Nickel plating salt spray failure

A. Since the owner is telling you what to do, tell him that you will be happy to do whatever it is that he wants, but the results are his responsibility.
Case in point: I want to drive a race car, but can not afford a competitive engine. Result, I will never win a race.
The dichromate step is doing nothing for the nickel or the copper. It is a poor man's rust preventative for the steel that disappears in the salt spray test.
You do not have enough thickness to NOT have tiny pinholes in the plate.
There is an outside chance that pulse plating might help, but that equipment costs money. There is a really outside chance that ultrasonics might help, but that also costs money.
Finally, talk to your nickel vendor. There is a remote possibility that adjusting the macro or micro throw additive might reduce the problem, but I suspect that it will increase the brightness of the nickel.

James Watts
- Navarre, Florida
April 27, 2009

A. DEAR KUSHWAHA,
Alone nickel can't give you the corrosion resistance.You should apply a top coat either of chrome or some stoving lacquer.

Gulshan sachdeva
- Moradabad, Uttar Pradesh, India
April 28, 2009

A. Hi. I agree with James. Nickel is a "barrier" layer plating rather than a "cathodic protection" plating like zinc or cadmium. Any porosity or pinholes forces the steel to corrode as it attempts to sacrificially protect the nickel plating. Coating a "porous look" substrate with a single 10 micron layer of nickel sounds unfit for much because, as you've learned, it is offering no corrosion protection at all.

The fact that 20 microns survives 24 hours, whereas 10 or 15 microns doesn't, is evidence that 20 microns provides at least a little corrosion protection. Best of luck.

Regards,

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
September 2013

A. Hi Geetha. If the parts have a good smooth finish such that 10 microns will provide freedom from porosity, then 24 hours sounds reasonable, maybe 48 hours.

We appended your inquiry to a thread where a reader was unable to get 24 hours due perhaps to the roughness of his substate. Again, nickel plating on steel is a barrier layer plating that is worse than worthless if it's too thin to be pore-free and pinhole-free.

Regards,

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
May 2014

A. Hi Arvind. Just test it until it fails (although how will you judge failure -- the first appearance of green corrosion products?). 8 to 10 microns isn't much and it won't take long.

Then if the plating is delivering satisfactory life for your purposes, that's the salt spray hours you can test to.

If you feel that I misunderstood the question, please double check your assumptions and premises ... because salt spray testing cannot be used to predict real-life except within very closely watched interpolations based on a wealth of very similar experiences. Best of luck.

Regards,

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
June 2014

June 2016

A. Hi Lynda. To get the explanations out of the way first ...

1. We've appended your letter to a thread which tries to make the point that such questions are usually based on a bit of a misunderstanding of salt spray testing. Ideally, the design sequence would be:
a. Ascertain the anticipated corrosion situation (mild, moderate, severe, very severe), and then choose a substrate preparation and plating sequence likely to be appropriate for such exposure condition.
b. Do real-world testing to verify that the corrosion resistance is satisfactory.
c. If real-world performance is satisfactory, run a salt spray test on the parts to determine how many hours of salt spray resistance your process tends to deliver.
d. Periodically run the salt spray test as you produce parts; if the salt spray hours decline, take this as an early warning that something has changed and may need attention.

2. Another thing to realize is that copper-nickel plating is a "barrier layer" plating. Copper and nickel are cathodic to steel, not anodic like zinc. If the steel underlying zinc plating is exposed though a pinhole or porosity, the zinc preferentially corrodes to protect the steel (similar to how zinc anodes protect steel boats). But if the steel underlying copper-nickel plating is exposed, the steel becomes the sacrificial anode, and pinhole rust is accelerated rather than retarded as the steel tries to prevent the copper and nickel from corroding. Copper-nickel plating protects steel from rusting only if it is porosity-free and pinhole-free.

3. Thirdly, adequate preparation and adequate plating thickness are not independent. A thin plating on a very smooth surface may be porosity-free & pinhole-free and prove adequate ... whereas the same thickness on a rougher surface can be inadequate.

Taking all of this into account, the salt spray resistance of your plating thickness on hardware (which is probably not polished and buffed, and which is probably banged about during barrel plating) is probably very close to zero hours. Manoj achieves 24 hours with 20 microns, and you can probably find other examples if you search the site. Good luck.

Regards,

Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

P.S. I'm not sure if "micros" is supposed to mean microns, or microinches or something else. In private mail you listed the thicknesses as .00381 and .00254, but I'm certain that you are not applying .00381 inches of nickel, so I think you mean 3.81 microns.

A. Regarding Salt spray and ENP coating the problem is it is bright nickel (Low phos) and too thin even at 50 µm it may do 90-100 hours salt spray.

Another thing as already mentioned would be the porosity of the base substrate under 20 µm coating thickness must not be creating a good enough barrier to seal the porosity.

Try using high phosphorous ENP as this offer far superior corrosion protection to low or medium phos. If done correct high phos ENP should do over 100 hours salt spray above 15 µm if treated correctly.

Paul Rennison
- Leeds United Kingdom
August 24, 2016