Page 176 - Corrosion Engineering Principles and Practice

150 C h a p t e r 6 R e c o g n i z i n g t h e F o r m s o f C o r r o s i o n 151

Corrosion fatigue,
23%

General corrosion,
22%
Other,
8%

High temperature
2%
Erosion,
cavitation, 6% Stress
corrosion
Intergranular cracking, 19%
corrosion, 8%
Pitting, 11% Hydrogen
embrittlement, 1%
(b)
FIGURE 6.2 (continued)

data for two large chemical plants on two different continents, one
located in Germany [Fig. 6.2(a)] and the other in the United States
[Fig. 6.2(b)] [3].

6.2 General or Uniform Attack
Uniform corrosion corresponds to the corrosion attack with the
greatest metal weight loss and is a common sight where steel
structures are abandoned to rust (Fig. 6.3). In fact, the rich hues
produced by the corrosion of some metals have been put to use in
notable outdoor applications, for example, copper as a long-lasting
roofing material and weathering steel in buildings and sculptures
(Fig. 6.4). From a corrosion inspection point of view, uniform attack is
relatively detectable and its effects predictable hence it is deemed to
be less troublesome than other forms of corrosion unless the corroding
material is hidden from sight. The internal corrosion of pipeline, for
example, or the corrosion of hidden components and that of any other
buried or immerged structures are good examples that even the
simplest corrosion process needs to be monitored.
Designing in a system a corrosion allowance based on the possible
loss of a material thickness is one of the simplest methods for dealing
with uniform attack. Ultrasonic inspection has been used for decades
to measure the thickness of solid objects. A piezoelectric crystal serves
as a transducer to oscillate at high frequencies, coupled directly or
indirectly to one surface of the object whose thickness is to be measured.
The time the wave of known velocity takes to travel through the
material is used to determine its thickness. Since the late 1970s,