Page 238 - Corrosion Engineering Principles and Practice

212 C h a p t e r 7 C o r r o s i o n F a i l u r e s , F a c t o r s , a n d C e l l s 213

Contaminants may also drastically change the corrosion charac-
teristics of nitric acid. Although the conventional type 300 series aus-
tenitic stainless steels, such as annealed type 304L and as-welded
types 304L, 321, and 347, are resistant to nitric acid in many instances,
there is a general accretion of hexavalent chromium ions (Cr VI) if the
acid is recirculated with no provision for dilution elevated tempera-
tures, as with nitric acid condensers. In other instances, contamina-
tion of nitric acid with chloride ions results in their being oxidized to
nascent chlorine. This is the basis for aqua regia* used to dissolve
gold. In 42 percent acid, excessive corrosion of stainless steels results
when chloride ion contamination exceeds about 3000 ppm. High ini-
tial rates are observed to diminish with time, probably due to the loss
of volatile chlorine formed by reaction with the chlorides.

7.2.4 Presence of Microbes
Microbes are present almost everywhere in soils, freshwater, seawater,
and air. However, the mere detection of microorganisms in an
environment does not necessarily indicate a corrosion problem.
Nonetheless, it is well-established that numerous buried steel pipes
have suffered severe corrosion as the result of bacterial action.
In unaerated or anaerobic soils, this attack is attributed to the
influence of the sulfate-reducing bacteria (SRB). The mechanism is
believed to involve both direct attack of the steel by hydrogen sulfide
and cathodic depolarization aided by the presence of bacteria. Even in
aerated or aerobic soils, there are sufficiently large variations in aeration
that the action of SRB cannot be neglected. For example, within active
corrosion pits, the oxygen content becomes exceedingly low.
Bacteria, fungi, and other microorganisms can play a major part
in soil corrosion. Spectacularly rapid corrosion failures have been
observed in soil due to microbial action and it is becoming increasingly
apparent that most metallic alloys are susceptible to some form of
microbiologically influenced corrosion (MIC).
The presence of aggressive microbes may also be quite severe in
industrial water-handling systems, for example, cooling water and
injection water systems, heat exchangers, wastewater treatment
facilities, storage tanks, piping systems, and all manner of power
plants, including those based on fossil fuels, hydroelectric, and
nuclear [8]. Table 7.2 lists potential problem areas by industry [9].

7.2.5 Presence of Stray Currents
The corrosion resulting from stray currents coming from external
sources is similar to that from galvanic cells that generate their own
current. However, the amplitude of stray currents may be much

* Aqua regia refers to a mixture in a 3:1 ratio of hydrochloric (HCl) and nitric
(HNO ) acids.
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