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Adhesion failures Ni on Al

2007

I am having a strange issue with adhesion plating nickel onto aluminum. The substrate is extruded aluminum 6061(?) that is anodized prior to assembly to limit tooling marks in the substrate.

Our facility gets the final anodized product for Ni/Cr plating. The process is as follows.

-Strip the anodize off in a hot NaOH solution.

-Polish the high cosmetic areas of the part for any defects from shipping/stripping.

-Parts are cleaned in a mild alkaline cleaner solution 135F for 6 min.
-Immersion in 50% Nitric 30 sec
-Etch: Using EPi's Epik-232 solution 30-40 sec
-50% Nitric for 1 min
-Zincate 25 sec
-50% Nitric 7 sec (part takes on white appearance)
-Zincate 10 sec

-Electroless Nickel Strike (Alkaline) 10 minutes
-Proceed with Bright Ni plate followed by Chrome

Parts look good after plating with no visible abnormalities but during adhesion testing (bake 430F for 1hr, cold water quench) blisters appear everywhere some of them even "popping" off the part. I would say the problem is definitely in the cleaning of this particular part as we run other aluminum parts this way with no failures. We are just having trouble determining where exactly the problem is. One interesting thing to note is wherever the parts are polished there is absolutely no adhesion issues---those particular areas of the part are perfect every time. I would polish the whole part but it has recesses which are not accessible.

Is it possible we are not getting all of the anodize off the part in the first step leading to failures later on but where the part is being polished is getting down to the base aluminum?

J. Ott
plating shop - Middletown, Pennsylvania

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I'd say almost certainly the fault lies with the stripping of the anodic coating. NaOH does not strip uniformly, therefore there may be areas that are completely stripped and the NaOH has begun to etch the surface, and other areas where stripping is incomplete.

If possible, trial a phosphoric/chromic acid solution as used in MIL-A-8625 / MIL-PRF-8625 [on DLA]F, which will allow complete stripping without etching the part in the manner of NaOH.

The other possibility is the microporosity created by the NaOH stripping is making rinsing more difficult, trapping solutions and causing blistering after plating. Once again, changing your stripping method may be able to help this.

The final option is - would it be simpler just to polish out any marks left on the substrate if not anodised prior to assembly? This is something you would need to discuss with your customer.

John Reid
- Brisbane, Queensland, Australia
2007

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Your second nitric should be only 5% or so, and the second zincate time seems short. You may be getting a poor zincate prior to plating. Acid EN may work better.

Jeffrey Holmes, CEF
Spartanburg, South Carolina
2007

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It looks to me as if the problem is with the stripping of the anodised layer. I know of a couple of ways of stripping this layer, one being 3% sodium hydroxide at about 35-40C. Other methods include a mixture of 10% sulfuric acid with 1% hydrofluoric acid (i.e., 100 ml/l H2SO4 and 10 ml/l HF), or 10% sulfuric acid with 40g/l potassium fluoride ⇦this on Amazon [affil links] (100 ml/l H2SO4 and 40 g/l KF). In all cases of stripping, the treated part must be rinsed in nitric acid to remove any residual surface debris. I suspect your polishing stage replaces the nitric acid dip, but where you cannot get to the surface, you leave some debris that later results in delamination. I would therefore try giving your parts a good dip in nitric acid prior to polishing with a lot of gentle agitation - hopefully that will solve your problem. By the way, it is NOT recommended that the sodium hydroxide route is used for stressed parts or anything for the aerospace industry - in fact I think it may be prohibited!

Trevor Crichton
R&D practical scientist
Chesham, Bucks, UK
2007

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Have you considered the differences in the coefficients of expansion at 430 °F of all those metals you have in the sandwich?

Robert H Probert
Robert H Probert Technical Services

Garner, North Carolina

2007

430 degrees F. Expecting a lot for nickel on aluminum. Even when everything is done properly.

Frank DeGuire
- St. Louis, Missouri, USA
2007

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We electroless nickel plated tens of thousands of aluminum pvc glove molds which are continuously cycled to 360 °F+. Not quite 430, but I don't recall a single adhesion failure, and these ran many thousands of heat/quench cycles.

Jeffrey Holmes, CEF
Spartanburg, South Carolina
2007

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2007

dear sir
please follow this sequence
1 pre clean (mild alkaline)
2 hot soak (" " " ")
3 cathodic cleaning if decorative purpose,or for functional purpose it is only immersion
4 nitric acid 50%
5 zincating
6 nitric acid 50%
7 zincating
8 alkaline nickel (ENP)
9 bright nickel (ENP)
every step in between require rinse well.
please use zincating solution which contain Ni ion composition .

Vijay Parmar
- Mumbai, Maharashtra, India

The issue has seemingly been resolved but using a very thin layer of Electroless Nickel. With this thin layer we can achieve the quench test with little or no blistering of the parts. Thanks to everyone who replied.

J. Ott
plating shop - Harrisburg, Pennsylvania
2007