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Chrome Stripping: is NaOH or HCl better?


A discussion started in 2004 and continuing through 2017 . . .

(2004)

Q. I'd like to know the pros and cons of both NaOH and HCl chrome stripping. Someone told me that Anodic NaOH stripping is prone to base metal corrosion. Is this correct? Also, which is lower in waste management costs? I appreciate responses from experienced chrome platers.

Syed Salim
plating shop - Kuala Lumpur, Selangor, Malaysia


(2004)

A. I'm not an experienced chrome plater, but I think you've got it somewhat backwards. HCl can corrode steel, and almost anything else, whereas alkalies are essentially harmless to steel.

Anodic stripping generates hexavalent chromium, which is more difficult to treat, whereas stripping in HCl doesn't (see letter 12044 )

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


(2004)

Q. Ted, Thanks for your input. You mentioned NaOH does not attack the base metal. But under anodic current, doesn't base metal react with -OH ions to form hydroxides or metal chromates just like chrome, which then dissolve into solution? How would the base metal, say steel, react differently from chromium when faced with -OH ions?

Syed Salim [returning]
plating shop - Kuala Lumpur, Selangor, Malaysia


(2004)

A. The short answer is that steel doesn't do that (go into solution) in alkaline solutions, as we can see from the millions of steel anodes and steel anode baskets that have been used in electrocleaning tanks and alkaline plating tanks for decades.

The slightly longer answer is that the interface between an alkaline solution and a chrome anode raises the chrome oxidation state to +6, where it is then soluble in solution, whereas the interface between an alkaline solution and a steel anode liberates nascent oxygen gas instead.

Now, just why the electrochemical potentials make this the case is something you'll have to wait on one of the electrochemists to answer. I could probably find the answer with some research, but it's not on the tip of my tongue right now :-)

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


(2004)

A. Syed and Ted,

For a job shop a HCl with a wetter or inhibitor is best in my opinion; if you get any leaded material NaOH with current will attack parts a lot quicker.

Chris Snyder
plater - Charlotte, North Carolina


(2004)

A. In my opinion, NaOH is better for stripping chromium than HCl. The reason being that NaOH will not attack as many metals as HCl and will not attack steel. The downside is that NaOH needs anodic polarisation and a slightly elevated temperature, whilst HCl will attack most metals at little over room temperature and no extra power.

trevor crichton
Trevor Crichton
R&D practical scientist
Chesham, Bucks, UK



(2007)

Q. Hi everyone,

Thanks for the information on chrome stripping. A great deal of it was Greek to me as I am in the beginning stages of investigating a cost effective way to strip motorcycle parts for personal restoration purposes (antique Harley-Davidsons to be specific).

For the past 9 or so years I have been sandblasting most of my parts (chain guards, oil tanks, visors, etc.) outdoors, but would love to bring the operation inside and speed up the process.

Would anyone out there have advice on solution-types, effectiveness with and without electricity and differentiating between steel and aluminum?

Thanks in advance for your time and consideration...Carole

Carole Del Vecchio
- Enfield, Connecticut in the good 'ol US of A

(2007)

A. The biggest issue, Carole, is that we are talking about stripping chrome here whereas the plating on your Harley is nickel-chrome: a heavy layer of nickel followed by a thin layer of chrome.

So, you take the chrome off with HCl (because you certainly don't want to generate hexavalent chromium in your neighborhood, which electrolytic stripping with NaOH will do). But then you still have to strip the nickel with another chemical (either proprietary or very dangerous cyanide). And there may be copper plating under that too. The best plan is, as Chris suggested, taking it to a plating shop. Second best may be blasting the plating off of steel as you have been doing.

Aluminum is much lighter than steel, and it is non-magnetic whereas steel is magnetic. Good luck.

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



November 14, 2011

Q. I understand that a product called Cuflex Dechrome-Ac, which is purportedly a buffered Hydrochloric Acid can safely strip chrome without damaging the underlying metal (caution is still paramount), but I have been unsuccessful in locating this products in quantities less than 55 gal.

Bill Rinz
- Croydon, Pennsylvania, U.S.

----
Ed. note: CuFlex is an Atotech trademark, so this process is available only from them or their distributors. But other major plating process suppliers offer inhibited HCl optimized for stripping chrome as well, under different trade names.



September 26, 2013

Q. We are seriously contemplating setting up an NaOH stripping bath in our hard chrome shop. Could anyone inform us where to find the best information on solution and equipment. We already have tanks and rectifiers. Thanks.

Mark May
plating shop - La Vergne, Tennessee


September 27, 2013

A. Hi Mark. The equipment will look just like an electrocleaning tank. As for the best operational parameters, both Guffie's "Hard Chromium Plating" [link is to info about book at Amazon] and Weiner & Walmsley's "Chromium Plating" [link is to info about book at Amazon] books have a little to say. Good luck.

Regards,

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



June 25, 2014 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. As a small hard chrome plater, I run a closed loop line which generates very little, if any, hazardous waste on the plating side, but I incur anywhere between 10 and 24 gallons of hazardous rinse water and spent acid from our stripping process (monthly) which we barrel and pay to have treated. Yes, we are a very small plater! Is there an advantage for me to be treating (permitted) this limited waste stream on site plus any other small chrome waste I may generate? At present, we don't have any waste treatment capabilities. Also, what is the difference, if any, between the hazardous chrome waste produced by acid stripping compared to electrolytic stripping in a caustic bath? Thanks!

