In many industrial processes, undesirable excess heat may be removed by the use of heat exchangers in which aqueous systems may be used as the heat exchange fluid.  Various metals and metal alloys, such as copper and copper-bearing alloys (e.g., brass), may be used in the fabrication of such heat exchangers, as well as in other parts in contact with the cooling water, such as pump impellers, stators, valve parts, etc.  Aqueous systems such as those cooling fluids may be corrosive towards these copper-containing metal parts due to the presence of aggressive ions and by the intentional introduction of oxidizing substances for biological control.  The consequences of such corrosion are the loss of metal from the equipment, potentially leading to failure or requiring expensive maintenance, creation of insoluble corrosion product films on the heat exchange surfaces, potentially leading to decreased heat transfer and subsequent loss of productivity, and discharge of copper ions which may then “plate out” on less noble metal surfaces and may cause severe galvanic corrosion, a particularly insidious form of corrosion.  Also, copper is a potentially toxic substance, so its discharge to the environment is undesirable. 
It is common practice to introduce corrosion inhibitors into such cooling water systems, as well as other aqueous and nonaqueous systems.  These corrosion inhibitors may interact with the metal/metal alloy to directly produce a film which is resistant to corrosion, or may indirectly promote formation of protective films by activating the metal/metal alloy surface so as to form stable oxides, other insoluble salts, etc.  Such protective films may not be completely stable, but may instead degrade under the influence of the aggressive conditions in the cooling water.  Because of this degradation, a constant supply of corrosion inhibiting substances in the cooling water may be required to inhibit corrosion of the metal/metal alloy surface. 
In general, the corrosion inhibiting performance of these corrosion inhibitors in industrial water systems (as well as in other systems, for example, other heat transfer systems, lubricant systems, hydraulic fluid systems, etc.)  may be judged by their passivation and persistency characteristics.  Improved film persistence is recognized as one of the criteria for film-forming corrosion inhibitors in view of the economic and ecologic advantages of the commensurate low dose or charge required for corrosion inhibiting compositions that may attain such persistence.  Passivation rate is also a relevant criterion for the same reasons.  In other words, those compositions that provide the most valuable corrosion inhibiting films are those which both form quickly, thus minimizing the presence of the corrosion inhibiting composition in the effluent, as well as persist for greatest length of time, likewise minimizing the need to continually charge the corrosion inhibiting composition into the system.