As was just discussed, chromate is sometimes combined with zinc for corrosion control in open recirculating cooling systems. Zinc is a cathodic corrosion inhibitor that inhibits corrosion through the formation of zinc hydroxide or zinc bicarbonate at the cathode of metal surfaces. This barrier film inhibits the passage of oxygen at the cathode, thereby, reducing the rate of corrosion.

Because the barrier films formed are not very durable, zinc is not usually used alone and is most often combined with chromate, phosphonate, and/or aromatic azoles to form a synergistic blend. Because zinc tends to precipitate at pH's above 7.8, most of the synergistic programs are designed to operate at a pH below this point. However, when combined with specific enhancers, the solubility of zinc can be extended, thereby, allowing program operation at pH values up to 8.5 or 9.0.

Zinc, like chromate, is a heavy metal that can pose toxicity problems when discharged to the environment. However, it must be emphasized that the regulations governing the discharge of zinc vary significantly throughout the country. For example, some cities still permit the use of zinc in domestic water systems.

Polyphosphates

Polyphosphates were one of the first chemicals used as corrosion inhibitors and later as chromate substitutes. The exact mechanisms of polyphosphate film formation is still subject to debate. However, most authorities recognize polyphosphate as a cathodic inhibitor, which forms a durable polarizing film on the cathodic surfaces of most metals by an electrodeposition mechanism. Theoretically, the molecule adsorbs or bonds with calcium Ions to form a colloidal particle; these positively charged particles migrate to the cathode and form a film. There are also some anodic effects because metal Ions can be included in the film. Polyphosphate has the added benefit of being a scale inhibitor at threshold levels as low as 1-5 ppm. As an additive to potable water supplies, it stabilizes iron and eliminates "red water" problems.

Polyphosphates are supplied in many forms and various chain lengths. The figure below depicts the structure of sodium polyphosphate molecule.

The chain length of the molecule is determined by the amount of repetition of that portion of the structure denoted by an "X". When X = 2 or 3, the polyphosphate structure is crystalline. These are the pyrophosphates and tripolyphosphates, respectively. As the chain grows in length, the characteristic "glassy" structure of amorphous polyphosphates is seen. One of the most commonly used polyphosphates is sodium hexametaphosphate.

The principal problem associated with the use of polyphosphates is hydrolysis of the phosphorus-oxygen bond. This reverts the polyphosphate structure to the simpler orthophosphate molecule. While orthophosphate is an anodic inhibitor, which does provide protection on mild steel surfaces, the presence of orthophosphate can present a number of problems in an open recirculating cooling system. This will be discussed in more detail later in this manual.

The primary causes of reversion are high temperature and pH excursions. There is no specific temperature at which reversion begins; this is generally a function of other system parameters. However, pH limitations can be defined more specifically. The potential for reversion increases as pH rises above 7.5 or drops below 6.0. The stronger electrodeposited film is replaced by a weaker adsorbed film. Depending on the other inhibitors blended with polyphosphate, a pH range of 6.8-7.5 is usually best. pH values below this guideline can also hydrolyze the molecule's bonding structure.

It is important to note that the best corrosion results are not usually obtained when all of the inorganic phosphate (polyphosphate and orthophosphate) is in the polyphosphate form. Specifically, laboratory studies indicate that a ratio of 65% polyphosphate to 35% orthophosphate will provide the best corrosion inhibition on an inorganic phosphate treatment program. To minimize the potential for excessive reversion of polyphosphate to orthophosphate, a chain length (denoted by "X" above) of 5-7 is usually chosen since these polyphosphates are the most stable.

Metal ions in water occasionally affect polyphosphate. Dissolved iron in water will have both positive and negative effects on the inhibitor. The obvious beneficial effect is the strengthening of the film resulting from the inclusion of iron. Iron, however, can complex polyphosphate, thereby rendering it useless as a corrosion inhibitor. Dissolved copper can pass through a polyphosphate film and plate out on iron, forming a highly active galvanic couple. In this situation, the underlying portion of the plated copper becomes the cathodic portion of the couple. Because the inhibitor cannot physically reach this area, corrosion may progress rapidly.

Orthophosphate

As was mentioned previously, orthophosphate is an anodic inhibitor, which can provide the protection to mild steel surfaces. In certain situations, calcium and/or iron can react with orthophosphate to form calcium phosphate and ferric phosphate sludges, respectively. The solubilities of these compounds are inversely related to pH, orthophosphate concentration, calcium and/or iron concentration, and temperature. Since an increase in temperature induces the precipitation of these materials, the sludges formed are usually found first along heat transfer surfaces thereby, impeding the heat transfer process.

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.