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Staining of Aluminum in DI and distilled water

Hello all,

I am having a recurring problem with staining of aluminum-1100 (99% pure) parts during an alkaline detergent ultrasonic cleaning process. We are using the process to remove dirts, oils, machining lubes, etc. We use an alkaline cleaner that is formulated with inhibitors to prevent corrosive attack on aluminum. Following the wash, the parts are rinsed in two stages of resin-bead filtered DI water, also in ultrasonic baths. The process works well for the most parts, but we will intermittently get batches of parts that are stained brown, sometimes severely so. We have successfully been able to recreate the staining to match what we think is happening, but we have also seen staining for parts that are put directly in DI water of ~1.0 microseimen conductivity at ~140° F after a certain amount of time. The staining was much more severe for distilled than it was for DI, although our DI results have varied a bit (sometimes very little after 3 hr, sometimes noticeable after 1 hr). Our test have been done in thoroughly rinsed lab beakers [beakers on eBay or Amazon]s. The pH of the pure water baths are always close to neutral. I have been reading into pure water corrosion of Al, but this seems to be mostly concerned with material loss versus staining. Does anyone know what the stain is, e.g. oxide or hydroxide buildup? Is this normal for DI water at this temperature or might we have some nonionic contaminants in our DI water? Thanks

Mikhail Kuffel
Engineer - Sacramento, California
August 25, 2011