Coordinated phosphate corrosion

Fig. 5. Coordinated phosphate—pH control avoids both acid and caustic corrosion. To convert psi to kPa, divide by 6.895.

Coordinated phosphate control charts assume either that all contribution to pH level is derived from phosphate or that the buffering action of phosphate is sufficient to overcome the presence of other alkaline species, such as amines. Neither assumption is true. This may lead operators to conclude perhaps that a higher than anticipated bulk water pH level (caused by the presence of amine) should be rectified by the addition of MSP. This action may lower localized Na P04 ratios below 2.2 1.0, producing acid phosphate corrosion (phosphate wastage) and resulting in tube thinning and ultimately tube failure. 

A final step in boiler feedwater treatment consists of pH adjustment as a further aid to corrosion control. Usually the pH is adjusted to a range of 10 to 11 with trisodium phosphate (or combinations of caustic with sufficient mono- or disodium phosphate to form trisodium phosphate upon inadvertent evaporation of the water). This "coordinated phosphate" treatment is intended to preclude the environmental cracking of steel by free sodium hydroxide (caustic embrittlement), a catastrophic form of corrosion described in Chap. 6. Caustic carryover with the steam can present severe corrosion problems (Fig. 8.16). 

For both coordinated and congruent control, the pH depends upon the phosphate concentration and the sodium to phosphate ratio. Generally, however, phosphates are unsuitable for use at boiler pressures above 100 bar as their low solubility and high concentration factors developed lead to corrosive conditions. 

The problem of corrosion, because of high localized NaOH concentrations, is generally attacked by one of a number of methods, all of which rely on proper ratios of various salts and alkalinity in the boiler water. Thus, the need for close control of boiler-water composition and frequent analysis to verify the control becomes apparent. The pH control situation is very complicated because, while the hydroxyl ion will passive the surface, too much will cause cracking. The problem then is to use a system that substitutes something else for most or all of the NaOH as a source of alkalinity. The coordinated pH approach rests upon the premise that the alkaline pH should come from trisodium phosphates as much as possible, rather than from NaOH. 

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