What Part Does Voltage Play in Electroplating?

A. Hi Agata. Kirchoff's Current Law has a lot to do with parallel circuits, but I am not seeing its applicability to your situation.

You are correct that Faraday's Law says nothing of voltage; rather it speaks about current & time (ampere-seconds, ampere-minutes, or ampere-hours), and this is why: Amperes are a measure of electron flow rate, and Faraday's Law says if you move electrons from anode to cathode, the anode metal becomes positively charged ions which dissolve into the solution, migrate over to the cathode, and when they meet the electrons they become neutrally charged metal atoms on the cathode -- in proportion to that current flow.

Current flow = Voltage/Resistance, and most of the resistance is in the plating solution. So if you keep the anode and cathode a consistent distance away from each other, the resistance should be close to 'fixed' (once the nail has a thin coating of zinc on it, and you are no longer fighting against the opposing voltage of that zinc-iron battery)

If I am understanding you, you will be trying to plate with one 1.5V AA battery, then with 2 in series for 3.0V, then possibly with 3 in series for 4.5V? Although the resistance of the circuit is not exactly 'fixed', it's probably close. If you can measure the resistance of the system with a simple VOM multimeter [on eBay or Amazon] (after the nail has some zinc on it), you can calculate the current for each of your situations by Ohm's Law, A = V / R.

Luck & Regards,

A. Hi Vashist,
Firstly, you should not allow your anode baskets to get so empty that the voltage climbs to 6 volts in a situation where 4.9 volts should have been enough :-)

You should be deciding what amperage to apply per square foot or per square decimeter, and setting your amperage to that value. About 40 ASF (4 A/dm2) is a typical value for bright nickel plating. If you are running consistent loads of about 7 square feet, then 285 Amperes might be fine. Whatever voltage is required to maintain that is the what you need. Amperage and voltage are not independent, they are tied together; and you should be trying to control the amperage not the voltage.br Luck & Regards,


Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

A. Electricity cannot be seen, so it can be rather abstract. Therefore, for ease of visualization, terms and units that relate to the flow and quantity of electricity are often compared to terms used to describe the flow of water.

The corresponding term to voltage is water pressure.

If you lived in a high-rise apartment alongside a lake and decided to hang a hose out your window to draw up some lake water for your flowers & plants, you would recognize that the water is not going to run up that hose by itself, rather that you'll need a pump, and that how much pressure that pump has to put out will depend on what floor of the high rise you live on.

You need voltage in order to overcome all of the resistances involved in plating, which include the resistance of the copper wires that the electrons flow through, the electrical resistance of the plating solution which the ions flow through, and other resistances. I can answer more deeply what those other resistances are, but that may be beyond the scope of your project.

A. Hi Scott. Faraday's Law says you will ideally deposit one gram equivalent weight of metal for every 96,485 ampere-seconds (coulombs) you apply.

When you double the voltage from 1.5 volts to 3 volts you'll probably more or less double the current because of Ohm's Law, I = E / R, so you more or less plate twice as fast.

But at 20 volts you will deplete the solution of copper ions in the area of the cathode almost instantly. But the electrons you are making available have to go somewhere--with the result that the electrons will reduce the hydrogen in the water to nascent hydrogen gas instead of depositing copper which isn't there. This formation of copious hydrogen gas, plus atoms of metal being yanked out of solution instantly instead of slowly growing as crystals, is called "burning".

A. Hi, Philip. You can't do what you are thinking of. If you keep the current that is going through the solution constant, you are also keeping the voltage drop across the solution constant, again because of Ohm's Law.

You can set the voltage at 20 volts and add a big external resistor to eat up a big voltage drop there, but if the current through the solution is constant, the voltage across the solution is constant, and the rest of the voltage drop is across the resistor.

Regards,

A. Hi, Philip. I think you're still not quite accepting all of the implications of Ohm's Law :-)

If you put the anode and cathode further apart to keep the current constant as you increase the voltage, you're still just looking at a voltage drop across an "external resistor", although in this case the "external resistor" is the extra plating solution between the anode and cathode rather than an actual external resistor. And in fact, practical experience shows the qualities of the electroplated metal to be the same whether the anode to cathode distance is a couple of inches or several feet.

There is a diffusion reaction in the boundary layer around the cathode as the plating proceeds. If you plate very slowly you tend to get large crystals because there is copper available when the electron touches it. If you plate faster, but still in conventional plating range, you get more adatoms and smaller crystals. If you try to plate still faster, you get burning. Good solution agitation keeps plenty of copper available just outside of the boundary layer, and extreme agitation (directing a high pressure stream of fluid at the cathode) reduces the thickness of the diffusion layer and allows faster plating. Pulse plating produces some changes in plated layer properties, but some people feel that the only real mechanism behind that is that the pulses cause agitation in the boundary layer -- but maybe not; maybe if you had pulses of microsecond duration instead of millisecond duration you could engineer the plating at a nano level.

Regards,

A. I believe you have the half cell potential for Copper correct at +0.34 V. If you were plating copper onto silver, which has a half cell potential of +0.80 V, you would need a minimum of 0.46 V to get it to deposit. If you were plating onto iron, which has a half cell potential of -0.41, the copper will spontaneously immersion deposit onto the iron while generating 0.75 V.

