nickel-plated aluminum 4

[Cokoliso]

We build sensors of T6065 aluminum and have them nickel plated. One was returned from central Florida with many spots of white corrosion, which appear under a microscope to be growing from a small point where the plating may have been pitted. But since aluminum forms an oxide layer so quickly, and the oxide is not soluble, how can the aluminum be corroding even if the nickel plating has a pit? The device was outdoors in the rain/dew but salt should not have been present.

http://files.engineering.com/getfil...dc4-4f27-838f-3a86ad57d093&file=corrosion.JPG

 

[stanweld]

The nickel coating is anodic to the Al substrate, similar to tin plate on steel. Once the coating is broken (pore/scratch, etc.), the substrate is exposed to moisture and will preferentially corrode to protect the coating.

 

[swall]

Nickel is cathodic to aluminum and a breach in the plating will lead to rapid corrosion of the aluminum. I have seen aluminum components with electroless nickel extensively pitted in this manner.

 

[CoryPad]

swall is correct, nickel is cathodic (aluminum is anodic) in this couple. The aluminum will corrode preferentially at any discontinuities.

 

[knowlittle]

I have seen aluminum components with electroless nickel extensively pitted in this manner. [end quote]

Was the part wet constantly?
I would anticipate aluminum to acquire protective oxide film at nickel discontinuities, unless the part stays wet. Am I mistaken?

 

[Cokoliso]

The part was not wet continuously, but drenched every night with dew and probably frequently with rain (outdoors in central Florida). After lab analysis and microscopic photos, the consensus is that the pre-electroless prep was not adequate, resulting in pits, allowing the cathodic corrosion. Thanks for everyone's input.

 

[macmet]

I've been following this thread, and I'd appreciate some clarification.

If the nickel is chips, or somehow exposes the aluminum. The aluminum then oxides (which is evident by the white marks).

Once the aluminum oxidizes, is that mark then stable? Or will it continue to pit deeper into the aluminum?

 

[swall]

As long as an electrolyte present, the reaction will continue, as the aluminum oxide formed is not a dense deposit, like you would have with an anodize film. Once the spot has corroded, you also have the possibilities of crevice corrosion and pitting corrosion mechanisms. And potential filiform corrosion undermining the nickel layer.

 

[knowlittle]

Thanks for the explanation. I have an aliminum part with electroless nicked coating for scratch resistance. I need to guard the part from moisture.

 

[DesignerGuy16]

We use Endura 202P coating on some aluminum parts with good success. It's an electroless nickle with a teflon adder. The teflon fills the pores in the nickle coating. Advantages are high hardness, great release properties (water will wick off the surface), and generally a stronger bond than traditional electroless nickle plating.

I've seen some other proprietary coatings out there that use an electroless nickle with teflon on aluminum that the teflon will actually self heal to the point that any surface defect will not propagate. I've seen these on Italian gearboxes, pretty sure it's from an Italian plater. The claims are legit though, we do a lot of destructive testing with chemicals, etc and can get most anything to fail. This stuff is as advertised though.

James Spisich
Design Engineer, CSWP

 

[unclesyd]

If Nickel isn't specked for the product you may want to talk to Alumniplate. This process is very effective dockside in NW Florida

http://www.alumiplate.com/html/properties.html

 

[Cokoliso]

Two questions now: Why plate aluminum with aluminum? Maybe we should not nickel plate at all, and just leave the aluminum exposed? And second, the aluminum shell is held together with stainless steel screws. Is there a galvanic action between SS and aluminum if we didn't plate?

 

[israelkk]

Cokoliso

The plating is pure aluminum that has an excellent anti-corrosion properties. Aluminum alloy will corrode without some kind of surface treatment. This is why in aircraft the use of clad aluminum sheets is common. The clad is a ~0.1 mm layer of pure aluminum on the sheet that protects the alloyed sheet.

 

[swall]

Yes, you have a potential issue with ss fasteners in aluminum. The ss, being cathodic, will cause localized corrosion of the aluminum right around the fastener head. There is also the possibility of hydrogen charging from galvanic corrosion, which could lead to delayed fracture of the fasteners if the stress level is high enough. You have several options to mitigate this. 1--use plastic washers to isolate the fasteners.2--Use a carbon steel fasteners plated with zinc, cadmium or IVD aluminum. Or use a coating such as Dacromet on the fasteners.