Why can higher voltages lead to less efficient electroplating?

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I was reading up on electroplating and I came across some things which really confused me:

Faraday's First Law says that the amount of metal plated onto the cathode is proportionate to the current, so it should be that a higher current= more metal plated onto the cathode (I assume). And from Ohm's Law, a higher current= higher voltage so (by syllogistic reasoning) a higher voltage should give you more metal plated onto the cathode.

However this isn't the case, because I did an experiment recently where the highest voltage didn't produce the highest amount of metal plated onto the cathode. I did some reading on why, and I read (on the internet) that it's because when you increase the current by a lot, the ions formed at the anode can't keep up with the rate of electron flow and electrons just end up combining with other things in solution and no metal forms.

I have some issues with this though; I used a solution with ions already in it, so shouldn't there be no problem for me? Even if the ions cannot be formed at the anode fast enough, I have ions already in solution, so I thought it would be fine. Plus, the experiment was run for only 25 minutes, so I doubt they would've all been used up or anything.

Also, even if it were true that the current was just too high for the ions to keep up; wouldn't the amount of metal formed be at least equal to or greater than the amount formed for lower voltages? It's just that you're forming more metal in less time, ie it gets faster (I think); so you should get the same amount at say 5 V that you do that 3 V, even if towards the end of the experiment in 5 V it gets inefficient and you stop plating. Because the amount of ions or anything doesn't change.

Sorry for the huge amount of writing, I just thought it'd be easier if I explained my thought process. I'd appreciate it a lot if anyone could help me! Thanks so much. :)

What’s gold, shiny, and resides in hundreds of celebrity homes? The answer is the iconic OSCARS® statuette — the most recognized award show trophy in the world. Their famous golden appearance is achieved through electroplating (Ref. 1). These awards are electroplated individually, but when multiple components need to be electroplated simultaneously, they are typically mounted on a rack and put into an electroplating bath in a process called rack plating. Let’s discuss how simulation can help optimize the rack plating process for multiple components.

Electroplating 101

Electroplating, or electrodeposition, is the process of coating a thin layer of metal onto another metal object. It’s known for making inexpensive metals look expensive, but it can also help prevent corrosion and rusting and augment undersized parts, among other benefits.

Fun fact: Modern tin cans are actually made of 98.5% steel, but are electroplated with a thin tin coat.

Common electroplated items include kitchen utensils, car parts, and everyday accessories.

Italian chemist Luigi V. Brugnatelli invented electroplating in 1805. He used a steel wire to connect a Voltaic pile, the first electrical battery, to a solution of gold and a metal object. When the Voltaic pile was activated, it released a current, which resulted in gold attaching to the metal object, giving it a lustrous finish. In the Belgian Journal of Physics and Chemistry, Brugnatelli wrote:

“I have lately gilt in a complete manner two large silver medals, by bringing them into communication by means of a steel wire, with a negative pole of a voltaic pile, and keeping them one after the other immersed in ammoniuret of gold newly made and well saturated.”

Portrait of Luigi V. Brugnatelli. Image by Rijksmuseum — Own work. Licensed under CC 1.0, via Wikimedia Commons.

During the electroplating process, a positively charged electrode (aka an anode) and a negatively charged electrode (aka a cathode) are placed into an electrolyte bath. Both electrodes are connected to a power source, like a battery. When activated, the power source applies a voltage between the electrodes, which eventually results in the anode dissolving into the electrolyte bath and the cathode getting plated with the desired metal finish.

Electroplating of a metal (Me) with copper in a copper sulfate bath. Image in the public domain, via Wikimedia Commons.

In a previous blog post, we discussed at length how the electroplating process works, using the example of how the U.S. Mint electroplates pennies with copper. In this post, we’ll focus on how simulation can be used to efficiently electroplate multiple components in a rack.

Modeling the Electroplating of Multiple Components in a Rack

Rack plating is a popular method when electroplating intricate, complex, or large metal objects. When compared to barrel plating, rack plating is considered the preferred choice in industries where a high-quality finish is paramount, such as automotive, medical, military, and electronics manufacturing. Rack plating is also much faster than electroplating items one at a time.

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When rack plating, it’s challenging but imperative that all of the substrates, or items getting plated, get coated in a uniform metal layer. The Electrodeposition Module combined with numerical modeling make it possible to investigate the effect of geometrical and operational parameters on electroplating uniformity, which helps optimize this electroplating process.

Model Geometry

In this model, 20 oil pump covers are mounted in a rack (Ref. 2). The covers are all electroplated with nickel. Nickel plating is popular because it offers superior chemical and corrosion resistance along with greater wear resistance, which increases product life cycles.

The anode is a planar dissolving anode, and the cathode is the array of oil pumps covers.

The model geometry is parameterized, which allows the oil pump covers to be displaced toward or away from the anode surface. As shown in the image below, one oil pump is displaced toward the anode in order to demonstrate the geometrical effect on the current distribution in the electrolyte bath.

Below, you can see the close-up geometry of an individual oil pump cover.

In this model, which uses a secondary current distribution interface, you apply the Butler–Volmer kinetics to the anode as well as the cathode in order to compute the deposited layer’s thickness at the cathode surface.

Analyzing the Results

Below, you can see both a streamline and surface plot of the electrolyte current density at the oil pump cover, or cathode, surfaces. In the electrolyte, the current flows from the anode surface to the cathode surface. The surface plot of the total current density shows a nonuniform density distribution across the shape of the oil pump covers (and between them). The current density is highest at the displaced oil pump cover, located closer to the anode. The oil pump covers along the edge of the rack receive a higher current than those at the center of the rack.

The inconsistent current distribution is due to the:

1. Complex shape of the oil pump covers
2. Mounted position of the oil pump covers on the rack

Current density distribution in the rack. The plot shows the high current density on the protruding cathode and also the edge effects on the components placed on the edge of the rack. These components get a slightly higher current density, since there is space on the side for some of the current to have a longer path.

Next, let’s look at the electroplating thickness over the cathode surfaces, which is directly related to the aforementioned current density distribution plot. Below, you can see the lowest deposition thickness is found at the bottom surface of the cathodes, and the highest deposition thickness is found at the top of the surface of the displaced cathode.

Electroplating thickness at the cathode. The nonuniform current density distribution in the previous plot is also revealed in the deposition thickness. The cathode with the highest current density obviously also gets the thickest deposit. Also, the edge effects on the components placed on the edge of the rack result in a thicker plating.

Numerical models, like this one, are useful in optimizing the deposition process because they allow engineers to change design and operation parameters, such as:

• Plating rack configuration
• Plate cell geometry
• Distance between the anode and cathode surface
• Conductivity of the electrolyte
• Applied current
• Applied potential

Next Steps

Try modeling the electroplating of multiple components in a rack by clicking the button below. Doing so will take you to the Application Gallery.

Further Reading

Learn more about electroplating on the COMSOL Blog:

References

1. C. Bell, “What You Probably Never Realized About Award Show Trophies“, Reader’s Digest, 2019.
2. J. Deconinck, G. Floridor, B. Van den Bossche, L. Bortels and G. Nelissen, “Numerical 3D BEM Simulation of the Chromium Layer Thickness Distribution on Parts in a Rack Plating Configuration”, Simulation of Electrochemical Processes, vol. 48, p. 173, 2005.

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