Corrosion Passivation

[joebaldauff]

I have a machine that operates in a water environment. The two materials are aluminum and brass. The aluminum is anodized but is corroding (piting) due to large difference in the electronegativity scale and the resulting difference in the anode to cathode ratio. I am considering plating the brass with .0005" to .001" of cadmium. The result should be that the machine is passivated and the corrosion currents should be reduced to near zero. Am I on the right track. What about other plating options?
Thanks in advance,
Joe B

 

[TVP]

Reducing the current is a good idea. You may want to consider zinc plating rather than cadmium plating. Either cadmium or zinc should have a chromate conversion coating applied as well. Olive drab is the thickest and most corrosion resistant chromate. For the aluminum, what type of anodizing has been used? Sealing of the anodic layer will improve corrosion resistance. Painting on top of that will provide additional resistance.

 

[unclesyd]

What type of Aluminum and Brass are you using?
What type of water.

 

[joebaldauff]

Aluminum is 7075-T6, Brass is whatever we could buy at the time. The aluminum is black annodized but not hardcoated because of dimensional requirements. Will the cadmium erode away in time hense the cromate conversion coating?

 

[unclesyd]

As long as there is any difference in potential as you know the anode will go. Plating at its best is usually a stop gap measure. A good Al anodizing with a sealer and a cadmium plate with a conversion coating are going to be your best bet as you already have the components made. You must remember that a pinhole in either coating can will be very detrimental.
As “TVP” mentioned zinc is a potential plating tough I’ve found that cadmium platers tend to do a better job. Talk to the plater and anodizer about their capabilities and experience with the different systems.

Here is website posted on another thread that maybe of assistance.

http://anodizing.org/index.html

Also check out:

http://www.finishing.com

If you can post the type of water the component is exposed to, salt , fresh, etc.

 

[Metalguy]

*IF* your water is not very corrosive to bare Al itself (soft/demin. water), your orig. idea of plating the cathode is good. Pinholes in cathode coatings don't mean much-it's pinholes on the anode (Al)that are bad. Given non-corrosive water, you're better off with bare Al so that you have a large anode and tiny cathodes (pinholes).

 

[joebaldauff]

The water is tap water. It gets dirty and contaminated so it is changed once a week approximately. We have some machines that experience no corrosion and some that corrode like the dickens. Different parts of the country with different water sources. I checked ph, clorine and alkalinity. PH was neutral, clorine low but alkalinity was 400 ppm. I suspect that any sort of conductive water will enable the corrosion.