Page 314 - Corrosion Engineering Principles and Practice

284 C h a p t e r 8 C o r r o s i o n b y W a t e r 285

Water has been treated for thousands of years, but only when the
connection between bacteria in sewage and severe epidemics was
made were procedures developed for safe water in the nineteenth
century [8]. Today the World Health Organization (WHO), European
Union (EU), and the United States Environmental Protection Agency
(U.S. EPA), along with many other organizations, have a well-defined
set of limits and standards for microorganisms and toxic substances
in drinking water.
The resistivity of potable water is usually between 1000 and 5000
Ω cm. By definition, certain mineral constituents are also restricted.
For example, the chlorinity will be not more than 250 ppm chloride
ion in the United States or 400 ppm on an international basis.
Distilled or Demineralized Water
The total mineral content of water can be removed by either distillation
or mixed-bed ion exchange. The level of purification may be described
qualitatively (e.g., triple-distilled water). However, the most accepted
description for both distilled and demineralized water is in terms of
its specific conductivity or resistivity. Demineralization actually
removes the dissolved minerals and represents the most efficient
treatment of hard water. The two most common and cost-effective
methods are deionization and reverse osmosis.
Deionization removes minerals by passing water through a mixed
resin bed in which two ion exchange resins selectively remove both
cations and anions. Cations are replaced by hydronium ions and
anions are replaced by hydroxyl ions. Mixed-bed units are extremely
effective in reducing hardness levels to almost zero ppm. Such water
exhibits a zero ECA* value (neutral ionic charge). Resin beds need to
be flushed or backwashed on a regular basis to remove all contaminants
and to prevent microbiological interference. Beds also need to be
regenerated on a regular basis.
Reverse osmosis forces water through a semipermeable membrane
under high pressure and varying flow conditions. This process can
remove as much as 95 percent of dissolved minerals. Reverse osmosis
units are often used in conjunction with a water softener pretreatment
stage to reduce the demand on the osmosis process itself. Filters need
to be flushed and replaced as necessary. Microbiological contamination
also needs to be regularly monitored.

Steam Condensate
Steam condensate produced from industrial steam approaches
distilled water in purity, except for contamination by dissolved
gases and the deliberate presence of additives (e.g., neutralizing
or filming amines). Corrosion in steam condensate systems is

* ECA (electrokinetic charge) is an indication of the charge strength of the water
(degree of anionic or cationic charge).