How to prevent electroless nickel coating from flaking after forming?

[bugeaterfan]

We have a hollow cylindrical part made from electroless nickel plated carbon steel that gets formed at one end over another part at about a 45 degree angle. The EN plating sometimes flakes off in the formed area during this operation. The base material is 1018 steel and the plating is done per MIL-C-26074, class 1. The plating is less prone to flake when the thickness is .0005"-.001" compared to .0015"-.002", but the spec doesn't require the plating to pass the corrosion test with the thinner plating. We recently changed to a high phosphorus plating, but this didn't seem to make much of a difference with a plating thickness of .0015"-.002". Any suggestions?

 

[israelkk]

MIL-C-26074E requires:

4.5.2 Adhesion test
The test specimen shall be bent 180° over a mandrel
diameter 4 times the thickness (0.4 inch minimum) of the specimen.

The EN coating should not flake after the adhesion test. I suspect that your forming procedure is more severe on the EN and the base metal than just the required adhesion test.

 

[israelkk]

Just wanted to add that in the adhesion test the base metal and the EN are basically stretched, while when you close your hollow cylindrical part over the other part the EN coating is compressed and may be even "buckles". Therefore, there is a greater tendency to flake from the base metal.

 

[IRstuff]

Is the nickel "shiny?" We used to have a sub that LOVED "shiny" electroless nickel. The problem was that shiny nickel turns out to have A LOT of stress, causing our substrate to actually warp. This might explain why the thinner plate has less of an issue.

Another issue, perhaps, is that the nickel just isn't adhering properly and there may need to be some sort of intermediate layer that sticks to the nickel better, but also sticks to the substrate better.

TTFN

FAQ731-376

 

[bugeaterfan]

We have noticed that parts with a dull finish form better than those with a shiny finish. We concluded that the appearance difference was due to differences in thickness.

 

[IRstuff]

I'm not sure what process to change to make it not shiny, but the shiny stuff we had could potato chip our mirrors pretty easily.

TTFN

FAQ731-376

 

[Riadsala]

Try heat treating it at around 600 deg.C for 8-10 hrs(creates an interdiffusion zone between the nickel and the substrate which greatly improves the adhesion). Of course you will then be out with the mil-spec for the coating... Heating and cooling should be controlled so it is not too quick.

The ENP coating ends up with a green oxide finish if not in a controlled atmosphere but which can easily be removed with a mildly abrasive pad (scotchbrite or such like).

Check the plating rate also - if it is too fast it is more likely to have adhesion problems.

Riadsala
Metallurgist in the Offshore O&G industry

 

[bugeaterfan]

Will the heat treat process you describe result in a harder coating? Heat treating is the method described in the spec to achieve a class 2 (harder) coating, which I don't want.

 

[Riadsala]

It can be high (>600 HV0.3) after HT. If that's a problem then maybe a look at the process controls may help.

Riadsala
Metallurgist in the Offshore O&G industry

 

[kenvlach]

bugeaterfan,
Have you solved this problem yet?
It is definitely an adhesion issue as suggested above. Electroless nickel should never flake off of 1018, regardless of degree of forming, merely crack, as its adhesion strength should be 50-60 ksi.

If the plater has poor pretreatment techniques or the EN bath has been heavily used (ask "What is the number of metal turn-overs? as by-products build up), two ways to improve:
1) Use a Wood's nickel (electrolytic) strike plating, and
2) Do a post-EN plating adhesion bake of 2-4 hours at 180-200 °C [ASTM B733 Class 3 adhesion bake for EN on steel).

The maximum hardness is usually achieved by 1 hour at 400 °C (depends somewhat on [P]) but is accompanied by a decrease in corrosion resistance. Don't put plated parts into a hot furnace -- best to heat up from room T. Longer times & higher temperatures result in softening -- grain growth, coarsening of the Ni₃P precipitates.

For some good info on EN, go to

http://www.nickelinstitute.org

(free registration req'd) and download 'Properties and Applications of Electroless Nickel' (10081).

For pretreatment for proper adhesion, see
'Basic Substrate Strategies and Approaches for Electroless Nickel,'

http://www.pfonline.com/articles/web0301.html