DI Water Quality

Question:

I read in one of your articles that “high-quality” deionized water (DI) should be used to make up a chromic acid anodizing bath (www.pfonline.com/articles/clinics/0801cl_alum1.html). I would like to know the specs/char-acteristics of “high-quality” DI water. R.C.

Answer:

I’m going to use up some magazine space by expanding on your question somewhat to pass along what I hope will be beneficial information about the general use and quality of deionized water in the anodizing line.

There are four areas of an anodizing line where the use of so-called “high-quality” DI water is of utmost importance. These are in any type of anodizing electrolyte, dye baths, sealing baths and in the rinses before dying and before and after sealing. The use of high-quality DI water in all of these areas will go a long way to help insure very high-quality anodized parts. I do not intend to address in this article how the DI water is best used in these applications but, rather I will talk about the DI water itself. Perhaps the “how” can be the topic of a future clinic.

DI water used to make up most baths in the anodizing line should have a resistivity of at least 200,000 W/cm. This is the same as five micro-Seimens/cm specific conductance. Most DI equipment used on the line or in the lab can be set to the level of water quality desired. While the DI equipment is producing that quality, or better, the discharge valve remains open and water flows through the DI system to the process. When the water quality falls below the setting the valve closes so that low-quality water will not be introduced into the process.

If a commercial bottle-change-out system is used for your DI water, the system's water quality capability should be discussed with the vendor to make sure that it is capable of producing water of >200,000 W resistance over a large portion of its cycle. If these systems are used directly “from the bottle,” the flow of DI water is usually not continuous, however, and the quality of water suffers greatly from starting and stopping the system to meet point-of-use demands. Even if a water quality monitoring system is installed, high-quality DI water does not immediately flow from the resin tanks when the flow is turned on. Water must flow through the resin tanks in order for it to be deionized. Every time the flow is stopped and then restarted the water must flow for a while to become “good” again. Thus, every time the DI unit is stopped and started, the efficiency of the system is reduced.

The use of a deionized water storage tank to store DI water of high quality is recommended. This can provide the convenience of good quality water at the push of a button or the opening of a valve directly from the storage tank. This allows long runs of DI water to be made and stored for use when it is needed. The longer the uninterrupted flow of water through the DI unit, the more efficient the system becomes by producing the maximum amount of DI water between regenerations or change-outs. There is a detrimental effect of storing DI water but it is relatively minor. When DI water is exposed to the air, it absorbs carbon dioxide, forming bicarbonates that lower the quality of the DI water to some degree. Most of this detrimental effect can be avoided by introducing the DI water at the bottom of the tank with a fill pipe that extends nearly to the bottom. It is true that the surface of the water in the storage tank can absorb CO2, but the effect is minimal on the overall quality of the water in the tank.

The value of 200,000-W resistance comes from experience and is a good number that excludes water that may have any level of chlorides, sulfates, iron or silicates. This is the recommended minimum water quality that should be accepted in the anodizing line. Depending on the quality of the feed water, properly specified DI equipment will produce water well in excess of one million W for at least a portion of its run. The water should be tested periodically for chlorides, as they can be particularly harmful to the anodizing process. Since the cation resin in the DI unit is usually regenerated with HCl (hydrochloric acid) it is possible for some chloride to appear in the final product especially if the resin quality is marginal, or if the regeneration rinse cycle is not functioning properly. Chlorides can also leak in from municipal water systems that are heavily chlorinated to pass water quality standards. This seems to include just about all municipal water systems these days. Test some DI water from the storage tank, or directly from a spigot on the machine or in the DI water line, for chlorides by adding a few drops of silver nitrate to the sample that is contained in a clean glass beaker. If any cloudiness is produced from the silver nitrate, it indicates the presence of chlorides. The presence of any chloride is enough to cause an alert.

Silicates are also detrimental to DI water if it is being used in the sealing bath. Levels of greater than five-ppm silicates in the sealing bath are unacceptable. This is a common concern in the winter because the microbes in the public water supply, which normally feed on silicates, are not present when the water supply drops below 45°F or so. The DI equipment doesn’t do a very good job of removing silicates. They are among the first ions to leak through the resin after some percentage of resin exhaustion. It is a good idea to check for silicates once or twice during the cold-weather months. I have a stock number for a silicate analyzer from a company called Scientific Instruments. The stock number is #180-A001-01, CFA-1030. The phone number is (800) 431-1956. I’m sure there are other suppliers of silicate analyzers but I just happen to know about this one. I do not recommend one brand over another.

You asked about making up a chromic acid anodizing bath. In this bath sulfates should be held below 100 ppm and chlorides below 200 ppm. If your DI water quality standard is at least 200,000W specific resistance, you should not have any problems staying below these limits for chromic acid anodizing or even the lower chloride limit of 50 ppm for sulfuric anodizing.