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Polyphosphates

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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.

 

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