Hard chrome plating: Volts, Amps, Anode-to-Cathode Distance

A. Hi Ahmed, First calculate the surface area of that I.D. you want to plate. Then the required amperage will be an absolute minimum of 1 amp/sq.in. of surface area (chrome, unlike some other metals, will not plate at all if the current density is too low), with 2 amps/sq/in. probably being a good value.

Hang the part vertically so the hydrogen you will be generating can most easily escape. The anode should be a lead rod centered in the I.D., with somewhere around 2-1/2" of exposed lead and the rest masked to prevent dog-boning at the ends of your 3" length.

The voltage will be whatever is required to hold the 2 amps/sq.in. but probably well under 6 volts.

You should calculate the required plating time from Faraday's Law, assuming probably about 10-12% efficiency unless you are using a HEEF-25 bath, in which case something closer to 20-25% can be assumed. Advise if you don't understand Faraday's Law or efficiency.

You will need to clean, and then either Wood's Nickel Strike or etch the stainless steel before it will be plateable.
Luck & Regards,


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

A. Hi again, Ahmed.

If you were doing production plating the diameter of the anode would matter towards controlling the gradual build-up of trivalent chromium. When you're a student or hobbyist doing one or a few items, this isn't very important; if you had a lead rod or tube of 3/8" or 1/2" diameter, that would be fine.

I gave you the formula to calculate the plating time (Faraday's Law with efficiency of about 10-12%), and asked you to advise if you didn't follow it. But there is a chart on page 866 of the on-line version of the Metal Finishing Guidebook ⇨
which does most of this calculation for you, simplifying it down to the fact that it takes 51.8 amp-hours of electricity per square foot (at 100% efficiency) to deposit 0.001" of thickness. Since the efficiency will be, say, 10%, it will take 10X that many amp-hours. You get plating time by dividing amp-hours per square foot by the amps per square foot you are plating at. I previously advised that should plate at about 2 amps per square inch (or 288 amps per square foot. So the plating time for 0.001" thickness would be 51.8 hours x 10 / 288 = 1.8 hours = 108 minutes. You can scale down to 50 microns (0.2 minutes) or any other thickness.

You said you wanted to plate "internally", so the only anode will be internal, running through the center of your item. If you were to use a bare anode, you would get a little bit of plating on the outside of the tube near the ends; so you would want to mask the anode except for the 3" portion within the item you are plating. Even then, the plating would be a bit thicker near the ends, so we usually mask a little bit more, exposing only, say, 2-1/2" of anode. But for a first time effort, masking the anode and other fine detail is probably overkill :-) Luck & Regards,


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

A. Hi, Remberto. We're always delighted to answer questions here, but you should probably start your investment in your chrome plating operation with the purchase of some books on the subject, and hiring an experienced chrome plater and/or retaining a finishing consultant to get you started because a few paragraphs will prove a very poor substitute for the broad knowledge you will need to bring to this enterprise.

"Hard Chromium Plating" by Robert K Guffie is certainly the indispensable premier resource on this subject, but "Chromium Plating" [on Amazon, or AbeBooks] by Weiner & Walmsley is very valuable too. For general historical background "Electrodeposition of Chromium" Electrodeposition of Chromium from Chromic Acid Solutions [affil link on Amazon] by George Dubpernell is informative reading (although I've personally come to feel that it overstates the importance of M&T Chemicals and understate the contributions of everyone else :-)

1). There are at least three very different general types of plating baths based on their catalysts (1. sulfuric acid - the traditional 'Sergeant' bath; 2. fluoride - such as the 'Self Regulating High Speed' bath; 3. proprietary catalysts - such as the High Efficiency Etch-Free bath). Please see thread 35184, "Hard chrome plating catalysts" for an introduction to that topic.

2). A large range of chromic acid concentrations are employed from about 28 oz./gal to 54 oz.gal.

3). A number of different general approaches are used, for example, tank plating with stick anodes vs. Peger's reversible rack system.

4). Rollers are sometimes plated vertically, and sometimes plated horizontally while slowly rotating, with the bottom half (or so) submerged and the top half above solution level.

