Organophosphorus Compounds

A large family of organic inhibitors, known as organic phosphates or organophosphorus compounds, have become popular in recent years. These would include aminotrimethylenephosphonate (AMP), hydroxyethylidene diphosphonate (HEDP), phosphonobutane carboxylates and phosphate esters, the structures of which are shown in the figures, which follow. Because of their low environmental impact and their effectiveness as deposit control agents, organophosphorus compounds are often blended with other corrosion inhibitors and polymeric antifoulants.

The relative corrosion inhibitory capabilities of each of these substances varies as do their structure and chemical stability.

HEDP and AMP phosphonate are most often used in inhibitors with the former exhibiting better chemical stability. In the presence of calcium, the phosphonates generate film formation in much the same way as polyphosphate, combining with metal cations and migrating to the cathodic surfaces of the metal. Because this film can be easily ruptured, normal recommendations would include pretreatment at high concentrations to lay down the initial protective coating. It is believed that these substances also put down an adsorbed film on the entire surface of the metal in soft or low calcium waters. Elevated pH levels are also desirable when using these organic inhibitors to further reduce the corrosivity of the system.

The organophosphorus compounds have been found to be aggressive to copper and copper bearing alloys. This is especially true of AMP phosphonate, which is a nitrogen-containing molecule. To reduce copper corrosion in systems using phosphonate inhibitors, specific copper corrosion inhibitors are often utilized; e.g., benzotriazole, tolyltriazole and mercaptobenzothiazole.

One advantage organic phosphates hold over their inorganic counterparts is their superior resistance to hydrolysis. Whereas polyphosphate will break down to orthophosphate under normal water conditions, the organophosphorus compounds will remain intact for a longer period of time. However, it has been found that specific contaminants will accelerate the breakdown process, and these contaminants have various degrees of impact on the individual materials. For example, strong oxidizers like chlorine will rapidly degrade phosphate esters and AMP phosphonate while phosphonobutane carboxylates show better resistance to breakdown by strong oxidizers. The presence of some dissolved metal cations like calcium and zinc tend to stabilize organophosphorus compounds under most conditions encountered in cooling water systems, further reducing their tendency to revert to orthophosphate.

Prior to their use as corrosion inhibitors, organophosphorus compounds were widely recognized as deposit control agents. Their use and effectiveness in this role is discussed in the deposit control section of this manual.