Page 181 - Corrosion Engineering Principles and Practice

156 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 157

FIGURE 6.9 Pits in the galvanized water pipe that contributed to the erosion–
corrosion of the gas line and the subsequent leak in a sewer main that
caused the Guadalajara 1992 explosion [6]. (Courtesy of Dr. Jose M. Malo,
Electric Research Institute, Mexico)

April 1992, also caused a series of blasts that damaged 1600 buildings
and injured 1500 people. At least nine separate explosions were heard,
starting at approximately 10:30 a.m., ripping a jagged trench that ran
almost 2 km. The trench was contiguous with the city sewer system
and the open holes at least 6 m deep and 3 m across. In several
locations, much larger craters of 50 m in diameter were evident with
numerous vehicles buried or toppled into them. An eyewitness said
that a bus was “swallowed up by the hole.” Damage costs were
estimated at 75 million U.S. dollars.
The sewer explosion was traced to the installation of a water pipe
by a contractor several years before the explosion. This water pipe
leaked water on a gasoline line lying underneath. The cathodically
protected gasoline pipeline had a hole within a cavity (Fig. 6.9) and
an eroded area, all in a longitudinal direction. A second hole did not
perforate the internal wall. The galvanized water pipe obviously had
suffered stray current corrosion effects which were visible in pits of
different sizes [6]. The subsequent corrosion of the gasoline pipeline,
in turn, caused leakage of gasoline into a main sewer line.
Pitting cavities may fill with corrosion products and form caps
over the pit cavities sometimes creating nodules or tubercles (Fig. 6.10).
While the shapes of pits vary widely (Fig. 6.11) they are usually roughly
saucer-shaped, conical, or hemispherical for steel and many associated
alloys. The following are some factors contributing to initiation and
propagation of pitting corrosion:

• Localized chemical or mechanical damage to a protective
oxide film