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Is plasma cleaning compatible with chromate conversion coated aluminum parts?

September 26, 2011

We have a supplier who manufactures an aluminum 5052-H32, .063 thk bracket that gets chromate conversion coated per MIL-C-5541 Class 1A (would now be MIL-DTL-5541 Type I, Class 1A). Another of our suppliers has proposed to perform a plasma cleaning operation on the bracket so that a bonding material will stick to the bracket better. We are concerned that the plasma cleaning process might reduce the corrosion inhibiting characteristics of the conversion coat.

The proposed plasma cleaning process will use oxygen. This plasma is said to remove only organic compounds. On some test parts, the before and after visual appearance of the brackets is unchanged. However, that in and of itself probably says very little about how the coating may have been compromised (if it is susceptible)

Any suggestions, experiences or ideas on where to find reliable information? It would be greatly appreciated.

Russ McCrum
Product Manager - Redmond, Washington, USA

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September 28, 2011

It is true that plasma cleaning is expected to remove organic contaminants from the surface. It is generally used to enhance the bonding properties of the surface. But the question if it modifies the surface in any other way needs to studied. You may try it on a test piece.
Plasma treatment of polymer surfaces
induces cross linking and enhances surface energy. This increases wettability and hence bonding. On metals it is just a cleaning step.
Two surfaces are involved in bonding. If you are bonding a metal to a plastic/polymer it would be more beneficial to plasma treat the latter than the metal.

H.R. Prabhakara - Consultant
Bangalore Plasmatek - Bangalore Karnataka India

We are a plasma surface chemistry company. Chromate conversion coating for aluminum is an inorganic mix of chromium(IV) oxide, barium nitrate and sodium silico fluoride. In our lab we have experimented with 0 min, 10 min and 60 min 150W, 13.56 MHz 02 plasma treatments of chromate finished aluminium surfaces. We subsequently analyzed the surface using XPS. We saw no elemental degradation of the Chromate. What we did find was that the organic residues or elemental carbon organic residues were significantly reduced with both 10 and 60 min plasma treatments while the chromate peak was un-changed. The finish and color of the parts was also unchanged. A final conclusive would be a salt spray corrosion test. However our test show a strong indication that the Chromate conversion coating is not compromised.

James Bond
- Corona, California, USA
November 9, 2011