Clean Wastewater Attacking Zinc Plate

Q. We use ion exchange treatment for wastewater from our nickel, hexavalent chrome and zinc plating rinses. We have been trying to reuse this “clean water” but have found that it is attacking the finish from our bright zinc plating. What kind of characterization should we do to identify the pollutants that are affecting our products, and what steps are needed to reuse this water? H.S.

A. The very likely issue is not that there is some kind of pollutant in your “clean water” that is causing the problem but that your ion exchange effluent is too clean and is very “aggressive” towards metals. Since your ion exchange system has essentially removed all ions, including metals, from the wastewater, the wastewater wants to restore its ion balance and will attack metals to replenish its ions. Also, if you check the ion exchange effluent’s pH, you will likely find it below 5 and perhaps as low as 3. If you are using this water in your nickel and chrome plating rinses and not experiencing the same issue, the reason is that these metals are able to resist the water’s aggressiveness during the relatively short rinsing time.

Here are several alternatives for you to consider:

Do not use this de-ionized water for zinc rinsing,
Supplement potable water for a portion of the rinse water flow, thereby reducing the rinse water’s aggressiveness. Once you have established the desirable ratio of potable water to deionized water, you can use a conductivity sensor and controller that adds potable water when the water’s conductivity (a gross measure of the water’s dissolved solids) reaches a low set point.
Adjust the deionized water’s pH with sodium hydroxide to maintain a pH of 7.0–7.5. (We like to use 10–25 percent sodium hydroxide because of its rapid reaction rate and low freezing temperature.) Be careful not to raise pH above 8, since this may tend to dull your desired bright zinc finish.

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