Coordinated pH-phosphate control
Coordinated pH-phosphate control was introduced in 1942 as a method of protecting boiler tubes from "caustic embrittlement," as well as protection from the effects of condenser in-leakage, primarily water hardness contaminants.
Boiler systems containing copper alloys should have a condensate/feedwater pH within the range of 8.8 - 9.2. All-steel boiler systems are best protected when the condensate/feedwater pH is within the 9.2 - 9.6 range.
A compromise condensate/feedwater pH control range of 8.8 to 9.2 is usually established where both metals are present.
The basis of a phosphate- pH control is that sodium phosphates are pH buffers.
The addition of phosphoric acid or any one of the orthophosphates to water produces a hydrolysis reaction that yields phosphate ions and hydrogen ions, sodium ions, hydroxide ions or combinations, while caustic hydrolyzes to sodium and hydroxide ions.
Within a pH range of from about 9 to about 10.5 the distribution of the
different ionic species of phosphate is at most 1 or 2 % of the mono- or
tribasic phosphate ions, while the dibasic hydrogen phosphate ion, HPO4-2,
is more than 98% present.
Thus substantially all phosphates that are added to water within that pH range will hydrolyze to the dibasic ion., and Tribasic sodium phosphate will hydrolyse releasing sodium hydroxide.
The solution (boiler water) pH that will result from the addition of the various orthophosphates therefore can be predicted, and graphs showing the phosphate-pH relationship in terms of phosphate concentration as PO4 (in ppm) versus pH of an aqueous solution for various Na:PO4 ratios are available in boiler literature. (Since orthophosphates are comprised of sodium and phosphate in molar ratios of 1, 2 and 3, it is common practice to describe an orthophosphate or a mixture of orthophosphates in terms of its sodium-to-phosphate molar ratio, Na:PO4). Sodium hydroxide generated solely by the hydrolysis of trisodium phosphate is sometimes called "captive" because it will revert to trisodium phosphate at any site of localized evaporative concentration.
Such reversion of sodium hydroxide to trisodium phosphate in confined areas such as within and/or under surface deposits avoids the formation of pockets of concentrated sodium hydroxide and thus prevents caustic-gouging type of metal attack.
A complete evaporation to dryness would leave a residue of trisodium phosphate free of sodium hydroxide, but incomplete evaporation is the more likely condition beneath a porous deposit.
Incomplete evaporation produces a liquid underneath the deposit that is rich in sodium hydroxide, particularly if incipient localized corrosion is already occurring. Thus the maintenance of a 3:1 sodium phosphate ratio in boiler water may not provide positive protection against caustic-concentration-type corrosion damage.
One form of pH/phosphate control maintains a Na:PO4 ratio that does not exceed 2.6. (A Na:PO4 ratio of 2.6 corresponds to a 3:2 blend of trisodium and disodium phosphate.)
The differing hydrolysis effects of different sodium phosphates when selectively adjusting pH, PO4, or both, to keep pH and PO4 coordinates within the desired range are shown in a control diagram discussed below.
The primary objective of a pH/phosphate program, whether of the Coordinated or Congruent Control type, is controlling the presence or absence of free sodium hydroxide.
At a 3.0:1 Na:PO4 mole ratio or greater, only the equivalent of trisodium phosphate plus caustic exists. At Na:PO4 mole ratios below 3.0:1 (down to a 2.0:1 mole ratio), a mixture of disodium and trisodium phosphate is present.
Water Services Ltd offers a full range products for coordinated phosphate control, under the trade name of WSB COP-2XXX. Please contact us for more details.