Page 324 - Corrosion Engineering Principles and Practice

294 C h a p t e r 8 C o r r o s i o n b y W a t e r 295

through the bed, sodium ions are exchanged for the calcium and
magnesium ions which comprise the water hardness. Other cations
are also exchanged. The water then exits from the Zeolite bed
softened and at a higher pH than it entered. The beds are regenerated
intermittently with sodium chloride solution to backwash the
hardness salts and reform the sodium salt.
For purer water quality, the water may be totally demineralized by
mixed beds of polymeric resins which exchange in turn hydrogen ions
for all cations and hydroxyl ions for all anions, effectively producing
pure H O from a raw water stream. Such highly purified waters are
2
required for boilers operating from about 6 MPa and for all nuclear
boilers (to avoid radioactive half-life of water-borne salts).
The softened water is now in its most corrosive state, being still
saturated with DO and having no hardness to form a protective
scale. For reasons of economy, the removal of DO is usually first
effected in part by thermomechanical deaeration. The boiler
feedwater is preheated, then flashed in a deaerator to remove any
free carbon dioxide and most of the dissolved oxygen. Then the last
traces of DO are chemically scavenged with sodium sulfite (Na SO )
2
3
that reacts with oxygen to form sodium sulfate (Na SO ) as
4
2
described in Eq. (8.22):
2 Na SO + O 2 → 2 Na SO (8.22)
2
2
4
3
or, if a complete absence of solid precipitates is desired, with hydrazine
(N H ) that forms water and gaseous nitrogen in the presence of
2
2
oxygen:
2 N H + O → 2 H O + 2 N (g) (8.23)
2
2
2
2
2
Both sulfite and hydrazine are available in catalyzed form to
promote more rapid reaction rates. A boiler designed to be operated
with a catalyzed oxygen scavenger must never be operated on the
uncatalyzed grades, or severe corrosion will be encountered in the
economizers or even the steam drum.
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).
Nuclear requirements are such that zero solids treatment is
required, precluding the addition of sodium salts and necessitating
the use of ammonium hydroxide for pH adjustment.