Water Quality in the Anodizing Process

Q. I would like to know more about the physical and chemical parameters that are used to evaluate the quality of water used in the anodizing process. What are the standard limits of water quality for each step in the process? Also, why do we need to check for silica on the process water?

A. First, I’ll categorize the rinse waters. I will detail the chemical process baths of a typical Type II or Type III (hardcoat) anodizing process, with dye, and comment on each one.

Rinse Tanks. It is common to use either tap (city) water or well water for rinsing in the clean-up side of the process as well as after the anodizing step. This is detergent clean/degrease rinse either alkaline or acidic, alkaline or acid etch rinse, deox rinse and anodize rinse.

There are usually no water quality parameters required for these tanks. Some specifications, especially aerospace, may require monitoring and control of TDS and/or pH, especially in the alkaline etch rinse. It is not uncommon for certain processing specifications to require control of these factors in all rinse tanks.

Some companies that specialize in aerospace anodizing will use RO water to supply every rinse tank except those requiring higher quality water. Good quality RO water is generally in the quality range of 5 to 50 microSeimens (µmS). Some RO systems are capable of producing water quality of less than 5 µmS.

When a double counterflow rinsing system is employed, e.g., etch, deox, anodize, usually the specs will call for only controlling the water quality of the second (clean) rinse bath.

The rinsing step immediately before dying and sealing should be high-quality (less than 5µmS) water. This is important because it helps prevent dragging contaminants into these baths. The usual tap water contaminants that are downstream of the anodize tank can include ions such as calcium (carbonates), iron and sulfates and others. If these are rinsed off in a high-quality water rinse before the dye or seal, it helps prolong the life of these baths. This means that fewer expensive chemicals are used and these baths (dyes and seals) will do a better job if they are kept clean and free of contaminants.

The rinse immediately after dying can be a single or double tap water rinse followed by the high-quality (DI) rinse before sealing.

After sealing, parts may be tap-water rinsed or go straight to a hot DI rinse. This step heats up the parts to help them dry and, if the hot rinse is clean, virtually eliminates water spotting on the parts.

Chemical Baths. The detergent cleaning bath, alkaline or acid etch, deox and Type II and III anodize baths are commonly made up with tap water. Many anodizing facilities use RO water for this purpose. RO water is better, but usually not necessary.

Type I (Chromic Acid) anodizing tanks are usually made up with RO or DI water. This bath is more sensitive to certain contaminants than the Type II and III baths.

Boric-Sulfuric (BSA) and Tartaric-Sulfuric (TSA) anodizing baths are normally made up with RO or DI water.
Dye baths and sealing baths should always be made up with high quality DI water less than 5µmS specific conductance). Some shops use RO water for these baths. The better the water,the better the dying and sealing, as long as the dyes and seals are properly maintained.

It is important to test for silica in the DI water. If the water pretreatment plant uses good quality RO water to make DI water, periodic testing for silica should still be done, however, the probability of finding silicates in the DI water in that case is low. Silicates in the seal bath inhibit the sealing process in as low a concentration as 10 ppm (10 mg/l). As the DI resins age, they can lose their ability to remove silicates from the water. When this happens “silica break through” can occur at any time. In some geographic locations, the presence of silicates in the water supply, particularly if the supply source is ground water, is a bigger problem in winter than it is in summer. In locations where granite rocks are present, silicates in the water supply can also be exacerbated.