Letter 22346

Permalloy plating bath stabilization  

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I am trying to plate permalloy (80/20 Ni/Fe) on a small copper substrate. The plating bath is a mixture of nickel and iron(II) sulfates, NiCl, boric acid, saccharin, and SLS wetting agent. The problem that I have is that FeSO4 turns from green-blue to yellow-brown as soon as it comes in contact with water. It looks like rust floating in water. I use DI water boiled to make sure it has no dissolved oxygen, etc. This process starts immediately after dissolution of FeSO4 and continues for days, filtering does not get rid of all the particles. Is there any way to sot this without dramatical changes to the plating chemistry?

Konstantin Glukh
- Research Triangle Park, NC, United States

Iron must be complexed to keep it in the ferrous state. Once oxidized (by air or anodic current) ferric iron forms ferric hydroxide a red-brown precipitate.

Make up the plating solution without Iron, add sodium gluconate as the complexing agent, then add ascorbic acid (or its isomer) ascorbic acid will help prevent air oxidation and will help keep the iron from oxidizing at the anode. Use a large anode area, of both nickel and iron. Contact a supplier of bright nickel iron plating solutions for proprietary sodium gluconate mixtures desighed for nickel-iron plating, and for suitable anodes.

Don Baudrand Don Baudrand, Consultant   Poulsbo, Washington

Three items to look at without major changes in chemistry:

1. Make sure the bath pH is kept below 3.0. Oxidation of ferrous ion is much faster at higher pH.
2. Make sure you use a dissolving nickel anode in good working condition and of an area at least as large as the cathode - any anodic curent not used for dissolving nickel generates oxygen bubbles, which oxidize ferrous ions.
3. Check your source of ferrous sulfate. There should be very little yellow-brown stuff , if any, visible in it. Crystals should be clear and light green. Plenty of opaque, dehydrated crystals usually come with elevated ferric ion content. A fresh bath should be clear. (This will not prevent oxidation as the bath ages, but at least you'll start from a lower level.)

Emanuel Cooper
- Yorktown Heights, NY

I used a Pt anode in a permalloy plating, it seems cause some problem of Ferric ions after short time of plating. I hesitate to use a nickel/iron anode because I want to avoid the chloride salts in the bath, which might be need to keep the anode from passivation, but also degrade the magnetic property of the permalloy. Anyone has experience on this and comments is appreciated.

Q Huang
- Baton Rouge, LA

I've had the same problem of rust formation for a while, until I started bubbling dry nitrogen in the bath. The problem, as confirmed by F*A*C*T, is that the Fe2+ tends to transform into Fe3+ in the presense of dissolved O2 in the solution.

And after that, we all know what'll happen. But ever since I started bubbling N2, the life of my plating bath is virtually quite stable, as the N2 drives the dissolved O2 from the electrolyte. Hope this helps. :o)

Cedric Cheung
University - Toronto, Ontario, Canada

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