Electrostripping of Nickel coating

[aspearin1]

I am investigating the option of using electrostripping to remove a nickel alloy coating. Right now, we use strictly a chemical strip process. My thinking is that if I can continuously strip and plate my nickel to a getter cathode, then I can prolong the life of my acid by keeping a low equilibrium of dissolved metal ions. Does anyone have experience with this? (Also known as reverse plating.) Thanks in advance.

ChemE, M.E. EIT
"The only constant in life is change." -Bruce Lee

 

[kenvlach]

I've done similar things a number of times, sometimes to clean up solutions by 'dummy plating,' sometimes for wastewater treatment. The electrical efficiency drops way low as the concentration decreases, so usually not practical ( unless you can concentrate from rinsewater via IX first). There is also a silicate product which can precipitate metals out of acidic solutions – if you're interested I can look up; they send me email every few months.

You can electrostrip (anodic dissolution) nickel from many substrates (aluminum, brass, copper, steel, zinc die castings) using 60 vol.% sulfuric acid at room T, with 6 V & lead* cathodes according to the 2005 Metal Finishing Guidebook. Several additives to reduce substrate pitting are given.
* I suggest graphite for environmental reasons.

For electroless nickel, a heated alkaline solution is listed, but I have never seen it used anywhere. Most commonly, nitric acid or proprietary stripping baths are used. Also note, you cannot plate out of nitric acid – it is too strong an oxidizer.

What are your alloy & substrate(s)?
What are you presently using for stripping?
Do you already have the Guidebook mentioned? If you're doing metal finishing, should qualify for a free subscription at

http://www.metalfinishing.com/

 

[kenvlach]

I should mention a practical dilemma is that the higher the acid concentration, the better it is for stripping & the worse for electrowinning the dissolved metal. Diluting the acid slows things down and can cause pitting (although there are anti-pitting additives). I guess, no such thing as a free lunch.

Also, check with these people:

http://www.phibrotech.com/

“Phibro-Tech is the most experienced –and the largest – hydrometallurgical waste recycler in North America.”
--- Don't know if they still do it, but at one time they had a nickel stripping solution that you could return to them for recycling.

 

[aspearin1]

kenvlach,
Thanks for the response and advice. My stripping solution is concentrated HCl; ~30%. My substrate is Hastelloy. I'm newer to this area I'm working, and what I'm witnessing is a degradation of stripping efficiency, followed by a dump and refill. I believe there are a number of control and cost savings opportunities for me to explore. Ideally, if electrostripping is successful, I will maintain a nominal level of metallic ions in solution, but not accumulate to a point of no return. I'd rather lose my acid to volatile emissions than metal saturation. There was some history here using chelaters, but it turned out that it was more expensive to process the concentrated sludge waste than it was to dispose of the spent acid. (This may have been a hexavalent chromium issue, though.)

ChemE, M.E. EIT
"The only constant in life is change." -Bruce Lee

 

[kenvlach]

HCl isn't often used for stripping – it pits most substrates and has nasty fumes.
On the plus side, Ni can be plated from HCl solution, at least from the Wood's Ni solution of 10-12 vol.% HCl (36 wt.%). Also, graphite electrodes work, at least in Wood's Ni solution.

I suggest a benchtop trial, under a fume hood. Get some graphite pieces off eBay. If you don't have a plating power supply, use a 12V battery charger.

If it doesn't work, well, the cost of sending spent Ni-containing solution to a metals recovery place is at an all-time low due to the high price of Ni.

 

[kenvlach]

aspearin1,
Let us know your results, good or bad.

The product which extends the life of acid stripping & pickling solutions almost indefinitely by precipitating out the metals as insoluble silicates is called
PRO • pHx Acid Life Extender.
Lots of customer results & documentation on their website:

http://www.pro-phx.com/index.htm

 

[aspearin1]

Thanks for the additional info. Right now, it's pie in the sky until I make a business case to get permission to even look into it. Lot's of red tape here. I'll make sure to post any results I get. Thanks again.

ChemE, M.E. EIT
"The only constant in life is change." -Bruce Lee

 

[MMATguy]

For a stripping solution I wouldn't recommend just HCl because as kenvlach stated HCl will pit a metal. Chlorides are some of the worst pitting ions. The reason is that they are relatively small and their diffusion is fast thus help keeping the pitted regions neutral. Also, the chlorides are great at removing the passive oxide films, which is necessary when stripping off Nickel. To strip nickel off I would use a 50/50 H2SO4/HCl mixture in reverse current. This way you will get oxide film removal but the pitting will be minimal.

MMATguy

 

[kenvlach]

aspearin1,
If you are dealing with one of the higher Cr (16-30%) Hastelloys C or G (& if plating out is a no-go), nitric acid might be preferable for stripping Ni. Use at full concentration (67-70 wt%) if you have an acid fume scrubber (seems likely since you're using hydrochloric) or diluted to 50 vol% if not. Concentrated HNO₃ should have at least 1.3x the dissolution capacity of concentrated HCl (30%). With nitric, add a little concentrated hydrogen peroxide to prevent NO_x fumes & extend the solution life.

Get any process change approved by Haynes Int.'s technical support. Maybe the best way to get your own company's approval.

http://www.haynesintl.com/

MMATguy,
Ni is relatively noble (i.e., non-passive) compared to Al, Cr, stainless and Ti. Please give some references for the use of HCl (chlorides) for stripping off Ni – I think maybe the Hastelloy substrate is a special case.