Anodizing Q&A: Rinsing Blind Holes Prior to Dying

Q. I have problems with leaching from blind holes in black anodized parts. The front plates I am running have side holes and top holes, mostly M2 and M3 diameter (≈ 1-64 and 4-40 Imperial screw size). I could not find a way to rinse out the sulphuric acid after the anodizing tank and just before the dying tank. We are rinsing them in two cold rinse tanks before the dye by dipping the spline with the front plates about 50 times in each tank. Any other suggestions?

A. Rinsing blind holes prior to dying can be a real problem and it is a common one with anodized parts. There are a few techniques that may be used to thoroughly rinse the acid out of blind holes and other recesses in the parts. The practicality of some methods depends on the size tanks in the anodizing line. With smaller tanks in a hand rack line, or a small hoist line, it is usually easier to use special rinsing techniques. With large tanks some of the “hands-on” techniques may be more difficult.

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One way to get all of the acid out of blind holes is to rinse by hand with a water hose and nozzle, like a garden hose. Using the spray nozzle, you can make the effort to individually rinse out all the holes under pressure with clean water. Do this once and then dip the load or rack in the rinse tank. Then rinse with the spray nozzle one more time and go to the dye tank. After this second spray nozzle rinse do not go back into a rinse tank. The water from the hose is cleaner than the rinse tank, increasing the chances of getting a good dye job without bleed-outs.

Sometimes an effective way to help rinse the acid out of blind holes and other folds and crevices in the parts, whether they be machined holes or fabricated sheet metal, is to alternate back and forth between a hot rinse tank and a cold rinse tank. Of course, you can always use compressed air to blow out each hole or fold, but really clean compressed air is required for this. Otherwise there is a risk of contaminating the parts with oil from the compressed air.

Another method, although labor intensive, is to use cotton swabs (Q-Tips) to soak up the dilute acid in each hole and then rinse with a hand-held spray nozzle.

In the end, it requires whatever techniques and persistence needed to get the job done. Some parts are just more difficult than others.

Originally published in the February 2016 issue.