Page 234 - Corrosion Engineering Principles and Practice

208 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 209

investigation providing information on the root cause of failure is
much more valuable than one merely establishing the corrosion
mechanism(s). Establishing the real causes of corrosion failures (often
related to human behavior) is a much more difficult task than merely
identifying the failure mechanisms.
In contrast to the traditional scientific mechanistic approach,
systems engineers prefer the “top-down” approach that broadens the
definition of the system and is more likely to include causes of
corrosion failures such as human behavior. This is consistent with the
lessons to be learned from the U.K. Hoar Report, which stated that
corrosion control of even small components could result in major cost
savings because of the effect on systems rather than just the
components [2].

7.2 Information to Look For
It is not always possible to anticipate the actual environment in which
a metallic structure will operate. Even if the initial conditions were
known completely, there is often no assurance that operating
temperatures, pressures, or even chemical compositions will remain
constant over the expected equipment lifespan. The complexity of
corrosion processes and their impact on equipment often buried or
out-of-sight complicates many situations, rendering simple life
prediction difficult.
Even corrosion tests carried out in controlled conditions often
yield results with more scatter than many other types of materials test
results because of a variety of factors, an important one being the
effect on corrosion rates due to minor impurities in the materials
themselves or in the testing environments [3].

7.2.1 Temperature Effects
In most chemical reactions, an increase in temperature is accompanied
by an increase in reaction rate. A rough rule-of-thumb suggests that the
reaction rate doubles for each 10°C rise in temperature. Although this
rule has many exceptions, it is important to take into consideration the
influence of temperature when analyzing why materials fail.
Changing the temperature of an environment can influence its
corrosivity. Many household hot-water heater tanks, for example,
were historically made of galvanized steel. The zinc coating offered a
certain amount of cathodic protection to the underlying steel, and the
service life was considered adequate. Water tanks seldom were
operated above 60°C. With the development of automatic dishwashers
and automatic laundry equipment, the average water temperature
was increased so that temperatures of about 80°C have now become
common in household hot water tanks.
Coinciding with the widespread use of automatic dishwashers
and laundry equipment was a sudden upsurge of complaints of