current efficiency on the relative pulse duration feature a maximum that corresponds to a relative pulse duration of 2 (at τ_pulse = τ_p = 1 s). Light chromium coatings with good adhesion to the substrate are formed under pulsed electrolysis and their thickness is several tens of μm. In order to prevent pitting formation, it is expedient to introduce a special wetting surfactant additive into the electrolyte. As follows from the data of small-angle X-ray scattering, the deposited coatings are nanocrystalline. An increase in the relative pulse duration promotes a decrease in the nanocrystal size. The application of pulsed electrolysis leads to a decrease in the anodic dissolution rate of coatings in an acidic medium. Chromium electrodeposition decrease under pulsed current results in a decrease in internal stresses and in an increase in microhardness of coating.