Jeff Hanaway
General Manager - Meadville, Pennsylvania


A. Hi Jeff. We appended your question to a thread which answers it. Stripping chrome in acid generates trivalent chromium, which can be precipitated by pH adjustment. Alkaline electrolytic stripping generates hexavalent chrome which must be "reduced" before precipitation is possible. Good luck,

Regards,

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



Intricate chrome stripping problem

March 24, 2015

Q. Hi Guys. My company is having a nightmare with a job we are currently processing. We have chrome plated cylinder approx. 155 mm in length by 38 mm diameter. Thickness of deposit is approx. 0.15 mm. There is a 9 mm hole at one end of the cylinder. We need to strip a circle around the hole. This circle needs to be 10 mm - 12 mm dia.

29778

Metal tape will be attacked by the stripper, and plastic tape tends to mis shape. We are worried about the stripper attacking the chromed and ground shaft outside of the circle area. Any advice will be appreciated. Thanks. Shaun

Shaun McDade
- UK


March 26, 2015

A. Hindsight, you would have been far better off to have masked the area before plate.
It is a fairly heavy coat, so I think that whatever stripper will undercut the masking.
If it were my problem, I would grind it with lots of fluid and a proper stone so that it does not burn or flake. something like a drill press might work.
If you have to strip it, you might consider using brush techniques to electrolytically strip it. Still an unpleasant masking chore, but a bit more selective than an immersion stripper.

James Watts
- Navarre, Florida


June 29, 2015

A. As earlier suggested, it would be fair to pre drill / bore and then grind the surface; if this is not possible, then wire cutting or chemical machining is alternate. Best luck.

Mahendra gargatti
- Belgium Karnataka India



May 24, 2016

Q. Greetings. I need to strip off a chrome bore plating from a rifle barrel. Not the whole bore, just the chamber area. This is in preparation for using a chamber reamer made of high speed steel HSS. I have driven a hard plastic (nylon?) plug down the bore to seal the area needing treatment from the rest of the bore. I have access to both Hydrochloric acid and Sodium Hydroxide. So which should I use? I will hold the barrel vertically, and pour the treatment solution in. Will the NaOH eat the chrome at room temperature? The area needing stripping is eroded, and the chrome is in islands now, rather than being continuous, as it was when the bbl was new. Many thanks. Chris.

Chris Gray
- Florence, Oregon, USA


May 25, 2016

A. Hi Chris,
Hydrochloric acid will strip chrome, but it works best when there is brass underneath as it tends not to attack that once the chrome has gone.
Your gun barrel will be made of steel, the chrome will not be removed evenly over the whole surface. Some areas will become bare before others, so the acid will carry on etching the base metal. So that has to be considered. Also hydrogen will be impregnated into the metal making it brittle, something you don't want in a gun, unless you de-embrittle the gun afterwards.
If you wish to use sodium hydroxide instead, then you will need a power supply to provide a DC current for etching, I.E. the gun will be anodic and the chrome etched off electrically in the sodium hydroxide solution that has to be hot.
Sodium hydroxide alone will not do anything.
Is hot, strong caustic soda and DC anodic currents something that you are OK using?
If not, employ a plating shop to do it for you.
Good look
Mark

Mark Lees
aerospace - Isle of Man, Great Britain


May 25, 2016

Q. Thanks very much Mark. Sounds like Caustic Soda is the way to go. Will a 12VDC car bty charger provide the current I need? Is the anode the (-) side, and cathode the (+)? One side will be on the bbl, and the other will be a copper wire suspended in the NaOH hot solution? How hot does my NaOH solution need to be? And should I use a saturated solution? Thanks again from across the pond. Chris.

Chris Gray [returning]
- Florence, Oregon, USA


May 27, 2016

A. Hi Chris, the Gun needs to be positively charged for it to be anodically etched. We strip chrome in a 100 g/l sodium hydroxide solution at ~60 °C and 5 volts. The solution will go yellow, this is Chrome VI, so you might want to consider safe disposal of that, as well as the neutralisation of the caustic after you are done.
Cheers
Mark

Mark Lees
Aerospace - Isle of Man Great Britain


October 24, 2016

Q. Hello Mark
Have you ever seen black surface on steel after Cr strip with pure Sodium Hydroxide? I've observed it few times after overnight strip with NaOH (~50 °C, 60-70 g/L). Thank you.

Leon Gusak
- Winnipeg, Canada



October 26, 2016

Q. I have a strip of ceramic that has gold plating over chromium on it. I have to strip the gold and chromium for a customer without breaking the 0.100" thick ceramic strip. This strip is very fragile. Therefore, it cannot be sand blasted or even reverse current stripped. The gold was easy to strip. I already stripped the gold using Techni-Strip Au. My CRC handbook of Chemistry says that chromium is soluble in dilute hydrochloric and dilute sulfuric acids. I have tried 25% HCl at 140 °F for an hour and 20% Sulfuric acid at 140 °F for an hour. Neither solution has done anything to the chromium. Any suggestions?

Bruce Havens
Engineer - Tempe, Arizona, USA


October 27, 2016

A. Good day Bruce.
I really think that layer is not chrome. Your methods are indeed viable for chrome, but the results show otherwise.
Usually the underlying layer under gold is nickel. Maybe try a nickel stripper either sulfuric or cyanide based. I would be concerned about the ceramic substrate, if it will show signs of attack.
Maybe try 50% nitric, and check for attack.
Chrome is very passive, it will not accept any electrolytical deposition of metal.
I have plated over chrome, and it is very difficult.
Hope this helps.

Regards,

Eric Bogner, Lab. Tech
Aerotek Mfg. Ltd. - Whitby, Ontario, Canada


October 27, 2016

thumbs up signOur customer got back with us and said that the metallic coating is probably not chromium after all. That explains why the HCl did nothing. The coating may be Ti/W. Customer said not to try to strip it anymore. Just send the parts back.

Bruce Havens [returning]
- Tempe, Arizona

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