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topic 47124

Passive anodes in the acid copper plating bath

A discussion started in 2005 but continuing through 2020


I'm doing some experiments with copper plating basically for breadboarding circuits with plated holes and learning the PCB manufacturing process.

I got a fairly functional copper sulfate / sulfuric acid bath working from chemicals I got at the big orange home center and a constant current power supply. The solution is a basic copper sulfate / battery acid / HCl and uses a copper anode made from a piece of copper grounding rod.

I got really bad results until I added citrate ions to the solution, after which I got excellent bonding and an unoxidized (but not smooth and shiny) finish that I can polish fairly easily. Since I was trying to do this without ordering any specialty chemicals, the citrate ions came from calcium citrate tablets that I dissolved in distilled water and decanted into the bath, leaving the sand/talc/whatever behind. The solution should have 0.1% citrate ions by mole count. I figured the small amount of calcium would plate out, and the microcrystaline cellulose doesn't seem to be hurting anything.

But that's not my question, just what I'm working with. I read that for electronics work that I should be using phosphorized copper anodes. In one article at this site someone had trouble with his phosphorised copper anode in that it was turning non-conductive.

So being really cheap I used a copper phosphorus brazing rod as an anode just to see what would happen, and sure enough, it turned black and non-conductive in less than a minute. Probably because it's 7%(!) phosphorus. I'm quite sure that using a copper-phosphorus brazing rod is the wrong material and I may have screwed up the solution, but it led to some questions.

My questions are

1) what is the purpose of a phosphorized anode?
2) what the heck is the black stuff that seems to evaporate when exposed to the air?
3) what is meant by the "polarized anode" and by what physical mechanism does it arise?
4) ignoring the phosphorus fiasco, what is the appropriate level of citric acid?

Dennis Schmitz
SCHMITZ ENGINEERING, LLC. - Pinckney, Michigan



Beginners may find it hard to follow some of this dialog, so we've added some sidebar notes :-)

1. When electroplating, the metal which is plated out of the solution should ideally be automatically replaced by metal dissolving into the solution from the anodes.
In some plating situations (like nickel plating), if the anodes don't dissolve fast enough, the plating solution becomes "starved" of metal and the anodes oxidize, compounding the problem.
But in the case of acid copper plating the problem is usually the reverse: the anode dissolves too fast and creates a zone of copper ions around the anode, comprising an insulating film so that plating current can't flow. In either case we may say the anodes became 'passive'. In the case of nickel we may add sulfur to the anodes so they dissolve easier; in the case of copper we may add phosphorous so they dissolve slower.

2. 'Woods Nickel Strike' is often the first plating layer applied to stainless steel, and is designed to deal with the fact that it's hard to get good adhesion of plating onto stainless steel. The strike simultaneously dissolves the oxide on the stainless and lays down a thin layer of nickel.

3. A "Hull Cell" is a very small (usualy 267 ml) "test" plating tank; a sample of plating solution is put into it to try to determine whether the problem is with the plating solution vs. other steps in the process or features of the installation.

Q. Hello we have a problem with passive anodes in the copper acid bath.

We are copper plating on stainless steel.
Firstly we make Wood's Nickel strike, next we put copper layer in the copper acid bath.
During electroplating of small parts we don't have any problems; but in the case of 3X bigger parts, the current in a few minutes decreases and the copper anodes are covered with a black coating. We tried to solve this problem by using additional anodes and increasing the sulfuric acid H2SO4 but it doesn't work. Anode to cathode ratio is 1:1. The test in the Hull Cell goes correctly.
Composition of our copper plating bath:
Copper - Cu 60 g/l
Sulfuric acid - H2SO4 70 g/l
Temp 22 °C
Current at start 2 A/dm2
Mixed by air

Do you have any suggestions to correct this problem?

Waldemar Tomankiewicz
Plating shop employee - Cracow, Poland


A. Waldemar,
For general purpose acid copper plating solutions: CuSO4 190-250 g/l, H2SO4 150-225 g/l, Chloride ion 30-80 ppm. You did not mention the chloride concentration in the bath, I hope you have one. It is quite natural to gradually build up a dark film on the anodes as long as the sulfuric acid concentration is where it should be. Two other things to check if the solution parameters are in range: Make sure you are getting good contact on your anode bar and cable connections. Check current on both the cathode and anode bar(s) with the high area (cathode) load in the tank. The readings should match current readings on your rectifier. Good Luck!

Mark Baker
Process Engineer - Syracuse, New York


To prevent copper anodes from going passive in acid copper plating baths, two chemical factors are usually employed: a small amount of chloride ions should be in the solution as described by Mark; and the anodes should not be pure copper, but should be phosphorized copper as mentioned by Francisco.


A. Hello Waldemar

As Mark said, in fact you haven't mentioned the chloride content, and also you need to make sure that the anodes have a phosphorus content.

José Francisco Costa
- Aveiro - Portugal



Sometimes anodes are large slabs of copper; but more often they are golf-ball size or smaller chunks that are put into perforated titanium baskets. When anodes dissolve, particles can fall off; to prevent them from depositing on the work, the anodes or anode baskets are put into pillowcase-like anode bags.

A. Pull the anode bags, empty and clean the anodes of smut.
Then clean the baskets. Lastly, leach new bags and put the clean anodes in. Put them back on the anode bars. VERY easy problem. VERY easy cure. No chemical notions needed. Try it, you'll be thankful you did.

Rob Coffie
Warren, Michigan, USA


A. Dear Waldemar,
You have copper sulfate concentration too high. 60 g/L of copper in the bath means over 230 g/L of copper sulfate. Your bath cannot operate correctly at 22 °C because of thin layer of copper sulfate on the surface of anodes. The layer blocks current. You should reduce copper concentration or increase temperature up to 30 °C.
Best regards

- Poland


? Hello Waldemar,

which types of anodes are you using?


Dominik Michalek
- Mexico City, Mexico

September 20, 2008

A. Dear Waldemar,


DM = demineralized water

The formation of dye scale is a common practice in bright acid copper bath, due to its brighteners and additives. You can go for cleaning of your phosphorised copper anodes twice or thrice a week, I hope this may not be a problem, if you are using less quantities of anodes. And however, please maintain chloride content of your bath as prescribed and see that the DM water you are using should not contribute chloride ion to your acid copper bath.

Good Luck!

Shafiuddin A. Mohammed
Shafiuddin A. Mohammed
metal coating shop - Dubai, United Arab Emirates


Q. Dear all,
We are also facing the same problem as my friend. While the anodes are not in use, we find that the white paste like formation over phosphorized copper ball anodes and causing decreasing urrent density in spite of maintaining the required chemical concentration. We have taken out the anodes and washed and put it back into the tank. Now it is okay but we have to check whether the same problem will repeat.


- Chennai, TamilNadu, India



CD = current density.
"Dummy plate" means plating out onto waste cathode sheets at a low current density as a way of removing contaminants.

A. SG,
I cannot stress enough - maintain the chloride level in the bath per supplier's specs. If the bath will be idle for more than three days, pull the anode baskets and soak in 5% sulfuric until they are ready for use. Rinse well, dummy plate at low CD for an hour or so. The sulfuric holding tank will etch a slight amount of Cu, but not enough to cause any harm. Good Luck!

Mark Baker
Process Engineer - Syracuse, New York

January 16, 2013

Q. We are using acid copper plating bath for our printed wiring board copper plating process.

We are unable to maintain the copper sulfate concentration in plating bath and the concentration drops daily.
Control range :
CuSO4 = 9-12 oz/gal
H2SO4 = 24-26 oz/gal

After adjusting the CuSO4 concentration, it seems to drop to 8 oz/gal overnight.

There isn't any dilution on my bath.
There isn't leakage from the machine.

Ideally, the concentration should not drop as the process goes on daily.

Is this due to poor activation/anodizing of my copper anodes?
We've checked the copper anodes and they have a dark black film on the copper balls.

Jake Ng
- Los Angeles, California

January 20, 2013

A. Hi Jake,

You may verify chloride content (usually ~50 ppm) in acid copper plating bath which does involve in copper anode dissolution. Also, please check if the copper anode you're using contains phosphorous (~0.04-0.06 wt%) and any abnormal high rectifier voltage observed. Copper anode without phosphorous and/or abnormal high rectifier voltage may result poor anode dissolution.


David Shiu
David Shiu
- Singapore

January 21, 2013

Q. Hi David,

Thank you for your response.

My bath contains 80 ppm chloride.
And we are using copper balls with 0.04-0.06 wt%.

Meanwhile, when you mentioned the voltage to be high, what would be the normal operating voltage for the anodes?

What would be considered as high voltage?

Thank you.

Jake Ng [returning]
- Los Angeles, California

simultaneous January 23, 2013

A. Hi Jake,

Rectifier voltage depends on total resistance and current setting. We usually observe ~3V for 100-200 ampere setting. Imagine if anode basket is not tighten to contact bar well may results poor contact and high resistance to cause abnormal high voltage under same current setting and poor dissolution of copper anode.

Therefore, you should compare existing voltage (which you encounter low copper concentration issue) to last time voltage (e.g., during plating line setup time which you don't face such problem) to see if any abnormal voltage surge. Also, you should always do proper maintenance to ensure all contacts (e.g. wire cable to contact bar, contact bar to anode basket, etc) are in good condition.


David Shiu
David Shiu
- Singapore

January 23, 2013

A. First, a black film on the anodes in your bath is normal. Second, acid copper baths I've run pretty much stayed below 3V.
Finally, have you checked the ratio of the anode surface area to the cathode surface area?

James Totter
James Totter, CEF
- Tallahassee, Florida

February 2, 2013

Q. Hi David,

The voltage on my rectifiers are above 4V.
Is this considered high?

Meanwhile, there's only one contact point on my cathode bar to the rectifiers.
Will it be better to have both ends on the bar connected to the rectifier?

My plating current is only between 3 - 6 Amperes.
This is rather low compared to 100 A that you are doing right now.

Appreciate your comments and input.

Q. Hi James,

What do you suggest for the plating surface ratio between anode:cathode?
Is 1.5:1.0 good enough?
What will be the maximum ratio?


Jake NG [returning]
- Los Angeles, California

simultaneous February 6, 2013

A. Hi Jake,

4V is abnormal high (especially at such low current setting) which could cause poor anode dissolution. You should have both ends of contact bar connect to rectifier and ensure all contact points are properly cleaned and tightened.

For anode to cathode ratio, please try to increase to 2:1 to 4:1.


David Shiu
David Shiu
- Singapore
February 6, 2013

A. Try increasing the ratio to 2:1 anode to cathode, and the anodes should extend below the bottom of the cathode to some extent. 4V seems a little high, but increasing the anode area should bring that down some.

James Totter
James Totter, CEF
- Tallahassee, Florida

November 12, 2013

Q. Hello Sir/Madam,

bright acid copper anodes get gray instead of black and plating becoming very very slow. Rectifier voltage goes up while plating in ON ... normally it goes down (e.g., 1 volt goes 1.2 or 1.4). Before it was set to 1 V goes 0.9 or 0.8V

In my area no hull cell testing no lab for electroplating test. I did clean it was working fine mirror finish, but after few hours same problem.

Check attacked pictures. We use bags; they where normal from outside inside it was some black & gray. Using Phosphorized copper 0.04 to 0.08% anodes. Cu anodes are then put in Ti basket and then wear bags.

copper anodes turn grey-1 copper anodes turn grey-2
2nd picture was taken about 72 hours -- second time cleaning seems darker color than 1st picture.


Dipen Pattni
Dipen Pattni
jeweler/goldsmith - Dar-Es-Salaam, Tanzania

simultaneous November 15, 2013

A. Hello Dipen,
You didn't mention that you dummy plated after cleaning the anodes. This is very important before you start production plating cycles. I like to start at 2 ASF for 1 hr, increase to 4 ASF for another hour and then 10 ASF for the final hour. I have found this method builds a nice thin protective film (cupric oxide) on the anodes. Other possible causes are high chloride level in the bath, poor quality anodes, high anode current density, high CuSO4 or H2SO4 in the bath. Anode current density should not exceed 2.5 A/dm2. Anode to cathode ratio should be 1:1 - 2:1.

Mark Baker
Process Engineer - Mesa, Arizona, USA

November 17, 2013

A. Hi Dipen,

Have you added ~50ppm Cl in form of HCl or NaCl? Chloride is necessary to facilitate copper anode dissolution in acid copper plating.


David Shiu
David Shiu
- Singapore

December 11, 2013

thumbs up signDear Sir,
I did not reply until now because I had given Cu solution for test but seems they did mistake. Attached is analysis report.

I'm planning to do zinc dust, carbon treatment and then add 1st brightener and start working.

copper solution analysis report-3


Dipen Pattni
Dipen Pattni
jeweler/goldsmith - Dar-Es-Salaam, Tanzania

Ed. note: We cropped out the name of the test lab because they understandably wouldn''t want their name on copies they do not have control over or which could be altered.

January 1, 2014

Q. Hi,

Got new report from India for same Acid copper solution says:-

Copper Sulphate /229.59 (gm/lt)
Sulphuric Acid/ 79.35 (ml/lt)
Chloride / (49.63 ppm)
Copper metal 58.42 (gm/lt)

Advised to make 30% dilution ... add 3 ml/lt hydrochloric acid (32 ml/lt), carbon treatment ... add 1st & 2nd brightener.


Dipen Pattni
Dipen Pattni
jeweler/goldsmith - Dar-Es-Salaam, Tanzania

Current problem with acid copper

October 25, 2018

Q. Lately I have been having a problem with my acid copper. After I put in the pieces to copper plate, the amps go down to zero. It is like it loses contact. It resists for about five minutes then slowly lowers until zero. All contacts are clean but I noticed that the anodes blacken quickly.

John Perilli
Employee - Italy

October 2018

A. Hi John. It certainly sounds like your anodes have gone passive. What type of copper are they, and are you monitoring your chloride level?


pic of Ted Mooney
Ted Mooney, P.E. RET - Pine Beach, New Jersey
Aloha -- an idea worth spreading

October 31, 2018


popat patel
Popat Patel
finishing shop - Roseville, Michigan

October 31, 2018

Q. Sorry to ask another question, but I would like to know how to check chloride content and what it is. I have little experience in acid copper plating.

John Perilli [returning]
- Rome, Italy

November 2018

A. Hi John. There is an excellent discussion of chloride analysis in acid copper plating baths on thread 0885.

If you have little experience in acid copper plating, the digital version of the Metal Finishing Guidebook [ed. note: sorry, that book is no longer available on line] provides a good introduction, and there are specialized texts available such as "Electroplating and Electroless Plating of Copper and Its Alloys" [affil. link to book info on Amazon]. For more info about analysis of plating baths, Langford and Parker, "Analysis of Electroplating and Related Solutions" [paid link to info about book on AbeBooks] is the "go-to" reference. Best of luck.


pic of Ted Mooney
Ted Mooney, P.E. RET - Pine Beach, New Jersey
Aloha -- an idea worth spreading

November 1, 2018

thumbs up sign Thank you for your reply!

John Perilli [returning]
- Rome, Italy

July 2, 2020 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. Dear all experts,

I am performing copper plating on stainless steel wire with usual acid based copper plating solution with copper anodes in the basket. What I have observed yesterday that the copper anodes, part which are immersed in the electrolyte solution, is turning into black. It seems that it is a black film of something, unknown, is being deposited. That film can be removed by water rinsing or by rubbing on the surface. Does anybody has any idea what is that film and why it is being built? The concentration of H2SO4 is fine. Chlorine content are not measured yet.

Kind Regards,

Ridwaan Ali
- Karachi, Pakistan

July 6, 2020 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. My Copper sulphate content is continuously decreasing in acid copper bath. I have also monitored the drain valve, there is no leakage or wastage of the solution. Kindly suggest me a solution to overcome this issue.

Annie Tahir
- Lahore, Pakistan

July 2020

A. Hello Annie. You are electroplating copper out of the solution -- it is going onto the parts. But what is supposed to be happening is that the copper anodes should be dissolving into the solution in proper proportion to keep the copper concentration stable. It seems that your anodes have gone passive. Please study this thread and threads 31751 and 44771.

Luck & Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET - Pine Beach, New Jersey
Aloha -- an idea worth spreading

September 14, 2020

thumbs up sign Hello there,

We have the same problem with black film on the copper anodes. I just got the chemical analysis result which says that:
Cu = 53 g/L
H2SO4 = 128 g/L
Chloride = 48 mg/L.

It seems that the sulfuric acid amount is more than double of the suggested amount (60 g/L). I am going to take out some amount the copper bath and will dilute the rest with water to reduce the sulfuric acid value.
I will let you know guys.
Thanks for the wonderful thread on this issue.

Kind Regards

Ridwaan Ali [returning]
- Karachi, Pakistan

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