Film Former Cause Corrode

Among the four filmers used, ortho-phosphate is the most common. Glycol manufacturers widely use ortho-phosphate in the dual role of corrosion inhibitor and pH buffer in their formulations. At normal use concentrations (1000 to 5000 mg/L as PO4), phosphate protects against corrosion on ferrous and non-ferrous alloys. The primary mode of action is via precipitation at the anode to form insoluble metal phosphates. This low solubility of phosphate salts is why manufacturers recommend using good quality (i.e., soft or distilled/deionized) water12 for diluting phosphate containing products like glycols.

The ability of phosphate to form a protective film by directly precipitating is both its strength and weakness. While it will film the metal surfaces, it will just as readily precipitate with metal ions or hardness salts in the bulk water system. This competition between useful and non-productive reactions is the major liability associated with phosphate. Since ortho phosphate is an anodic inhibitor, if the concentration falls below the critical level (200 to 300 mg/L), rapid corrosion attack will occur, this mean that on the surface have oxidized. Phosphonates are related to inorganic phosphates, which include ortho and polyphosphate. HPA (hydroxy-phosphonoacetic acid) is the best example of a phosphonate. These chemicals are effective cathodic inhibitors. HPA can be employed where a product with a low environmental impact is preferred. Acceptable corrosion rates can be obtained at levels of 50 to 200 mg/L (as HPA). The phosphorous contribution of 25 to 150 mg/L (as PO4) is well below what would be necessary with inorganic phosphates.

A third class of film formers is the various dibasic acids. They can be used for pH buffering and corrosion inhibition, much like phosphate. Dibasic acids work because of their limited solubility with transition metals (iron and copper) and alkaline earth cautions (hardness). As the corrosion process takes place at the anode, iron ions go into solution. The dibasic anion reacts with the iron ions and precipitates at the corrosion site, stopping corrosion.

Unfortunately, dibasic acids are biodegradable. In chilled water loops this can pose a serious limitation. In particular, where there is already a supply of biologically available nitrogen (i.e., nitrite), rapid biological growth can quickly consume these nutrients in a few days.

Also, as they rely on iron ions to form the inhibitor film, losses can occur if a considerable amount of corrosion product is present in a system before adding the dibasic acid. In systems (new or old) with significant amounts of corrosion products present, loss of this type of inhibitor can be dramatic.

Substituted triazines are the final type of film former. They react selectively with iron to form an inhibitory film on the metal. Although they are widely used in oil-field applications as down hole corrosion inhibitors, they have not seen extensive use in closed loops. The hexanoic acid triazine compound is an example of this chemistry.