Page 340 - Corrosion Engineering Principles and Practice

310 C h a p t e r 8 C o r r o s i o n b y W a t e r 311

Corrosion inhibition in recirculated cooling water systems
historically depended on the oxidizing inhibitors such as chromates
and nitrites. These days are gone since nitrites have been found to be
highly conducive to organic growths and chromates have been
phased out for environmental toxicity reasons.
The higher the chlorinity of the water, the more it requires a good
dosage of corrosion inhibitors. Traditional glassy metaphosphates
lose much of their effectiveness because they revert in time to
orthophosphates. There is also an attendant possible danger of
precipitation of tricalcium phosphate. This is a function of pH,
calcium, orthophosphate, and solids concentration of the water.
Silicates have also been used in water systems, but tend to have an
adverse effect on heat transfer because of the deposition of relatively
heavy films.

8.6.2 Scale Control
Encrustation of tubing, boilers, coils, jets, sprinklers, cooling towers,
and heat exchangers arise wherever hard water is used. Scale
formation can greatly affect heat transfer performance. One mm thick
scale, for example, can add 7.5 percent to energy costs, while 1.5 mm
adds 15 percent and 7 mm can increase cost by over 70 percent. Many
factors can affect scaling. Scaling, which is basically the deposition of
mineral solids on the interior surfaces of water lines and containers,
most often occurs when water containing the carbonates or bicarbonates
of calcium and magnesium is heated. There are basically three alternatives
to prevent scale deposition:

• pH control
• the addition of scale inhibitors
• removal of scaling species

One of the most effective methods for controlling crystallization
fouling is by adding chemical inhibitors to a scaling water. Commonly
used antiscalants are derived from chemicals such as condensed
polyphosphates, organophospates, and polyelectrolytes.
Scale control in open recirculated systems is affected primarily
by limiting the concentration of the scale-forming species and
related parameters, especially by controlled acid additions to
bring the water to a suitable pH range. Many of the commercial
inhibitor formulations include scale-controlling additives (e.g.,
polyphosphates and chelating agents).
8.6.3 Microorganisms
Algae and slimes are naturally occurring where sunlight and airborne
contamination are to be expected. Slimes normally contain fungi,
yeasts, bacteria, and entrapped quantities of inorganic and/or organic
material. Some of these are described in more details in Chap. 10.