For these reasons,
- the ideal current density may range from a low of about 1-1/2 amp/in^2 to 3X that and more.
- the voltage is closely proportional to the anode-cathode spacing and solution conductivity, so it might actually be only 3-4 volts with a clean solution and close spacing or it might be 12 volts (maybe even more) in some cases.
- the temperature is usually between 130 and 140 °F, but again it can depend on the other factors.
- the concentration of chromic acid is usually, as previously mentioned 28-54 oz./gal. with 32 oz./gal. probably being most typical. But there will always be catalysts in the general range of 1% of the chromic acid concentration.

You might get hold of old issues of Metal Finishing magazine and read what Clarence Peger -- a strong proponent of reversible rack plating -- has to say. Best of luck!

A. In addition to looking at past issues of Metal Finishing magazine for Clarence Peger's articles about chrome plating, get a copy of Clarence Peger's books about chrome plating.

The manuals are available from:

Hard Chrome Plating Consultants , Inc., Cleveland Ohio

Ed. note: We're delighted to see you carrying on your father's legacy, Mary! In general we can't post advertisements in these forum responses (huh? why?), but out of great respect for his contributions we've posted your reply. Please become a supporting advertiser so you can advertise on this site; thanks; otherwise it gets hard to justify printing your promotions while refusing to print a dozen from China :-)

A. Hi, Rolando.

As with many businesses, labor cost is surely the biggest single item. It takes several hours (occasionally several days) in the plating tank, and hard chrome plating jobs often require careful masking, building of specialized auxiliary anodes and thieves, etc.

Electricity costs can be substantial: look into Faraday's Law to estimate electrical usage based on the amount of chrome you will deposit, remembering that the chrome is in hexavalent form, and deposition efficiency is probably only about 12 percent. The air exhaust system also uses substantial energy.

The chromic acid itself is a fairly minor cost, but it can be expensive to manage the handling of chromic acid (hexavalent chromium) to avoid worker exposure and pollution of the environment.

Although it might not be easy, or even possible, it would be best if you very clearly understand chrome plating from visits to several chrome plating shops. Before you try to estimate the operational cost, it's useful to know what operations are actually involved and in what proportion. There are many plating shops which make good money ... but there are no plating shops which produce substantial rejects and make money. Good luck.

Regards,

A. Hi, Mikki. If you have experience in this and you never use more than 8 volts, you should get an 8-volt rectifier. Simply, there is no value whatsoever in oversizing a rectifier :-)

But if this is a new installation and you are asking how much voltage an unspecified part will require, no one can answer exactly. But essentially the required voltage is proportional to the anode-cathode distance. If you use only very closely spaced conforming anodes, 8 volts is probably enough; if you do 'tank plating' with the anodes several inches to the better part of a foot from the part you are plating, it is probable that you will need 12 volts (maybe even more). The previously referenced book by Guffie suggests thinking very carefully before considering 9-volt rectifier because real-world contaminated plating solutions will require more voltage than new solutions. Good luck.

Regards,

A. Hi Gopal. Are you racking this part vertically or horizontally? If vertical, it's possible to arrange anodes in a ring around the part and get reasonable consistency of thickness. Anodes shorter than the part may help avoid dog-bone effect.

When you say you have "no idea" I'm not sure how literally to take that. If you are an experienced hard chrome plater looking for a tip based on this one particular shape of this part, hopefully someone can help you. If you literally mean that you have no knowledge at all of jigging & fixturing for hard chrome plating, you'll need to read some books or attend some classes as that would be way beyond what anyone can tell you via a public forum posting. Good luck.

Regards,

A. Hi Gopal: It is kind of difficult to help you without seeing a diagram or something about the part, but I am going to give you a few hints based on my experience. Hard chrome plating is a very tricky business, it is very complex and the bath is very poor in throwing, for every 100 amps you are "really" using 12 to 14 amps, with your bath in top shape. You should use 2 to 4 amps per square inch of surface to plate well.
Put your part vertical in the tank ; jig it up with heavy copper bar or hook; hopefully you have threaded holes on both ends of your part; use a stack of steel washers or nuts (to thieve the high current density of both ends, so you can get better distribution of the layer of chrome); Like Mr. Mooney said, set your anodes in a circular way around the part (use a heavy copper ring for your cell) and keep your part in the center about 3" from the anodes. Make sure your cleaning and activation process is good, good rinsing and give it a shot. This the best I can do for you. Good luck.

