Metal finishing Q&As since 1989

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DI Water is Etching Anodized Aluminum in the Cleaning Line

Q. Do I have an anodize seal or DI water rinse problem?

We are having some staining/etching issues with black anodize (organic dye) on 6061-T651 Aluminum after washing in a 5% concentrate cleaning solution (8 pH cleaner) and rinsing in DI water. Parts are being anodized per MIL-A-8625 Type II Class 2 Black.

We can hand-wash parts using a mild degreasing soap and hand rinse with DI water and not see any staining/etching. When we run the parts through our clean line, we see staining/ etching appear during the rinse stage.

We are currently using 18 MOhms DI water. I have heard that DI water this high quality can be corrosive because the water wants to replace the ions and cations that were stripped out. Could this be the case when cleaning black anodized parts?

The difference between the two parts is that the part on the left (part 1) ran our normal wash cycle and the part on the right (part 2), we added an extra rinse cycle.

The intent was to see if we were leaving a cleaning solution residue on the surface prior to drying. We thought that maybe the heat on the residue was causing the staining. This was not the case; we could see staining/etching on both parts prior to drying. Part 2 shows more staining/etching, as it had an extra rinse.

Our wash time, rinse, and dry time:
CL4 process
Detergent bath -> Rinse bath -> Rinse bath -> Dryer
Chemistry: pH neutral detergent, 18+ MOhm DI water rinsing
Bath temps: 122 +/- 5 degrees °F (or 50 °C)
Dryer temp: 200 +/- 5 degrees °F
Dwell times: 300 sec in each bath and in dryer.

Test part 1: entering dryer after final rinse (normal process)

Test part 1: post-dryer (normal process)

Any feedback would be appreciated.

Paul Panozzo
- Boulder, Colorado
April 3, 2024