Page 296 - Corrosion Engineering Principles and Practice

270 C h a p t e r 8

• Organic matter, including that of both animal and vegetable
origin, oil, trade waste (including agricultural) constituents
and synthetic detergents
• Microbiological forms, including various types of algae and
slime-forming bacteria

Table 8.5 lists typical analytical determinations made in the
examination of most natural waters. The list in Table 8.5 describes
the general categories of substances, the difficulties commonly
encountered by their presence and water treatment methods used to
alleviate the difficulties.
The pH of natural waters errs rarely outside a fairly narrow range
between 4.5 and 8.5. High values, at which corrosion of steel may be
suppressed, and low values, at which gaseous hydrogen evolution
occurs, are not often found in natural waters. Copper exposed to
acidic waters might suffer a slight corrosion attack putting small
amounts of copper ions in solution that may in turn cause green
staining of fabrics and sanitary ware. In addition, redeposition of
copper on aluminum, a common radiator material, or on galvanized
surfaces might set up a very aggressive corrosion cell resulting in
severe pitting of the metal.
Pure water, without dissolved gases (e.g., oxygen, carbon
dioxide, and sulfur dioxide) does not cause undue corrosion attack
on most metals and alloys at temperatures up to the boiling point of
water. Even at temperatures of about 450°C, almost all of the
common structural metals, with the exception of magnesium and
aluminum, offer adequate corrosion resistance to high-purity water
and steam.
From a corrosion standpoint, a significant water component is
dissolved oxygen (DO) from ambient air. Oxygen acts both as a
cathodic depolarizer and as an oxidizer. As a cathodic depolarizer, DO
can remove hydrogen from the cathode during electrochemical
corrosion and accelerate the corrosion attack. As an oxidizer, DO can
be reduced on the metallic surface and participate directly to the
electrochemical processes as described in many examples of Chap. 5.
The effect of DO on the corrosion of carbon steel is illustrated
in Fig. 8.5 [10]. It should be noted in Fig. 8.5 that an increasing
temperature is accompanied by an increase in corrosion rate of
the steel due to faster reaction kinetics. The decreasing solubility
of oxygen with temperature and salinity depicted in Table 8.6
only explains the upper limits of each of the three curves in
Fig. 8.5.
The effect of oxygen on corrosion with increasing temperature is
also shown in Fig. 8.6 that compares the results obtained in a closed
vessel with those obtained with an open container that favored deaera-
tion of the water by ebullition [10]. In a closed vessel, the solubility of
DO increases with pressure and corrosion continues to increase with

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