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Voltages and rectifier selection for chrome plating process




An ongoing discussion from 2000 through 2015 . . .

(2000)

Q. I am looking to purchase a rectifier for the purpose of chrome plating, I have discovered that there are different methods of control, but I don't know the pro's and con's of each. Could somebody please explain the differences between variable transformer, tap switch, and SCR control? Also, I assume I'll need up to 1500 Amps output to handle auto bumpers, if this is incorrect please let me know. Thank you.

Brian C [last name deleted for privacy by Editor]
- Surrey, BC Canada


(2000)

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.

Ted Mooney, finishing.com
Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey


Hard Chromium Plating


Chromium Plating
by Weiner & Walmsley

(2000)

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 Watts
- Navarre, Florida


thumbs up sign 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,

Ted Mooney, finishing.com
Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey



(2000)

Q. What would be a correct choice for voltage whilst selecting a Decorative Chrome plating Rectifier?

At many job shops Chrome burning is a regular feature, despite other parameters being in control. I doubt if this is on account of using a higher voltage rectifier such as 16 or 24 Volt, when in actual use the required Voltage will be only 4-6 Volts.

Does burning occur if a higher Voltage is used to obtain the required amperage? Is using an 8 or 12 Volt Rectifier a better choice for obtaining higher amperage at a lower voltage or vice versa?

Thanks in advance.

khozema
Khozem Vahaanwala
Saify Ind supporting advertiser
Bangalore, Karnataka, India

saify logo


(2000)

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).

Ted Mooney, finishing.com
Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey


(2000)

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.

James Watts
- Navarre, Florida


(2000)

Q. In India the general rule is to use only 3 phase Rectifiers on account of incoming line voltage problems hence I'm reasonably certain that ripple is not a major concern. However people tend to use higher Voltage rectifiers on account of versatility .

I wish to add a sequel to my above question :

When general parameters are normal, then what should be the correct distance of the component from the anode at say 150 amps /ft2 to avoid burning without compromising coverage?

khozema vahanwala Khozem Vahaanwala
Saify Ind 
Bangalore, Karnataka, India


(2000)

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".

Ted Mooney, finishing.com
Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey



(2002)

Q. Sir,

We are designing a new setup to hard chromium plate hydraulic rams. We are in two minds about the rectifier we should choose. Our requirement is about 3200-3500 Amps and are in two minds whether to have one rectifier of 4000 Amps capacity or can we club two 2000 Amps rectifiers together. What is the possibility that the second option works as we have two rectifiers available one having 2000 Amps 12 V and second of 2000 Amps 8 V. I have heard that it is indeed possible to club rectifiers together but not aware of how it can be done. Do they need to be clubbed electrically having a single current controller or can we club the cathodes together to the job and have independent/ clubbed anodes. I hope you can understand what I mean .

Your expert advice will be appreciated.

REGARDS,

vikram dogra Vikram Dogra
- Chandigarh, India


(2002)

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.

Ted Mooney, finishing.com
Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey


(2002)

Q. Dear Ted,

I am afraid but answer to my query No : 16431 has not been satisfying. You have not mentioned which pole to club and what "little" problem will it cause. Also what will be the best way to club them.

Hope to receive a satisfying answer.

REGARDS,

vikram dogra Vikram Dogra
- Chandigarh, India


(2002)

thumbs up signHi 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!

Ted Mooney, finishing.com
Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey


(2002)

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.

James Watts
- Navarre, Florida


(2002)

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,

asif_nurie
Asif Nurie
- New Delhi, India

With deep regret we
sadly advise that
Asif passed away
on Jan 24, 2016




(2006)

Q. Can I do hard chromium electroplating with a 1000 A / 6 V rectifier ?

NAOUI KADOUR
SARL SODEC - Algeria


(2006)

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.

Ted Mooney, finishing.com
Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey



(2007)

Q. Sir,

In chrome plating rectifier how much ripple is allowed? in 250 kw rectifier how much ripple will generate in 10,000 Amps current rating. Transformer primary wave is pure sine wave form.

