Electrostripping of Nickel coating
Electrostripping of Nickel coating
(OP)
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





RE: Electrostripping of Nickel coating
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/
RE: Electrostripping of Nickel coating
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.
RE: Electrostripping of Nickel coating
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
RE: Electrostripping of Nickel coating
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.
RE: Electrostripping of Nickel coating
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
RE: Electrostripping of Nickel coating
ChemE, M.E. EIT
"The only constant in life is change." -Bruce Lee
RE: Electrostripping of Nickel coating
MMATguy
RE: Electrostripping of Nickel coating
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 NOx 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.