Keywords: Functional chromium plating, hexavalent chromium alternatives, trivalent chromium plating, pulse plating, electrically-mediated current waveforms
The US EPA has identified chromium as one of 17 “high-priority” toxic chemicals. The US EPA selected the high-priority chemicals based on their known health and environmental effects, production volume and potential for exposure.1 Under former US EPA administrator William K. Reilly's Industrial Toxic Program, the high-priority toxic chemicals were targeted for 50% reduction by 1995.2 Additionally, worker exposure to Cr(VI) plating baths is regulated by OSHA, exhaust/scrubber systems must be installed for Cr(VI) plating operations and the exposure limit is 0.01 mg/m3.3 The Clean Air Act, as well as local constraints, regulates the emission of chromium to the air and water. Since Cr(VI) plating produces hazardous air emissions, all Cr(VI) platers must control and monitor the bath surface tension and report the results to the EPA. In contrast, less hazardous trivalent chromium [Cr(III)] platers are not required to monitor bath surface tension.3
The chemistry of chromium provides a basis for understanding the toxicology. Chromium can exist in oxidation states ranging from II to VI. However, only Cr(III) and Cr(VI) are stable enough to actually be used in electroplating. Cr(VI) is readily reduced to the more stable Cr(III) and in this process, substances in contact with the Cr(VI) are oxidized. Cr(VI) compounds are very soluble compared to Cr(III) compounds. Therefore, in the environment Cr(VI) is much more likely, upon release into a stream or an aquifer, to dissolve and move with the flow. In fact, one method that has been used to stabilize Cr(VI) (make it less mobile) in the environment is to reduce it to Cr(III).4
Cr(VI) is widely recognized as a human carcinogen. In a recently published study,5 Cr(VI) was reported to cause increased incidences of lung cancer in a group of 2,357 workers at a chromate production plant. In the same study, the carcinogenic property of Cr(III) was also examined. Gibb, et al.5 found, “Cumulative hexavalent chromium exposure was associated with an increased lung cancer risk; cumulative trivalent exposure was not.” This study “offers the best quantitative evidence to date of the relationship between Cr(VI) and lung cancer.” In previous studies, it was recognized that workers were exposed to a mixture of Cr(VI) and Cr(III). However, the relative contributions of each of the forms of chromium were not assessed. In the Gibb, et al. study, the relative exposure levels indicated that exposure of workers to Cr(III) did not result in an increase in lung cancer. When comparing the two forms of chromium, the chemistry and toxicology of each form is markedly different. Cr(III) is rather insoluble and does not oxidize organic material. Cr(III) is poorly absorbed from the gastrointestinal tract and is not considered to be a carcinogen. On the other hand, Cr(VI) is very soluble, penetrates cell walls, and is a human carcinogen. Cr(VI) is very reactive and causes ulceration of the nasal septum and other tissues upon exposure as well as reproductive, gastrointestinal and dermal effects. Even so, Cr(III) is not without toxic effects and should be handled appropriately. To put this last statement in perspective, a ubiquitous metal like aluminum, for example, is not without toxic effects. Although aluminum is formed into cooking utensils and soda cans, depending on the route of exposure and dose, aluminum can be potentially toxic (Occupational Safety and Health Guideline for Aluminum).
The use of Cr(III) in industrial and commercial processes is preferred over Cr(VI) on the basis of the comparison of the toxicities. Cr(III), like other metals (e.g., aluminum) must be handled appropriately since each of these metals is at some dose and via at least one route of exposure, capable of causing harm to humans.4
From an environmental perspective, plating from additive-free Cr(III) solution has several advantages relative to Cr(VI):
1. Cr(III) is non-toxic, non-hazardous and is not an oxidizer. Therefore, meeting air quality regulations is easier and working conditions are greatly improved. The exposure limit for Cr(III) is an order of magnitude higher than Cr(VI).
2. Disposal costs are significantly reduced for Cr(III) plating. Hydroxide sludge generation is reduced ten to twenty times because Cr(III) generally operates at a Cr(III) content of about 4-20 g/L vs. 150-300 g/L for a Cr(VI) bath.
3. Since there are no proprietary additives in the Cr(III) bath, the rinse water may be recycled.
In addition, Cr(III) has the following technical advantages:
1. The Cr(III) plating bath is not sensitive to current interruptions.6 Therefore, the innovative modulated reverse current approach used in this program is more suitable for Cr(III) plating than for Cr(VI) plating.
2. Drag-in of chloride and sulfate from any previous nickel plating operations into the Cr(III) process can be tolerated.7 By contrast, chloride and sulfate drag-in upset the catalyst balance in a Cr(VI) process.
3. Throwing power for Cr(III) plating, which is poor in a Cr(VI) bath, is good and similar to other metals such as copper.7
Chromium coatings are widely used in a variety of industries. Plating operations are used to fabricate two types of chromium coatings, functional and decorative. Functional chromium coatings consist of a thick layer of chromium (typically 1.3 to 760 μm8) to provide a surface with functional properties such as hardness, corrosion resistance, wear resistance and low coefficient of friction. Applications of functional chromium coatings include strut and shock absorber rods, hydraulic cylinders, crankshafts and industrial rolls. Carbon steel, cast iron, stainless steel, copper, aluminum and zinc are substrates commonly used with functional chromium. Decorative chromium coatings consist of a thin layer of chromium (typically 0.003 to 2.5 μm8) to provide a bright surface with wear and tarnish resistance when plated over a nickel layer. It is used for plating automotive trim/bumpers, bath fixtures and small appliances.
