Page 398 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

376 CONSEQUENCES OF CORROSION

The following equations represent the anodic reactions:

0
−
Fe –2e → Fe 2+
−
1 ∕ 2O + H O + 2e → 2OH −
2
2
0
1
Fe + ∕ 2O + H O → Fe 2+ + 2OH −
2
2
2+ −
Fe + 2OH → Fe(OH)
2
Pit propagation can become autocatalytic when one of the products of the reac-
tion accelerates it. Pitting occurs in three stages: namely, initiation, propagation, and
termination.
Pit initiation depends on: (i) imperfections or defects in the metal oxide, passive,
or protective layer between the metal and the environment; (ii) exposure of the metal
to an aggressive medium.
Pit propagation consists of corrosion driven by the potential differences between
the anodic base of the pit and the surrounding cathodic area. Furthermore, the reaction
products autocatalyze the propagation and make the environment more aggressive.
Pit propagation may terminate because of an increased resistance to local cell. This
can occur because of the filling up of the pit with corrosion product or the filming
of cathodic area with corrosion product. Pitting will cease when the metal is dry
and may reinitiate when the metal becomes wet. Pitting tends to occur at defects or
imperfections in the passive layer. Defects may be naturally occurring or as a result
of mechanical damage. In fluid containers, stagnation of liquid favors pitting. The
rate of pitting decreases as the number of pits increase following the availability of a
reduced cathodic area.
The storage tank failed by pitting corrosion from the inside because of the presence
of water. The source of water is not clear, although it is speculated that condensation
of water vapor could have been responsible for it (19).

5.3.7.4.15 Underground Corrosion of Water Pipes in Cities A total of 17 pipe fail-
ures and soil samples were studied by metallurgical and analytical evaluation and the
conclusions are: in almost all the failures, including the failure of the service saddle
on an asbestos–cement 10-in. water pipe, corrosion from the soil side was to a great
extent responsible for the failures.
The corrosion rates of cast iron pipes were in the range 0.21–0.58 mm/year in
a typical environment with the average rate of 0.41 mm/year. The failure appeared
to be some form of pitting, leading to perforation of the pipe walls. Graphitic cor-
rosion involving selective leaching of steel matrix resulting in a loss of strength and
mechanical properties ultimately leading to pipe rupture was observed. Both localized
corrosion and graphitization resulted in wall thinning and cracks in the pipe.
Loss in mechanical properties such as reduced tensile strength of cast iron pipes
contributed to the pipe failures.
The corrosion rate of ductile iron pipes in soils was in the range 0.62–2.5 mm/year
with an average of 1.11 mm/year, which is twice the corrosion rate of cast iron pipe.