Page 295 - Corrosion Engineering Principles and Practice

268 C h a p t e r 8 C o r r o s i o n b y W a t e r 269

monitoring program that does not consider these changes will have
an incomplete picture of the biology and chemistry of the system.
Fresh Water
Fresh water may come from either a surface or ground source, and
typically contains less than 0.1 percent sodium chloride (<1000 ppm). It
may be either “hard” or “soft” (rich or poor in calcium and magnesium
salts) and thus possibly forming insoluble curds with ordinary soap.
Actually, there are gradations of hardness, which can be estimated from
the Langelier or Ryznar indexes discussed later or accurately determined
by titration with standardized chelating agent solutions (e.g., versenates).
The two most important sources of fresh water are surface water
and groundwater. A portion of the rain or melting snow and ice at the
earth’s surface soaks into the ground while part of it collects in ponds
and lakes or runs off into creeks and rivers. This latter portion is termed
surface water. As the water flows across the land surface, the flow-
ing water dissolves some minerals and carries along finely divided
particles and organic matter in suspension. The character of the terrain
and the nature of the geological composition of the area influence the
nature and quantity of the impurities found in these surface waters.
That portion of water which percolates into the earth’s crust and
collects in subterranean pools and underground rivers is called
groundwater. This is the source of well and spring water. Underground
supplies of fresh water differ from surface supplies in three important
aspects, two of which are advantageous for industrial use. These are
a relatively constant temperature and the general absence of suspended
matter. However, groundwater may be higher in mineral content than
surface supplies in the same geographic area because of the added
solubilizing influence of dissolved carbon dioxide and the long
residence time.
The concentrations of various substances in water in dissolved,
colloidal, or suspended form are typically low but vary considerably.
A hardness value of up to 400 ppm of calcium carbonate, for example,
is sometimes tolerated in public supplies, whereas 1 ppm of dissolved
iron would be unacceptable. In treated water for high-pressure boilers
or where radiation effects are important, as in nuclear reactors, impurities
are measured in very small units such as parts per billion (ppb) or 1 mg
of contaminant per liter of water. Water analysis for drinking water
supplies is concerned mainly with pollution and bacteriological tests.
For industrial supplies a mineral analysis is of more interest. The
important constituents can be classified as follows [9]:

• Dissolved gases (oxygen, nitrogen, carbon dioxide, ammonia,
sulfurous gases)
• Mineral constituents, including hardness salts, sodium salts
(chloride, sulfate, nitrate, bicarbonate, and so forth), salts of
heavy metals, and silica