A. It seems unlikely that anything you did reduced the required amperage because that is dictated by Faraday's Law. With half the current, you'd put on only half the chrome, and I think you'd have noticed that thinness ... so I suspect the rectifier readings. The first and easiest guess about the problem is the ammeter and shunt leads.
The ammeter on your rectifier doesn't really measure amperage :-)
What actually happens is that there is a precisely sized block of copper alloy in your bussing called the shunt. When current flows through the bussing there is a voltage drop across the shunt just like any of the other bussing -- except that the shunt was sized to have a fixed voltage drop, usually 50 mV, when the rectifier's maximum current is flowing. At half current the voltage drop will be 25 mV, etc. So your ammeter is actually a 50 mV voltmeter. If that voltmeter is defective or the shunt is the wrong size for the rectifier rating or the meter, or even if the shunt wiring is loose or the connections dirty, the ammeter can read wrong.

A. Hi Pete,
I agree with Ted on his suggestion.
However you did not comment on the condition of your part.
Is the result same in same time or do you observe a difference? To my guess check your rectifier for a broken carbon leading to less current. Moreover in case you have a broken carbon it means that you are working on 2 phases which could harm the rectifier and the deposit too.

ALL THE BEST.

A. Hi Ezhil. If this problem developed as the plating solution aged, please consult Guffie for detailed comments on why this may happen with plating solutions as they accumulate impurities. An oscilloscope will show the actual voltage being delivered, and when sync'ed to your 50 or 60 Hz power, it can be very helpful towards revealing whether the rectifier is malfunctioning (these days some high-end digital multimeters can simulate an oscilloscope, plotting the waveform of the output voltage on an LCD display).

Luck & Regards,

A. Hi Aziz. Chrome will not deposit at all at low current densities and will burn at too high a current density. You will probably want to size things for around 250 to 500 Amps per square foot (25-50 Amps per square decimeter), although these are not firm limits.

Voltage is not an independent variable. It is whatever value is required to deliver that amperage under the actual conditions of anode-to-cathode distance and solution conductivity. Relatively small parts usually means relatively close anode-to-cathode spacing and relatively low voltage. A 9-volt rectifier is probably safe if parts remain small and anode-to-cathode distance remains short. But it is better to find someone who is doing a similar job and doing it in a similar way, and find out from them what voltage they have found necessary than to try to guess :-)

Luck & Regards,

A. Hi again, Aziz. I have no personal experience in chrome plating of gun barrels and can only report 'book knowledge'. Guffie says 2-4 Amps per square inch, which would mean 18-36 Amps for the small gun barrel and 57-115 Amps for the large. He also suggests that while 9 volts is usually enough, 12 volts is rarely not worth it.

But what he also makes a big point of is that the anode area should be as large as possible or the anodes will passivate and simply stop plating. That sounds like maybe that's a problem for you on the larger barrels.

Luck & Regards,

A. Hi Aziz. The most common hard chrome plating temperatures is probably 130 °F / 54-55 °C.
The allowable current density increases at higher temperature, but unfortunately the efficiency decreases, so changing the temperature isn't usually much of a strategy. Weiner & Walmsley has dozens of graphs covering the interaction of all the variables if you can find it in a library.

Luck & Regards,

A. Hi Hassan. Are you using only tank anodes? If so, my first suspicion would be that something is wrong with the rectifier or its gauges.

But A = V / R, and the bulk of that R is solution resistance across the gap from anode to cathode. So I suppose if you are using conforming anode surfaces very close to your component it's vaguely possible that as the surface of the plating grows closer to the anodes the resistance is being reduced by enough to affect the current.

Luck & Regards,

A. Hi again, Hassan. Yes, external devices have been used on plating rectifiers to stabilize the voltage for decades. But don't use the word "damper" in your search, do a google search for "filter chokes for plating rectifier ripple"

Luck & Regards,

A. Hi Ammar. I don't fully understand what you are describing, but if you are using the same chrome solution in both tanks, one is not plating much faster at the same amperage ... it sounds like your rectifier or meters (or the way they are installed) is wrong.

