Why Test for Azole in Cooling Waters?

Azoles, such as tolytriazole (TTA) and benzotriazole (BTA), are yellow metal corrosion inhibitors used in the water treatment field to prevent corrosion of copper and its alloys. These compounds have been used for many years, but only recently have they become the subject of discussion for testing and the target of water consultants. Why the change?

Adding non-oxidizing biocides (glutaraldehyde and DBNPA, for example) used to be the standard practice for controlling microbiological growth in open cooling water systems. These non-oxidizing biocides did not react with or degrade the performance of the azoles. So with the azoles not having a predator in the waters, it could be assumed they would remain in the water proportional to the other actives in the cooling water treatment chemicals.

Now, enter today’s world where we recognize Legionella as a risk to peoples’ health. To control against Legionella and other biological activity, the use of oxidizing biocides like chlorine and bromine have become much more popular. Chlorine can degrade azoles and inhibit their ability to protect copper from corrosion. The azoles now have a predator in the water! This in turn leads to the importance of verifying that azole compounds are indeed present in the cooling waters.

Case in point: I had a cooling tower system whose make-up water came from a fish farm. Imagine the “bio-loading” to this cooling tower from its use of water that fish were swimming in! Quite a bit of chlorine had to be fed into this water to keep the bacteria levels low. The cooling system cooled a condenser made of copper alloy tubes, so the treatment program supplemented TTA into the water. As an extra safety net for the system, a continuous corrosion analyzer with copper alloy tips was added to the monitoring station.

During a regular visit, my engineer told me the copper corrosion monitor was reading much higher than normal. A review of the testing logs determined the cause: Evidently the TTA levels were quite low (<0.5 ppm) while the free chlorine levels were quite high (>1.0 ppm). I printed a chart showing the free chlorine residual against the TTA residual and the corrosion rates. I showed the engineer that when the free chlorine went up, the TTA went down, and the corrosion rates went up. To me, this confirmed everything I had read in books: The high chlorine levels were “eating” the TTA and causing higher copper alloy corrosion rates.

So the moral of the story is that there is wisdom in periodically verifying the azole corrosion inhibitor levels. Not doing so is leaving your treatment results to chance.