Orthophosphate passivator

Both molybdate and orthophosphate are excellent passivators in the presence of oxygen. Molybdate can be an effective inhibitor, especially when combined with other chemicals. Orthophosphate is not really an oxidizer per se but becomes one ia the presence of oxygen. If iron is put iato a phosphate solution without oxygen present, the corrosion potential remains active and the corrosion rate is not reduced. However, if oxygen is present, the corrosion potential iacreases ia the noble direction and the corrosion rate decreases significantly. 

Precipita.tingInhibitors. As discussed earlier, the localized pH at the cathode of the corrosion cell is elevated due to the generation of hydroxide ions. Precipitating inhibitors form complexes that are insoluble at this high pH (1—2 pH units above bulk water), but whose deposition can be controlled at the bulk water pH (typically 7—9 pH). A good example is zinc, which can precipitate as hydroxide, carbonate, or phosphate. Calcium carbonate and calcium orthophosphate are also precipitating inhibitors. Orthophosphate thus exhibits a dual mechanism, acting as both an anodic passivator and a cathodic precipitator. 

Figure 10.1 Types of phosphate structures, (a) Where x = 12 to 14, the structure represents sodium polyphosphate, a phosphate typically used in HW heating and industrial steam boiler formulations. The structure is ill defined and described as glassy rather than crystalline. Where x = 2, it represents sodium tripolyphosphate, (b) This is the structure where effectively, x = 0, and represents trisodium phosphate (sodium orthophosphate), which is commonly supplied in either crystalline or anhydrous powder form and used as an alkalinity booster, boiler boil-out cleaner, and metal surfaces passivator.

Although orthophosphates are themselves passivating, anodic inhibitors (and also cathodic inhibitors, forming a calcium phosphate barrier film), the film strength is weak, even in simple HW systems and they are not used for this purpose. Nevertheless, despite the thermal instability of sodium hexametaphosphate and other polyphosphates, phosphates in general have several important properties that make them useful in boiler plant operations. These properties include ... 

Although chromate is the best aqueous corrosion inhibitor available, its use has been severely curtailed due to toxicity and environmental concerns ( ). One of the more successful non-chromate treatments involves the use of phosphate/phosphonate combinations. This treatment employs high levels of orthophosphate to promote passivation of the metal surfaces. Therefore, it is important to control calcium phosphate crystallization so that high levels of orthophosphate may be maintained in the system without fouling or impeding heat-transfer functions. 

The short-term application of an increased dose rate of chemical inhibitor enhances the corrosion inhibiting potential of an already passivated metal surface. Nevertheless, in order to change the metal surface from an active state to a passive state, the electrode potential must be raised to a level above that of the passivation potential. Typically this is achieved by the use of chromate, nitrite, and/or orthophosphate in the presence of oxygen. 

The presence of polyphosphate detergent in the cleaning formulation (some of which will revert to orthophosphate) will, under most circumstances, lead to passivation of the clean metal surfaces. Additionally, as part of the water treatment program start-up procedure, it may prove useful to supplement this initial passivation with 2 to 3x the normal reserve of maintenance inhibitor chemical treatment for a period of 1 to 2 weeks. 

Passivation Inhibitors. Examples of passivators (anodic inhibitors) include chromate, nitrite, molybdate, and orthophosphate. AH are oxidizers and promote passivation by increasing the electrical potential of the iron. Chromate and nitrite do not require oxygen, and thus, can he the most effective. Chromate is an excellent aqueous corrosion inhibitor, particularly from a cost perspective. However, owing to health and environmental concerns, use of chromate has decreased significandy and will probably he outlawed soon. Nitrite is also an effective inhibitor, but in open systems it tends to be oxidized to nitrate. 

There are two types of passivating inhibitors oxidizing anions such as chromate, nitrite, and nitrate, which can passivate steel in the absence of oxygen, and the nonoxidizing ions such as phosphate, tungstate, and molybdate, which require the presence of oxygen to passivate steel. Examples of passivators (anodic inhibitors) include chromate, nitrite, and orthophosphate (Dihua et al. 1999). 

Beland suggests that corrosion protection comes both from the ability of the tripolyphosphate ion to chelate iron ions (passivating the metal) and from tripolyphosphate ions ability to depolymerize into orthophosphate ions, giving higher phosphate levels than zinc or molybdate phosphate pigments [23]. 

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