Yes you can "plate by voltage", many people recommend it, but the required voltage will depend upon the anode-to-cathode distance, which may be different for different size cylinders.

Please read the rest of this thread, then get back to us with more detailed questions.

Luck & Regards,

A. Hi Ammar. I don't know what else can be said :-)

You already knew the amperage was too high; but it's confirmed as too high a second time from thickness measurements because it's plating too fast, and you know it's too high a third time because you are seeing 'burning' instead of shiny chrome. We can't troubleshoot your rectifier from here but it's apparently the problem.

Luck & Regards,

A. Hi again, Ammar. I apologize but I just can't understand your 4 postings. You were trying to chrome plate with a chromic acid concentration of 104.4 g/l when you know it's supposed to be 280 g/l? Are you trying to chrome plate without catalyst at all (not possible), or are you saying it's now at 104.4/280 of what it should be? You believe that your rectifier meter readings are wrong but you are using them anyway? I'm sorry but we're just not communicating. Sorry, but I can't understand your situation, or you can't understand what I've been trying to say, or both :-(

Luck & Regards,

A. In the photo, the part is after immediately after chrome coating (why is the insulation so clean from below), or after working in some kind of unit? Please describe the problem in more detail. The composition of the bath, application modes preparation.

A. A visual inspection of the provided blanks revealed non-coverage in areas with low current density. On the protruding parts (high current density areas) of the part, shiny chrome is deposited. At the same time, roughness is present on some of the surfaces where chromium has deposited.
This defect is known and its occurrence usually occurs when the covering power of the chromium plating electrolyte decreases or when the current source (if the current source itself is in good condition) cannot maintain the specified current due to the high resistance of the electrolyte.
A decrease in the covering power of the electrolyte is possible due to its contamination with iron ions, trivalent chromium, etc., with an increased content of sulfuric acid or a low content of chromic anhydride.
Also, ion contamination with ions of iron, trivalent chromium, etc. increases the resistance of the electrolyte and maintaining a given current (30A/dm2) requires more voltage than the current source can produce. For example, a current source can deliver 12 volts, and in order to maintain the desired current (30A / dm2), with a high electrolyte resistance, 15 volts are required, since the current source cannot give them out, it will reduce the current (for example, to 15 A / dm2) and the blanks will not be covered.

If you have a Hull cell (you can make it yourself), then the problem can be quickly identified and subsequently eliminated.
If the hiding power is reduced due to an increased content of sulfuric acid, then it can be precipitated with barium carbonate by determining its required amount from the result of the analysis or by selecting this amount using the Hull cell.
Sulfuric acid is precipitated with barium carbonate according to the reaction H2SO4 + BaCO3 = H2O + CO2 + BaSO4,
based on 1 g of sulfuric acid, introduce 2.012 g of barium carbonate.
If the hiding power is reduced as a result of the increased content of trivalent chromium, then the electrolyte needs to be worked out.
Working out should be carried out with an anode area 5-10 times larger than the cathode one (the part for working out should have an area ten times smaller than the anode area, but not less than 10 dm2). The study must be carried out with periodic stirring of the electrolyte so that trivalent chromium ions are constantly present in the anode layer. Current strength 10 -30 A / dm2. The duration of study is 24-72 hours (depending on the amount of trivalent chromium accumulated in the electrolyte). Efficiency should be controlled according to the readings of the voltmeter; in the process of working out, a gradual decrease in voltage should occur. This measure will increase the covering power and reduce the stress on the bath.
If the hiding power is reduced as a result of contamination with iron or copper ions, then a reduction in its content is required. Of the possible ways:
Diluting the electrolyte by 2 times with water and adding chromic anhydride there (the effectiveness of this method is first checked by the Hull cell).
Addition of complexing agents that bind iron and copper ions into complexes, which are partially deposited on the bottom of the chromium plating tank, and partially leave them in the electrolyte in complexes that do not affect the course of the chromium plating process
The use of special membrane plants that allow you to clean the electrolyte.
To determine the correct solution to the problem, first of all, it is necessary to conduct electrolyte studies on the Hull cell.