Pulse-reverse electroplating of chromium from Sargent baths: Influence of anodic time on physical and electrochemical properties of electroplated Cr

doi:10.1016/j.ijrmhm.2020.105213

International Journal of Refractory Metals and Hard Materials

Volume 89, June 2020, 105213

$International Journal of Refractory Metals and Hard Materials$

https://doi.org/10.1016/j.ijrmhm.2020.105213 Get rights and content

Highlights

•
Chromium film was prepared by pulse-reverse (PR) electrodeposition of hexavalent chromium using various anodic current time.
•
The crack density, hardness, and thickness was decreased with an increase in anodic time for PR electroplating.
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The optimal anodic time for PR electroplating was found to be 0.001 s based on physical properties.

Abstract

Herein, chromium film was successfully synthesized by pulse-reverse (PR) electrodeposition using various anodic current time in a Sargent bath which is mainly composed of hexavalent chromium (Cr⁶⁺) and sulfuric acid (H₂SO₄). The correlation between anodic time during electroplating and various physical properties was investigated. The crack density, hardness, and thickness of electrodeposited chromium was decreased with an increase in anodic time for PR electroplating. The chromium prepared by PR electroplating showed higher corrosion resistance than that prepared by direct current (DC) electroplating owing to low crack density. Consequently, the optimal anodic time for PR electroplating was found to be 0.001 s based on the crack density, hardness, current efficiency, and film thickness. The results obtained suggest that this optimized process is a promising route for electroplating chromium film with low crack density and high corrosion resistance.

Graphical abstract

Introduction

Chromium electroplating has been widely used for functional and decorative improvement of manufactured products in various fields because of its outstanding properties such as superior corrosion resistance, smooth surface, and high hardness [[1], [2], [3], [4], [5]]. Especially, its corrosion resistivity is extraordinarily higher than other commercial metals, which is attributed to the strongly passivated chromium surface in a form of chromium oxide, naturally formed in air [6]. In cases of other common metals, for example, iron, zinc, copper, and nikel, oxidized film also can be easily formed on their surface [7]. However, these metals tend to be oxidized partially and incompactly, allowing oxygen to penetrate into underlying metals and react with it continuously. On the contrary, the oxide layers on the chromium are so densely and compactly formed that the diffusion of oxygen is well blocked [8,9]. Moreover, passive film of chromium is highly stable in acidic condition.

However, in actual chromium electroplating process, the chromium coatings with excellent physical properties cannot be easily achieved due to the formation of cracks [1]. In general, electroplating of chromium from Cr⁶⁺ has been carried out by applying cathodic current, and an appropriate amount of sulfuric acid is added to obtain a bright chromium metal [[10], [11], [12], [13]]. Due to a high concentration of hydrogen ion in acidic electrolyte, a deposited crystal structure of chromium can be accompanied by numerous entangling of hydrogen atoms which leads to expansion of the chromium lattice and decrease in current efficiency during electrodeposition [14,15]. The entangled hydrogen escapes easily from the chromium lattice, leaving lots of vacancies. In this step, a tensile strain is generated to stabilize crystal structure, resulting in cracks on the deposit layer [2]. Consequently, a number of cracks on chromium coating caused by lattice contraction bring about decreases in originally desired properties, for examples, hardness, wear resistance, and especially corrosion resistance by facilitating infiltration of oxygen into underlying substrate [15,16].

Among the various electrochemical plating techniques, a promising candidate is pulse reverse (PR) electroplating to obtain crack-free chromium film [2]. The PR electroplating is a method to apply bipolar currents alternatively [14]. In this procedure, anodic current is used to re-oxidize hydrogen within deposited chromium layer through which the essential reason of cracks can be promptly eliminated [17]. In PR electroplating, there are several kinds of fabrication parameters, for examples, cathodic current density (i_c), anodic current density (i_a), cathodic pulse time (t_c), anodic pulse time (t_a), plating temperature and total passed charge [18,19]. In order to produce the electroplated film with a desired quality level in various applications, these process parameters need to be finely tuned to achieve desired properties [20,21]. Although significant progress has been accomplished in this field [2,22,23], the optimization of anodic time for PR electroplating is rarely reported. And the influence of electroplating conditions in case of PR process on film quality, such as crack density, film thickness, hardness, and corrosion behavior, has not been intensively studied.

In this study, physical properties and corrosion behavior of electroplated chromium were investigated with respect to variation of anodic pulse time in PR electroplating. Among four electrodeposition parameters (i_c, t_c, i_a, and t_a), we focus on anodic current time (t_a) for PR electroplating to reduce cracks on chromium electroplating. The density of cracks, hardness, wear resistance and corrosion resistance could be controlled by anodic current time (t_a). To the best of our knowledge, no previous study has been conducted to examine the mutual relationship between anodic current time in PR electroplating and the crack density of plated chromium. We clarified the correlation between effects of anodic pulses and physical properties of chromium film.

Section snippets

Electroplating method of chromium film

Electroplating was conducted with a three electrode system using a lead sheet (2 × 4 cm²) as a counter electrode and Ag/AgCl/KCl(3 M) electrode as a reference electrode. Ni-coated steel use stainless 304 (SUS304) plates were prepared as working electrodes for chromium electroplating in which electrode area was fixed as 2 × 2 cm² and edge parts of specimens were insulated with polyimide film (Du Pont; Kapton). Prior to the electroplating process, all the substrates for electroplating were

Determination of cathodic current density for PR electroplating

PR electrodepositions were carried out under pulse-reverse current system comprised of cathodic pulses and anodic pulses using a Potentiostat (PGSTAT 128 N), as shown in Scheme 1. DC electroplating experiments were carried out to determine the adequate value of cathodic current density (i_c) for PR electroplating. Only cathodic current was introduced for electrodeposition without any pulse current at current density of −0.1, −0.2, and −0.3 A cm⁻², which was denoted as DC1, DC2, and DC3,

Conclusions

In summary, the anodic time, considered as the most important factor in PR electroplating, was controlled to identify various physical properties and corrosion resistance of Cr deposits. The increase of anodic time plays a significant role in reducing the crack density, but it also interferes with the uniform plating of the surface at long anodic time. The hardness and thickness of electrodeposited chromium was decreased with respect to an increase in anodic time for PR electroplating. The

Declaration of Competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was supported by the National Research Foundation of Korea(NRF) Grant funded by the Korean Government(MSIP) (No. 2015R1A4A1042434) and by the Korea Institute of Energy Technology Evaluation and Planning(KETEP) and the Ministry of Trade, Industry & Energy(MOTIE) of the Republic of Korea (No. 20194030202340).

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Pulse-reverse electroplating of chromium from Sargent baths: Influence of anodic time on physical and electrochemical properties of electroplated Cr

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Electroplating method of chromium film

Determination of cathodic current density for PR electroplating

Conclusions

Declaration of Competing interest

Acknowledgments

J. Mater. Process. Technol.

J. Non-Cryst. Solids

Corros. Sci.

Electrochim. Acta

Appl. Surf. Sci.

Surf. Coat. Technol.

Corros. Sci.

Electrochim. Acta

Electrochem. Commun.

Corros. Sci.

Trans. IMF

Korean J. Chem. Eng.

J. Appl. Electrochem.

Plat. Surf. Finish.