As described above, Cr(III) plating has numerous environmental, health and technical advantages relative to Cr(VI) plating. Considerable research has been done to study Cr(III) plating, including the effects of the plating bath chemistry on plating thickness, brightness, hardness and corrosion resistance9,10,11 and the effect of current waveforms on chromium deposit structure, distribution, brightness and hardness.12,13 Currently, functional chromium plating from a Cr(III) bath is not commercially available because of the challenge to plate thick chromium coatings with the appropriate properties. In addition, the low current efficiency and low plating rate of Cr(III) baths lead to unfavorable economics. Due to the rapid drop in current efficiency, the practical limit for existing conventional DC Cr(III) plating is 2.5 μm.14 The plating thickness increases quickly at the beginning of the electroplating process. As plating continues, the deposition rate diminishes and becomes negligible. In this work we demonstrate a technique in which thick chromium coatings are formed.
Pulse/pulse reverse electroplating
During Cr(III) plating, chromium is deposited and hydrogen is evolved at the cathode, as described in the following reactions:
Cr+3 + 3e- à Cr (ϕo = - 0.74 VSHE) (1)
2H+ + 2e- à H2 (ϕo = 0 VSHE) (2)
The current efficiency for chromium plating from a Cr(III) bath is usually below 20%. Therefore, about 80% of the current is used for the hydrogen evolution reaction. As a result, the pH near the cathode surface increases dramatically and chromic hydroxide (Ksp = 5.4 × 10-31) precipitates in the high pH layer at the cathode. The sedimentation of chromic hydroxide covers the cathode surface and its thickness increases as the plating time and pH increase. This promotes an increase of cathode polarization, a further decrease of chromium plating efficiency (i.e., increase in the hydrogen evolution reaction) and the increase of impurities in the plating film. All of these factors retard the normal growth of crystals in the plating film, leading to the inhibition of chromium plating as the evolution of hydrogen continues. The precipitation of chromic hydroxide at the cathode also results in surface cracks and reduces the hardness and brightness of the chromium coating. Faraday’s approach deals with overcoming this hydrogen evolution problem by utilizing electrically mediated field**.
The process utilizes pulse and pulse reverse waveforms. Figure 1 is an example of a pulse / pulse reverse waveform, consisting of a cathodic (forward) pulse followed by an anodic (reverse) pulse and a relaxation period (off-time). The cathodic peak current (ic), cathodic on-time (tc), anodic peak current (ia), anodic on-time (ta), and the relaxation-time (to) are individual variables for process control. The sum of the cathodic on-time, anodic on-time and relaxation-time is the period of the modulation and the inverse of the period is the frequency. The cathodic duty cycle (γc) is the ratio of the cathodic on-time to the period, and the ratio of the anodic on-time to the period is the anodic duty cycle (γa). The frequency and duty cycles are additional variables for process control. The average current density (iaver) or electrodeposition rate is given by:
iaver = icγc - iaγa (3)
Just as there are infinite combinations of height, width and length to obtain a given volume, in pulse and pulse reverse processes there are unlimited combinations of peak current densities, duty cycles and frequencies to obtain a given electrodeposition rate. By controlling the cathodic on-time, anodic on-time, relaxation-time and the cathodic and anodic peak currents, precise control of the electrodeposition process is achieved and consequently the properties of the resulting deposit may be controlled or fine-tuned for a specific application. In conventional direct current (DC) electrodeposition, the current is turned on and held for the duration of the process. By interrupting this constant stream of current, as in the pulse / pulse reverse process, one may achieve results not possible with conventional DC electroplating, such as deposit property control and elimination of adverse side reactions, such as hydrogen evolution, which may become co-deposited with chromium, affecting the nature of the electrodeposit and resulting in samples with inadequate wear properties.
Elimination of hydrogen
Pulse / pulse reverse plating can be used to reduce the amount of H2inclusion in the coating. This section further explains how H2 can be incorporated in the coating and discusses how the density of these inclusions can be mitigated. The evolution of hydrogen during a metal deposition process results in a low current efficiency and a high power consumption. In addition, hydrogen evolution during metal deposition leads to hydrogen embrittlement in the substrate or hydrogen bubble inclusion in the metal deposit. This adversely affects the electrodeposit in terms of:
1. Low corrosion resistance,
2. Poor adhesion to the substrate,
3. High internal stress and
4. Excess porosity.
By properly adjusting the electrically-mediated parameters, one can alter the kinetics of both the metal deposition and hydrogen evolution reactions. While reaction kinetics can be modified using an electrically-mediated waveform with only a forward modulation, hydrogen bubbles adsorbed on the electrode surface can be released during the off-time period. By using an electrically-mediated waveform with a reverse modulation, one can “tune” the anodic or reverse part of the waveform to consume the nascent hydrogen according to the following reaction:
H2 → 2H+ + 2e- (4)
In this manner, a low pH is maintained and chromic hydroxide precipitation is avoided. Furthermore, hydrogen is not permitted to diffuse into the substrate or be incorporated in the deposit. Finally, by compensating for the anodic current with an increased cathodic current, the overall electrodeposition rate is maintained and the net current efficiency is improved.
The ability of pulse / pulse reverse waveforms to increase mass transfer to and from the surface during deposition is essential to producing a coating whose properties contain minimal side products, for instance H2 bubble inclusion which for chromium reduces its wear resistance and adhesion. Mass transport in the pulse reverse process is a combination of steady state and non-steady state diffusion processes. The mass transfer limited current density (il) is related to the reactant concentration gradient (Cb - Cs) and to the diffusion layer thickness (δ) by the following equation: