Removing chloride from Chrome plating solution

A. Hi Bek. In electroplating, the word "dummy" just means a dummy load as opposed to a real load.

To avoid confusing people who are familiar with dummy plating in other applications, we should start by noting that in most plating operations other than Chrome Plating "dummies" are used at low current density to plate out contaminating metals. For example, if a nickel plating tank is contaminated with copper, plating onto a scrap piece of corrugated sheet metal at low current density will plate out a lot of copper but waste only a little nickel because copper plates preferentially at low current density.

In the case of chrome plating however, things are different. Chloride is not a metal that can actually plate out. Rather, dummies are used at very high current density to release the chloride as chlorine gas. I've never personally done this or even seen it, but we appended your question to a thread where it is discussed.

Luck & Regards,

A. The best method that I have found is to do a draw off the bath.

Some people add silver oxide, but this should be done in a separate tank to ensure that the precipitated silver chloride is completely filtered out during the transfer back to the plating tank.

I have heard of people dummy plating the chloride out of the tank, but in an aqueous environment the chloride reacts with the water to form hypochlorous acid. I have not actually tried dummy plating to remove chloride, though.

One could probably remove the chloride with ion exchange resins, but that would probably remove the catalyst also.

A. Dummying is a good solution to chloride contamination. I do not remember the exact conditions, but I think it is done with a small cathode and high current density. I know it works. For more accurate conditions, leave a message for Jim Watts, he knows the answer for sure.

There is another possibility, to add barium carbonate. The chloride acts as a catalyst just like sulphate (actually a stronger one then sulphate) so by reducing the sulphate, you actually reduce the effect of the chloride. The chloride will decrease with time and then you will have to increase the sulphate again. It works!

A. To remove chloride from a chromium plating solution, you can add silver oxide or silver carbonate. Both methods are expensive due to the cost of silver salts. Use stoichiometric amounts

Electrolytic dummying of the solution is very effective, especially if the contamination is high. There are a variety of recommended electrolytic procedures and one that I am familiar with (and it works) is to Electrolyze the solution using an anode to cathode ratio of 10:1 while maintaining an anode current density of 25 ASF.

Trust this helps.

A. Interesting problem! It sounds like your chromium solution may not have been a brand new make up. This supposition is based on the fact that you had high chloride contamination.

If your solution came from another plating facility, knowing its history would be very helpful.

For example, if it had been an SRHS chromium (self regulating, high speed), the solution could be over suppressed. SRHS chromium solutions contained ingredients that helped control the sulphate and secondary catalyst if they started to get too high. These special ingredients would actually cause the sulphate to precipitate if it got out of range. If the sulphate got too low, it would allow the sulphate to come back into solution -- thus the "SR" part of the name.

If however excessive amounts of these special ingredients got into the solution, the sulphate content of the solution would be suppressed to a level below where acceptable work could be plated. To correct the condition, we would have to keep adding sulphate until we had consumed the excess special goodie.

This is one of a few scenarios that could be causing your problem. The important point is that if your solution came from another shop, you should investigate it's history, as it may point to the answer.

If its a new make up, that presents a more interesting problem.

Regards

A. Hi Dean. Since HEEF-25 is a proprietary product from Atotech, and nobody else admits to knowing exactly what is in it, I think that as the starting point should be to tell us what the supplier suggests. Thanks and good luck.

A. It is Silver Oxide, not silver carbonate to reduce or eliminate chlorides. The upper limit is 50 ppm chlorides in a bath when it may begin to interfere with the sulphate/chrome ratio. 150 GRAMS silver oxide added to 1,000 gals. chrome bath reduces chloride 20-30 ppm. Hope you are sitting down when you obtain the price for silver oxide. No it should not interfere with the HEEF process.

A. Hi Dean, to clean your electrolyte from chlorides, it is much cheaper and easier to work it out by dummy-plating. Putting silver-ions into the electrolyte precipitates AgCl AND AgCrO4. You get a lot of sludge and it is a very expensive treatment.

Best regards,

A. Dear Mr. VIKRAM,

For this kind of dummy plating you take a steel sheet or some steel rods and plate them with high current densities at your normal plating temperature for some hours. Then you work the chlorides out. It is not fast, but it works. You should control your effort by some chloride analysis.

Best regards,

A. I agree with Michael, dummy plating is the right treatment for chloride contamination.The treatment with silver is very expensive. As for the silver reaction, well it is true that you get Ag2CR2O7 at the beginning but after a while all the chlorides will precipitate and give AgCl before any silver ion precipitates the chromates. Just as with Barium carbonate addition, at the beginning Ba2Cr2O7 is formed as there are a lot of chromate ion and much less sulphate but it will shift towards the sulphate and create BaSO4.

A. Hi,

I'm agreed with both Sara and Michael how to remove chloride. The cathode area must be lower than anode area. Also check the temperature because this will be very high if you not have a automatic cooling system.

Regards,

A. Dear Selvaraj,

Removal of chloride requires a very high over voltage. Easy way to achieve this is swap the anode/cathode connections and place a former round Pb/Sn at the former cathode point. About 4 A/Sq in will exponentially reduce the chloride by about 20 ppm in 1st 2 hours.

Good Luck,

Ken

A. Silver salts are sometimes used to overcome chloride contamination but not silver nitrate as the nitrate can cause the chromium deposit to turn dark.

A. Chloride contamination will lead to hazy, milky, or even no deposit. You can add in silver carbonate to precipitate out chloride as silver chloride while carbonate will turn into carbon dioxide and water.

A. I think dummying at 1,000 ASF will remove chloride as chlorine gas at approximately 160 °F. Try it with good ventilation; the gas is very nasty. You can also precipitate it out with silver oxide. High chloride can cause dull or hazy deposits in high current density areas among other things.

Good Luck

A. Use small cathode surface area (4:1 anode to cathode ratio works well) heat solution to 150 °F dummy between 300-800 ASF; use light air agitation to help circulate solution. 4-6 hours will usually reduce chlorides by 10-20 ppm.

A. ion chromatography

A. If you are just checking to make sure that you don't have too much chloride as a contaminant in your chrome solution, I think the easiest method is a titration; but you don't need an electrode- just a good eye and some good clean glassware. I'm feeling generous, so I'll just give you a method that I have used for years.

Pipette [pipettes on eBay or Amazon] 10 mL bath sample and 40 mL of 25% nitric acid into two 125 mL Erlhenmeyer flasks [125 mL flasks on eBay or Amazon] . Set one aside as a turbidity reference. To the other add a few drops of 0.1 N silver nitrate. Carefully titrate this solution (in fine, precise increments) with standard 0.010 N mercuric nitrate from turbid to clear. Add a few more drops of silver nitrate, if the turbidity returns, continue the titration back to clear. Repeat that step until the solution no longer turns turbid. Chloride ppm = 35.6 x mL of 0.10 N mercuric nitrate.

A. Use RO or DI for make up and for water additions and you will eliminate the chloride problem as trace amounts for chlorine gas at the anodes.
This assumes that you are not using HCl to activate the metal.

A. Hi Elham. I think a small sheet would be good. Its surface area should be 1/4 of the surface area of your anodes according to J. Ott's posting. Dubpernell's "Electrodeposition of Chromium from Chromic Acid Solutions".
notes that the lead dioxide on the anodes is what converts the chloride to chlorine gas ... so perhaps the shape of the cathode isn't critical? Dubpernell also implies that you can't get all of the chloride with dummying, and may still want to precipitate the final small amount with a silver salt.

Regards,

A. Hi again, Elham. I've never operated a chrome plating line -- all I know is what I read in the papers :-)

Sara Michaeli says other salts are preferable to silver nitrate; David Shiu suggests silver carbonate; Trent Kaufman suggests silver oxide. And, yes, Michael Hekli says silver chromate is also precipitated. Good luck.

Regards,

Hi ted. Thanks for guidance.

Hi THYAGARAJ. We appended your inquiry to a thread which covers the subject exhaustively over a period of more than 17 years. I'm sure it will answer your general questions -- but get back to us with any remaining thoughts. Good luck.

Regards,

In my opinion, heat up the solution to around 100 °C for an hour and well agitate; will decrease chlorides as chlorine gas with dummy plating sheet.

A. I'd suggest it would have to come from the final rinse waters.
Make final rinse in DI water.

A. Hi Dinu,

chloride stain on chrome plated surface, hydrochloric acid or chloride related chemical drain into your chrome tank, from the broken rack, or some other reason
1) remove chloride from chrome, dummy bath or decant solution then readjust chemistry
2) silver nitrate take your chloride out and filter out, this method is expensive.

A. Hi,

I think it comes from how you store the part.

Best regards

A. Dear FRANCIS,
If your lab report is right you are in deep soup.
CHROMIC ACID around 250 gm/lt.
sulphate 2.5 gm/lt
Density 1.180 ( In your tank 2 density cannot be 1.094 that too in gm/lt. if chromic acid is 273).
All impurities like iron, copper well below 2-3 gm/lt.

Once you achieve this call the ATOTECH rep to do dosing for you.

Hope this helps.

A. Nakul,

Try a Silver Sulphide treatment and decant the tank.

Please try a small portion of 5 litres first, if it doesn't work please make up new solution.