Anodizing Clinic: Sealing with Distilled Water

Q. Can distilled water be substituted for deionized water? Our facility can create a distilled water heated bath fairly easily because we already produce distilled water for another process. A DI water bath would be much more expensive.

A. Yes, distilled water can certainly be used for sealing. In fact, distilled water is probably more reliable quality-wise than DI. It depends on the type of water pretreatment system. With DI, there can be a wide variability in quality during a given “run”. When regenerating the DI resin, the final rinse is directed to drain while the quality of water gradually gets better. When the water quality reaches the lower set limit for DI process water, it would typically be directed to the storage tank. The water quality gets better and better during the normal run. After reaching a peak, the DI water quality gradually subsides until it reaches the lower quality limit. Thus the quality of DI water in the storage tank is essentially a mixture within the acceptable range. Some DI systems also continuously recycle DI water from the storage tank back through the DI resin to polish the water. In this case, you can end up with a tank full of high-quality DI water.

A relatively new development in the production of DI water is called Continuous Electro-Deionization, also called CEDI, or just CDI. The CDI unit is comprised of cathodic-anodic cells; each cell contains a very small amount of resin. A DC electrical field is applied, which results in removing all, or nearly all, of the anions and cations in the water that passes through. The system does not require regeneration in the traditional sense. CDI units are particularly effective in removing dissolved silica in the water and CDI systems should be cleaned about once a year.

Distilled water should be good all the time. There is no such thing as “partially distilled” unless distilled water is mixed with other lower quality water. It’s all about the amount of dissolved solids in the water. Distilled water should have none, or virtually none. Whether or not DI or distilled is used, the lower limit of “high-quality” water is 200,000 Ohms resistance (5 microSeimens specific conductance). When the water quality in a run reaches these numbers, the water quality is no longer acceptable.

Originally published in the January 2017 issue.