These potentials are for standard conditions, and I believe that the Nernst equation can get you more realistic numbers for your actual situation.

Sorry, but I have no experience in the area that you are exploring (trying to fill nano voids). But "brighteners" have long been used in the plating industry to try to get more uniform plating. They work by (at least partially) blocking a surface so that the electricity is steered towards causing the metal to deposit elsewhere, resulting in smaller grain, brighter deposits, and more even distribution.

The following is from an engineer, not an electrochemist, and it is a simplification -- but here's the way I understand it: In order to copper plate thru-holes in thick multilayer circuit boards, where the current is very low in the hole but relatively high on the surface, they have developed developed special brighteners. These brighteners have molecules so large that they can't get into the hole to block deposition there, but cover everything else. The result is copper is deposited deeply into the thru-hole. Please talk to one of the major suppliers about their acid copper process for thru-hole plating. In may be what you need.

Also, please see our library article "A Review Of Copper Plating High Aspect Ratio Plated Through Hole Papers" contributed by Paul Stransky.

Regards,

A. Hi, Bryan. I'm not quite following your question. And in view of the previous rather long discussion, I am therefore not sure if you followed it and are asking a follow-up question, or you blew it off and are starting over :-)

Regardless, think of it this way. Each atom of a metal has electrons. If you use a battery or power supply to pull electrons away from a metal anode, pump them through the wiring, and over to the cathode, what will happen in electroplating is those metal atoms which are now short on electrons will dissolve into the solution as positively charged ions, and migrate through the solution over to the cathode -- at which point they will reacquire those electrons and the metal will deposit on the cathode. In brief, if you move the electrons, the positively charged ions will follow.

But this is not only qualitative, it is quantitative. In an acid copper plating solution, where copper ionizes to the +2 state, if you move 2 electrons from the anode to the cathode, one copper ion will follow. Faraday's Law is the mathematical expression of this.

This situation will persist over a fairly broad voltage range, i.e., the higher the voltage, the more electrons will flow, and the more metal will plate out. However, if the applied voltage is too low (in the neighborhood of less than 1-1/2 volts depending on the position of the metals in the EMF series) then no current will flow. That is simply because those two metals in solution are comprising a battery themselves, which you must first overcome. And if the voltage is too high, the electrons start piling up at the cathode and can't "wait" for the metal ions to get there, and will instead start pulling hydrogen out of the water, "depositing" hydrogen gas on the cathode.

Regards,

A. Hi, Spencer.

Electroplating is a science to a very good degree, but some aspects of it are still limited to the empirical level. We can tell you qualitatively what happens if you try to plate too fast (as we did above), and we can offer the Nernst Equation, Faraday's Law of Electrolysis, and Ohm's Law -- but we can't yet explain in terms of formula what the limiting current density for sound plating is based on concentration, temperature, pH, additive levels, and degree of agitation. The supplier of the plating solution has tech service people with a great deal of this empirical knowledge; and I can also say that pumping a vigorous stream of plating solution at the cathode will thin the boundary layer and allow the highest possible current density and fastest plating.

Defects like incomplete coverage, and peeling are more often related to pretreatment than the plating step per se. And burning is probably not the principal cause of roughness -- inadequate filtration can be the culprit there. We are still at the point where hands-on experience in tin plating easily trumps math skills :-)

Good luck.

Regards,

A. Your vendor for the solution can provide guide lines. After that it is trial and error as the barrels tend to have the holes peen shut with use and that reduces the refreshment of solution.

I would invest in a good book from the list of reviewed books at this site for a lot more information. Plating has a lot of black art as well as science that goes with it.

A. Hi Spencer. I see from your follow-up posting that you are talking about barrel plating rather than a hypothetical specialized strip plating. So directing a high pressure stream of plating solution at the surface may not be practical.

The way to get the highest possible plating rate and freedom from defects in commercial plating is to carefully follow the tech data sheets and supplier's advice. There are thousands of barrel tin plating lines in operation and the supplier's instructions are already optimized from decades of experience with them. Please don't imagine that there are shops out there saying "we don't need maximum production, we can just coast along making half what other shops make". Everyone strives to plate as fast as they can, so you are not going to find that raising the temperature or concentration beyond the supplier's recommendations will improve the operation.

Jim is of course right that the holes on polypropylene plating barrels peen shut or at least smaller from operation, and it may be possible to re-drill them or buy plating barrels with larger perforations for somewhat better plating speed. Good luck.

Regards,

A. Hi David.

Yes, the parts will plate without directly touching the cathode contact, as long as they are touching parts that are touching parts that are touching the cathode (like "electricity" from "home base" in a child's game of tag). But I think you may be re-inventing the wheel and are only up to 16 sides :-)

Please be aware that electroplating is a well-developed industrial science, and you don't need to start from what you may have seen in chemistry class; rather, you can start from catalog equipment. Polypropylene plating barrels are available in every size, and they will be a better starting point than a vibratory tumbler. Please see our library article on Barrel Plating. There are also catalog items called "vibratory barrels" for very small and delicate parts.

