Voltages and rectifier selection for chrome plating process

A. Hi Brian. The required amperage will depend on how many bumpers you plate at a time and how big they are. You should figure that you will probably not be able to chrome plate at all at less than 100 ASF, and 150 ASF is pretty typical, but 200 ASF is probably not excessive as a rectifier sizing specification.

SCR control means 'silicon controlled rectifier' control and is a solid state control method, the most modern style, and probably the least expensive and simplest of the automatic control methods. The incoming waveform is 'chopped' to control the outgoing current. However, this means that when the rectifier is operated at substantially below its rated current, the incoming sine waves are chopped into 'shark fin' shape, with the result that there can be substantial ripple (AC component) in the outgoing current, which can interfere with proper plating.

Variable transformer rectifiers achieve control by varying the number of turns in the primary vs. secondary coil, so they are better at achieving reduced ripple at low output. However, they are not solid state, but use a motor to adjust the transformer, so they have moving parts. They are probably more expensive and less reliable than SCRs.

Tap switch rectifiers are manual. As the name implies, there are taps in each of the three windings, and you manually turn a handle attached to brushes to vary the primary to secondary voltage. Nothing wrong with them as long as you don't need automatic control. Good luck.

A. Ripple is normally controlled by chokes or capacitors. Both are effective for a given percentage of the load that they are designed for. They all work fine at that point or at 100% output which is hard on the unit.

Solutions, buy a three phase unit. It will be a full wave unit, so you will be using 6 "phases" which have a significant overlap making ripple of no consequence unless you try running a 1500 amp unit at 50 amps.

The true state of the art is a supply that runs at mega HZ or giga HZ and uses a true chopper, not the one that Ted mentioned. These are tiny units vs a normal power supply. They generate very little heat, so are more efficient at all power settings. The electronics portion is a sealed unit with heat sinks so none of the lousy atmosphere gets to the critical components. Draw back-- They cost more. I am not sure how big they are now. 5 years ago, they were only available in the 300 amp area, but that has been steadily going up.

James is right that the state of the art is to use solid-state "chopper" circuits to generate high frequency rather that operating at 50/60 Hz because this greatly reduces the size of the components. In the plating industry, this technology is called "switchmode" rectifiers.

Regards,

A. Hi Khozema. As you know, Ohm's Law demands that voltage and current are directly proportional; so, for a fixed resistance, you can't raise or lower the current or voltage without proportionately raising or lowering the other. So the only way to vary the required voltage is by varying the anode to cathode distance.

But a rectifier should never be grossly oversized because it leads to extreme ripple, inefficiency and other problems. So, if you know that you never need more than 12 volts, get a 12 volt rectifier, not a 16 or 24 volt one.

The same things that cause burning in other plating solutions cause burning in chrome . . . the difference is that the operating window for chrome plating is very small because burning will occur at too high a voltage/amperage whereas no plating at all will occur at low voltage/amperage (say, less than 100 Amps/ft2).

A. I will carry Ted's reply one further step. If you currently never exceed 6.0 volts, buy a 6 volt supply. If you occasionally get any higher, buy a 9 volt unit. If you use a 3 phase unit rather than a single phase unit, you will virtually eliminate ripple without a choke or huge capacitors. Ripple is extremely bad for chrome.

A. I have seen anodes as far away from the cathodes as about 18" to 21" when plating bumpers or plating shafting vertically, but the rule in any plating is to get the anodes as close as practical, and 1-1/2 to 2 inches is common in hard chrome plating--so I don't think that there is a distance that is too close for decorative chrome plating as long as you can assure that they never touch.

But chrome has horrendously poor throwing power / covering power; so if one area of the part is significantly recessed you may need greater anode to cathode distance so that the relative distances from the closest and furthest anode to cathode distance don't vary excessively. For example, if the part has a 2" deep recess, it probably won't work for the nearer points of the part to be 2" from the anodes and the recess to be 4" from the anodes (twice as far away). In that case you might need to go to, say, 10" nominal anode to cathode spacing, so the ratio of closest to furthest anode to cathode is 10" vs. 12" instead of 2" vs. 4".

A. The best way is to tie only one pole of the rectifiers together, and use one rectifier for one bay/cell and one for the other, and you'll have little trouble. If you tie both poles together, you'll have nothing but trouble.

Hi Vikram, thanks for reminding me that American English can sometimes be a bit different than Indian English :-)

In American English, "You'll have little trouble" is an idiom meaning "You'll have [very] little [if any] trouble"; whereas I realize that in Indian English it could mean "You'll have [a] little trouble".

A. You can connect both rectifiers to the same anode busbar, or you can connect both rectifiers to the same cathode busbar, depending on what is convenient. But I do not think you should connect two rectifiers both to the same anode busbar and the same cathode busbar; that will be like being "in two minds" because two different control systems will be fighting to regulate the same system. Good luck!

A. Appropriate sized diodes in all 4 leads from the two power supplies will prevent current from flowing backwards thru the unit and will solve the problems Ted referred to. It takes a qualified person to determine the correct sizes.

A. VIKRAM,

YOU CAN INSTALL ONE 4000 AMP RECTIFIER. THAT IS THE RIGHT WAY TO GO IF YOU ARE SETTING UP A NEW PLATING FACTORY.

