Page 341 - Corrosion Engineering Principles and Practice

312 C h a p t e r 8 C o r r o s i o n b y W a t e r 313

Conditions at the base of even thin slime films (biofilms) can be
ideal for the growth of anaerobic bacteria, with high organic nutrient
status, no oxygen, low redox potential, and protection from biocides.
Figure 8.21 illustrates the steps in biofilm formation [18]. Sulfate
reducing bactreria (SRB), for example, can produce active sulfide
corrosion even in systems where the bulk liquid phase has a low
nutrient status, a high oxygen concentration, and will not support
growth of anaerobic aerobic bacteria [19]. Slime deposits can cause
oxygen concentration cell-type corrosion and pitting. If sloughed off,
they can contribute toward fouling of filters and other equipment
(Fig. 8.22).
Chlorination to residual chlorine content of a few tenths of a ppm
is the usual treatment for control of biological growths in open
recirculated systems. In order to discourage the development of
chlorine-resistant strains, it is advisable to surprise the microbes at
irregular intervals with other biocides (e.g., quaternary amines).
Other oxidizing biocides (e.g., chlorine dioxide and ozone) used in
municipal waters are normally too expensive for cooling water
systems.
Historically, copper sulfate had been used to control algae in
some specific applications. It is effective at concentrations as low as 1
ppm, but must be used at a sufficiently low pH to prevent precipitation
of insoluble copper hydroxide. The use of copper sulfate as a biocide
is incompatible with aluminum equipment because of deposition and
subsequent galvanic corrosion. Copper plating may also occur on
zinc, on galvanized steel, and even steel.

Aerobic bacteria
Organic matter 4 5
3
Anaerobic bacteria
2
1

Flow

Metallic surface

FIGURE 8.21 Steps in biofilm formation. Formation is initiated when small organic
molecules become attached to an inert surface (1) and microbiological cells are
adsorbed onto the resulting layer (2). The cells send out hair like exopolymers to
feed on organic matter (3), adding to the coating (4). Flowing water detaches some
of the formation (5), producing an equilibrium layer.