There have been numerous papers presented over the last thirty years on the performance of benzotriazole and tolyltriazole as yellow metal corrosion inhibitors. Many of these papers have focused on the interaction of azoles with halogenated biocides. There seems to be much debate on whether azoles are subject to degradation, when in the presence of these biocides. This debate ranges from those who claim the inhibitor is rendered ineffective due to this degradation, to those who agree that there is an interaction, but the resulting compound is still capable of protecting the metal. Many of these papers also tout the superior strength of the azole film, yet the recommended guidelines for treatment with azoles is to maintain a residual inhibitor in the cooling water for repair, especially when halogenated biocides are present.
This paper presents laboratory studies that will attempt to debunk some of the myths associated with azoles. Studies will be presented that show that azoles are not significantly affected by high concentrations of halogenated biocides (particularly bromine). The azoles actually perform remarkably well when residual inhibitor is present, even at extremely high dosages of free bromine. Unfortunately, the azoles have a much greater weakness that is not strongly emphasized in the industry today. The azole film has been found to be extremely delicate in a cooling water environment, regardless of the presence of halogenated biocides. The need to maintain a residual level of azole in the water is not simply a recommendation to provide an added sense of security; it is an absolutely critical measure to maintain acceptable protection of the yellow metal surfaces. As soon as residual azole is removed from solution, the film begins to break down, causing an almost instantaneous increase in corrosion rates. Any presence of halogenated biocides will only serve to accelerate these corrosion rates further. Electrochemical test methods and pilot studies will be presented that help to demonstrate these findings.
Copper alloys are a common component found within industrial cooling systems, especially at the heat exchange surface. The corrosion of these surfaces and resulting galvanic deposition of copper onto existing ferrous metal surfaces can have detrimental effects on the structural integrity and operation of the cooling system. As a result, copper corrosion inhibitors have always been a staple ingredient in most water treatment formulations. The most common of these corrosion inhibitors have been benzotriazole (BTA) and tolyltriazole (TTA), with TTA being the overwhelming industry favorite today. These inhibitors are generally dosed into cooling towers at a range of 2.0 to 5.0 mg/L. In closed loop, recirculating systems, their dosages can reach as high as 25 to 50 mg/L.
Both BTA and TTA utilize the triazole functional group as their binding site to the metal, resulting in a protective film on the copper surface. However, the properties of these two films are quite different. The film formed by TTA has been found to be more resistant to breakdown in aqueous environments. The methyl group on the TTA molecule is believed to sterically hinder the film's thickness.1