REGARDS,

A. Yes, Naoui

Although 12 volts is more typical for rack plating, if you have anodes very close to the part (maybe an inch or two), it is possible to plate with 6 volts. Please explain your situation in more detail.

A. Hi Natarajan. The ripple will depend on a number of things including what percentage of the rectifier's capacity you are using and what rectification technology is used, G., but the mathematical ripple in rectifying a 3-phase sine wave to DC is 5 percent. You can read a lot of detail about this, and a number of opinions in letter 1424.

One of the readers in the referenced letter offers his opinion that the shape of the disturbance is as important or more important than the actual amount of ripple. Further, opinions differ on how much ripple is allowable. But for the briefest answer, it is usually said that ripple should be held to 5 percent or less for chrome plating. Good luck.

A. 1. 6V is too small for hard chromium plating.
2. The solution plays an important role in the conductivity of this process. A properly operating solution should have a conductivity of 450-500 mS/m. Metallic impurities decrease rapidly the conductivity which means that you need more V for the same Amp.
3. Try to reduce Cr3 and iron.
4. If you are not able to get rid of the impurities then the remedy is increasing the CrO3 concentration.

A. Hi Abdul. I'm not sure if I understand your question, but you simply multiply the surface area of your job in square feet by the required current density in amps/sq.ft. (or the surface area of your job in square decimeters multiplied by the required current density in amps/sq.dm.). The current density requirement might vary depending on the workpieces you are processing and the method, but 200 Amps/sq.ft, 20 Amps/square decimeter is probably a mid-range value.

If you can explain the details of your situation, I think we can help you further. Good luck.

Regards,

A. Hi Tom. Voltage and amperage are locked together by Ohm's Law: V = A x R.

The plating solution will have a resistance that you can't change except by moving the parts closer to or further from the anodes. So if you choose the amperage, you have to let the rectifier produce however many volts it needs to get there. Chrome plates at about 1 Amp per square inch, so 2 Amps would be appropriate for a part of about 2 square inches surface area. If you are doing some kind of imitation chrome, it probably plates at about 1/3 as much current. Good luck.

Regards,

A. Hi Benjamin. If the part is 628 square inches, you'll need about 600 Amps at probably 12 volts. But a lot more equipment is required: please see our "Introduction to Chrome Plating". You should surely start with small scrap parts rather than with something large or important though.

A scalpel and needle & thread is not the heart of surgery, a skilled surgeon is ... similarly, some people feel that they are still relatively new to chrome plating after years at it and thousands of parts successfully plated. Good luck, but please start smaller -- and I'd suggest getting experience with other plating before trying to deal with toxic, carcinogenic hexavalent chrome plating solution.

Regards,

A. Hi Abbey,

Hexavalent chromium plating needs a very smooth (low ripple) DC supply. Ideally I would start with a 3 phase generator. The rectified DC output is suitably smooth for chrome plating without additional smoothing.

The ripple from a single phase power supply has a very high
ripple and needs a great deal of smoothing before it can be
used for hex Cr. Traditionally the smoothing circuitry
consists of inductors (chokes) and large value capacitors. For low current supplies, say up to 10 amps this is fairly easy. But when when we start to consider hundreds of amps the whole thing becomes heavy and expensive.

If single phase must be used I would look at switch mode power supplies and compare costings. They have smooth DC and can have variable voltage control. In fact I would consider switch mode for 3-phase as well.

I had a quick look for switch mode on Google. I found the following link helpful:
http://www.dynapower.com/products-switchmode.php
I'm not sure Ted will allow a commercial link. I'm certainly not trying to promote them! We have small SM units all around us for in computers, cell phones etc. but it is useful to see industrial size supplies.

Does this help? Have I missed to point of your question?

Harry

Thanks Harry.

Regards,

A. Hi cousin Ali. When you plate one metal with another you consume perhaps .2 to 2.0 volts for the actual reduction operation (depending on the half-voltages of the plated metal compared to the substrate). You might consume another small amount, probably not over 1 V, as resistance in the bussing system. The biggest voltage drop is due to the resistance of the plating solution, and is proportional to the anode-to-cathode spacing. As you will know, amperage and voltage are related by Ohm's Law of A = V / R

Practically, this means that if you want to control the amperage -- which you do -- you don't get to choose the voltage: it's just whatever voltage is required to deliver the desired amperage. Practically speaking a 12 V rectifier will be adequate for most plating operations except where you are processing large parts which demand a big anode-to-cathode spacing and therefore have a high solution resistance ... and with a 20 A rectifier you won't be doing big parts. I think you'll be fine.

Regards,

A. Hi again, Ali. Co-deposition of this sort involves occlusion. The particles are not electrically attracted to the cathode; rather, if they are in the vicinity of the cathode as the nickel or chromium is reduced onto the cathode, they become entrapped in the deposit.

This is not meant to imply that composite coating is as easy as pie; some companies have been in business for decades by licensing their knowledge and technology. But as far as I know, the particles do not add enough resistance to the passage of current through the solution as to require higher voltage rectifiers. Good luck.

Regards,

A. Hi,

12 voltage is enough for rectifier. To have more amperage you will destroy the rectifier, because a rectifier should be working at the high amperage that it's built for.

I have seen many connections from the rectifier to the chrome bath which are wrong connections. With wrong connection the rectifier loses so much of the power (amperage) through the cable. The + and -cable to the solution should be twisted from rectifier to the chrome tank.

Regards,