Page 323 - Corrosion Engineering Principles and Practice

294 C h a p t e r 8 C o r r o s i o n b y W a t e r 295

Chlorides in the water can cause corrosion, particularly in the case
of stainless steels. Typically 304 is satisfactory up to about 200 ppm
chlorides, while 316 can withstand around 1000 ppm and 4.5 percent
molybdenum austenitic stainless steels and duplex stainless steels are
known to have suffered from crevice attack at 2000 to 3000 ppm
beneath fouling. Titanium and the six percent molybdenum stainless
steels have been shown to resist crevice attack in seawater (1900 ppm
chlorides) under deposits.
Operational problems can cause corrosion of stainless steel and
copper alloy heat exchangers. Stagnant water left from hydrotesting or
from inadequate draining at shutdown can cause fouling and/or
microbiologically influenced corrosion (MIC). The design and production
of the tube to tubesheet joint are critical factors in the successful use of
shell and tube heat exchangers. Roll-leaks can be caused by tubes being
pulled out of the tubesheet under the action of thermal stresses. This
is particularly so where different metals with different expansion
coefficients are used for the tubes and the tubesheet. This effect can be
prevented by using an expansion joint in the shell. Properly designed
and executed seal or strength welds may prevent leakage at this joint.
Seamless, as-welded, bead-worked (locally cold-worked) or cold-drawn
joints are equally likely to perform well in most services. In critical
applications or where localized corrosion may be initiated by surface
defects, adequate inspection and testing are recommended regardless of
method of tube manufacture.

8.5 Steam Generating Systems
The greatest use of high-temperature water and steam is in electrical
power generation. Historically, fossil fuels (i.e., wood, coal, gas, and
oil) were used almost exclusively to heat water and make steam until
the introduction of nuclear-power steam generators in the second
part of the twentieth century. The two types of power plants have
much in common, but are sufficiently different to be discussed
separately. Both, however, presuppose technically advanced water
treatment and control for successful operation.

8.5.1 Treatment of Boiler Feedwater Makeup
Boiler feedwater make-up and boiler feedwater must be softened to
prevent scaling and deaerated to reduce the water corrosivity. The
extent of the treatment depends on the specific requirements based
on a boiler operating temperature and pressure ranges.
A number of lime-softening treatments were used in the past,
but these have given way to more sophisticated treatments. Probably
the most common for boilers up to 2.8 to 4.0 MPa is Zeolite softening.
In this treatment, a sodium salt of a long-chain polymeric organic
molecule comprises the ion exchange bed. As the feedwater passes