Natarajan.G
Maintenance incharge - B'lore, karnataka,india


(2007)

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.

Ted Mooney, finishing.com
Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey



November 23, 2009

RFQ: Looking for chrome rectifier min 5k 3 phase/220 north Jersey

Andrew M [last name deleted for privacy by Editor]
broker - Newark, New Jersey, USA
outdated



March 17, 2011

Q. I'm currently hard chrome plating with a 5000 amp, 6 volt, water cooled, tap switch rectifier. I have trouble getting past 2100 amps max. My busing, cabling, fixturing, etc. all seem to properly sized and there is less than 4 inches between my anodes and cathodes (parts). My parts are small diameter (1/4 to 5/8"), long length (4' to 12' long) and I suspect that they are a source of significant resistance. So, I think I need higher voltage to overcome the resistance. Two other plating shops I've spoken to say they do hard chrome plating with 12 volt rectifiers. Does anyone else out there use a 6 volt rectifier for hard chrome? If not, what voltage do you recommend?

Peter Krasucki
Plating shop - Webster, New York, USA


simultaneous March 18, 2011

A. Hi Peter,

6V rectifier may not enough to support 5000A working current, unless resistance is very small. For your case, better to switch to a higher voltage capacity rectifier or lower down working ampere by reducing total plating area.

Regards,
David


David Shiu
- Singapore


March 20, 2011

A. Hi Peter,
I think you don't need a 5000 amp rectifier to plate a 5/8" dia X 12' long job. You have an oversized rectifier. 6 volts is enough for a 4" gap, however for hard chrome application you need at least 8-12 V rectifier capacity if you plate at 1" or more anode/cathode gap just in case you get a job where area to be plated is more. Your job will not require more than 1500 amps, so if you are able to give it that much current and still stay below 6 volts you don't need to change your rectifier. If this is a production job try conforming anode(3/4" gap)- you might just plate it at 4.5-5 volts and not require more than 800-1000 amps... you save a lot on the electricity plus the part resistance will not trouble you much. Other plater as you say might be using 12 volts rectifier as they might be plating at say 6" gap or having jobs with more surface area..ALL THE BEST

vikram dogra Vikram Dogra
- Chandigarh, India


March 21, 2011

Q. Vikram - I forgot to mention that I'm plating 5 to 10 parts per load. So the total surface area for 5 bars at 1/2" x 12' is 1131 square inches. At 3 amps per square inch I would need 3393 amps. With the rectifier turned all the way up I only get about 2100 amps.

Peter Krasucki
- Webster, New York, USA

simultaneous March 22, 2011

A. 3 options: Decrease the anode to cathode spacing or use a more conductive solution or use a larger power supply.
Temperature adjustment might help, but I do not like the lower temp for hard chrome as it adversely affects the distribution.

The quickest and cheapest thing is to try decreasing the spacing by 1 inch and see what happens.

James Watts
- Navarre, Florida


March 22, 2011

A. Peter in that case you would def need a 12 V rectifier ORRRR...take my idea go for a conforming anode. 1/2 inch spacing would be too much to ask for for a 12' long anode so try a 5/8" or 3/4" spacing , you might just be able to plate it at 6 V with desired results.. (i mean faster plating rate / energy saving and above all cost towards a new rectifier.. go for it..)

vikram dogra Vikram Dogra
- Chandigarh, India


April 10, 2011

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.


 
Sara Michaeli
chemical process supplier
Tel-Aviv, Israel




April 9, 2014

Q. How to calculate the rectifier capacity in terms of amps?

How related to our job to be coated?

abdul nazar
- coimbatore, india


April 2014

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,

Ted Mooney, finishing.com
Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey



September 30, 2015

Q. Hi I have a triple chrome plating kit from a hobby plating company and I need help with volt and amps.
I have a power supply that has a gauge for volts and goes up to 25 volts and a gauge for amps that goes to 50 amps and a dial that is for adjustment. Now, say I need 2 amps to plate my part, do I just adjust the knob till I get 2 amps on the amp gauge and don't worry about what the voltage is, or must I worry about the voltage also.