One problem I foresee with a vibratory tumbler as a plating cell is insufficient solution volume for the area of the parts you are plating. If you quickly deplete the solution of its metal content, you won't plate properly.

Amperage rather than voltage is the salient variable in electroplating because the amount of plating that occurs is directly proportional to the ampere-hours of current (within the operational limits). Voltage should be whatever is necessary to support the amperage you are trying to draw in the particular configuration you have -- but yes, 4.5 V is probably in the ball park.

Likewise, plating solutions are catalog items rather than something you mix up from commodity chemicals, and the optimum operational temperature will be a function of the type of plating solution. Best of luck.

Regards,

A. Hi Spencer. There are actually two rather different components to your question. The first element is that, yes, there is a very clear scientific basis for how many ampere-hours must be applied to get a particular thickness, and it's already been mentioned twice on this page. It's Faraday's Law, which says that 96,485 ampere-seconds will deposit one gram equivalent weight of metal. It you want to do the longhand math, this will evaluate to thickness after you take atomic weight, valence, density, and the dimensions and speed of your strip into account in your calculation.

If you want to shorten the calculation, there is a chart entitled "Electrochemical Equivalents" in the back of the Metal Finishing Guidebook (all years), which integrates those constants for the atomic weight, valence, and density, so that you can directly read ampere-hours per square foot required to deposit 0.001" of the given metal. Get the book if you don't have it; you should not be plating without it.

But the second element is that the science has not yet advanced to the point where we can calculate from first principles the limiting current density (i.e., amperes per square foot, or ASF) that will still yield acceptable and burn-free plating. For that we still rely on experience and empirical data. In rack plating, it's probably in the ballpark of 20 ASF, and the supplier of your plating solution can probably give you a better estimate. In the case of reel-to-reel plating it is not uncommon to spray the plating solution onto the strip. If it is sprayed hard, this thins the "boundary layer" and makes plenty of full strength plating solution available right at the interface, and thus enables much higher current density. It wouldn't surprise me if you could plate at 100 ASF or more with a modern high-speed reel-to-reel plating machine, but it depends on how much agitation your machine applies. Good luck.

Regards,

A. Hi Sudhir. Bright nickel plating is usually done at approximately 40 amperes per square foot of surface area with vigorous air agitation; probably somewhat less with mechanical agitation instead of air agitation. As previously explained, you use whatever voltage is necessary to obtain the 40 ASF -- it will probably be somewhere between 3 and 12 volts.

Have fun playing with it and learning from your experiments on scrap material! But nickel plating is a very complex subject, with many people spending decades at it, so if you need to secure functional plating on that small object to not "spoil" it, please send it to a plating shop for professional plating. Unfortunately, the chance of good plating on your first attempt and with amateur equipment is nearly zero :-(

Luck & Regards,

A. Hi Stefan. You should not be attempting an experiment on electrolysis of NaCl except under close supervision by your science teacher. The chlorine gas you would generate is highly toxic to the point of even being lethal.

Your copper plating experiment should probably be done at 1-1/2 to 3 volts -- certainly not 9 volts. Severe burning instead of copper plating would be the sure result. Good luck.

Regards,

A. Hi again Stefan. This page already describes what burning is, and what causes it, twice. If it's not clear, please try to express your ongoing question in terms of what has already been said about burning. Thanks!

What is your science exhibit about? Is it about the development of ideal industrial plating parameters as a graduate student thesis, or demonstrating Faraday's Law as a high school/college project, or just demonstrating that you can develop a copper color on silver or nickel as a grade school project?

See, the thing is, temperature matters, concentration matters, pH matters, impurities matter, agitation matters, current density matters, addition agents matter -- just about everything matters -- and there are whole textbooks just on copper plating, so I can't condense a book down to a paragraph if you're trying to do robust plating to ideal conditions; but I can certainly point you to simplified presentations if that's all you need.

So what is the hypothesis that your experiment is attempting to verify or disprove?

Luck and Regards,

A. Hi again Stefen. Thanks for the clarification. CuSO4 is an acceptable plating solution for copper plating, but it is only one of 4 or 5 potential ones, so your response helps.

Typical concentrations are 195-248 g/l of copper sulphate, 30-75 g/l of H2SO4, operated between room temperature and 50 °C, with small amounts of generic or proprietary addition agents. Purifying copper from impure form is usually called "electrowinning" rather than "electroplating", so you may find some web info using the term "copper electrowinning". Letter 14472, "Electro-refining of Copper" should be a good intro.

Electrolysis of NaCl into caustic soda [affil links] and toxic chlorine gas is an industrial process, but probably one you want to show descriptions of, or pictures of, rather than trying to actually demonstrate at the high school level.

I don't know quite what you mean by a thin electrode, but the size of all current-carrying conductors must be large enough to carry the current in question with minimal to acceptable resistance. Good luck.

Regards,

Hi, Vladamire. I am a very old friend of the Von Hung family of London; please give my regards to your father and have him fax me a photo of your plating tank and I will be happy to calculate it for him.

Regards,