Page 338 - Corrosion Engineering Principles and Practice

308 C h a p t e r 8 C o r r o s i o n b y W a t e r 309

Physical procedures
In operation Reverse osmosis for demineralization by use of
diaphragms.
Transformation of the crystal structures of the
hardening causing substances:
• Magnetic field method by means of electrical
alternating or permanent magnet
• Electrostatic method by applied active anodes
Post-treatment Automatic cleaning of tubes heat exchangers by sponge
rubber balls or brushes without operating interrupt of
the plant.

TABLE 8.13 (continued)

8.6.1 Corrosion Inhibitors
By definition, a corrosion inhibitor is a chemical substance that, when
added in small concentration to an environment, effectively decreases
the corrosion rate. The efficiency of an inhibitor can be expressed by
a measure of this improvement as indicated in Eq. (8.24):
(CR − CR )
e
Inhibitor efficiency (%) = 100 uninhibited inhibited (8.24)
CR uninhibited
where CR uninhibited is the corrosion rate of the uninhibited system and
CR the corrosion rate of the inhibited system
inhibited
In general, the efficiency of an inhibitor increases with an increase
in inhibitor concentration (e.g., a typically good inhibitor would give
95 percent inhibition at a concentration of 0.008 percent and 90 percent
at a concentration of 0.004 percent). There is often some positive
synergism between different inhibitors and therefore mixtures are
commonly chosen in commercial formulations. The scientific and
technical corrosion literature has descriptions and lists of numerous
chemical compounds that exhibit inhibitive properties. Of these, only
very few are actually used in practice. This is partly because the
desirable properties of an inhibitor usually extend beyond those
simply related to metal protection. Considerations of cost, toxicity,
availability, and environmental friendliness are of considerable
importance.
The use of chemical inhibitors to decrease the rate of corrosion
processes is quite varied. In the oil extraction and processing industries,
for example, corrosion inhibitors have always been considered to be
the first line of defense against corrosion. Table 8.14 presents some
inhibitors that have been used with success in typical water
environments to protect the metallic elements of industrial systems.