Tom Barbush
- harrisburg Pennsylvania usa


October 2015

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,

pic of Ted Mooney
Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey
Striving to live "Aloha"



October 17, 2015

Q. Hello and good day to everyone here around. I am a newbie and I would like to do a small experiment about chrome plating at home but I don't have enough knowledge about what basic and right equipment to purchase and use.

I am hoping that someone could help me around here regarding the basic equipment. I am planning to chrome plate about a size of a body of an electric guitar. I think it's about 20 inches in height and 14 inches in width with a 2 inches thickness . Definitely, the container of the item would be a bit bigger than the item that I am going to plate. With that size, I would like to ask what kind of rectifier I'm gonna to buy? How many amperes? How much volts do I need to use to plate it properly? And lastly would that be all the equipment needed?

I hope I can get an answer regarding these matter. Thank you so much for taking the time reading my letter.

benjamin_lucero
Benjamin Lucero jr.
- Manila, Philippines


October 2015

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,

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



What kind of single phase rectifier can I use for chrome plating?

June 27, 2016

Please I need your advice. I want to start decorative chrome plating on a smaller scale. Because of the power source, which is going to be a petrol generator of about 7 kva, I want to go for a single phase rectifier. What is the type of single phase rectifier I will need that will reduce the ripple that is common with single phase rectifier in order to have good chrome plating? The input voltage range between 220 to 240 volts. I will prefer rectifier with variable voltage and current. The output will be direct current.
Thanks for your assistance
Abbey

Abiodun Jide
- Lagos, Nigeria


June 30, 2016

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

harry_parkes
Harry Parkes
- Birmingham, UK


July 2016

thumbs up signThanks Harry.

Regards,

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



Is 12 volts enough for composite plating?

November 1, 2016

Q. I am an interested researcher in the field of composite coatings such as Ni-TiO2 and Cr-Al2O3 prepared by electrodeposition. I want to buy a rectifier for this purpose. In most articles only the amount of applied current density has been reported and there is not any information about working voltage. Is equipment with maximum current 20 A and maximum voltage 12 V proper for composite electroplating?
Please guide me to select proper rectifier for electroplating.

Ali Jam
- Isfahan, Iran


November 2016

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,

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


November 2, 2016

Q. Thanks for your reply.
Based on your answer 12 A and 12 V is enough for electrodeposition of composite coatings on small parts, e.g., with less than 20 cm2 surface area. Again it is emphasized that the goal is co-deposition of composite coatings including a metallic matrix and ceramic particles as reinforcement. For example in Ni-TiO2 coating in addition of reduction of existing Ni2+ ions in the bath, TiO2 particles (added in the bath) also should incorporate in the coating.
In this condition, is it enough 12 V or not?
Yours Sincerely
Ali Jam

Ali Jam
- Isfahan, Iran


November 2016

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,

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



November 30, 2016
wikipedia
IGBT

Q. Hi all,
I have to select 20,000 amps 12 V rectifier for hard chrome plating. Which one will be good IGBT modular technology or SCR?
Thanks
Ramesh

Ramesh Yadav
- Brisbane, Queensland, Australia


January 19, 2017

Q. I have two hard chrome plate rectifiers, both are 12000 (6000+6000) Amps and linked with tank. If I have small job work I use 6000 Amps if I get big job I join them together like 12000 Amps. At present my Output voltage 18V, if I use 24V is there any reflection on my job, and what is the result will come up if I use more than this?

Razack Syed
Industry - chennai, Tamilnadu


January 24, 2017

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,

anders sundman
Anders Sundman
3rd Generation in Plating
Consultant - Arvika, Sweden


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