Environmental Engineering Reference
In-Depth Information
Figure 3.5
Schematic illustration of the sprinkler system in which the malleable iron
clapper latch failed from galvanic attack.
3.4 PITTING
This form of corrosion is associated with the formation of pits, i.e., small holes
or cavities with surface diameter equal to or less than the depth. The depth eventu-
ally increases, leading to a thorough perforation or a massive undercut in the
thickness of the metallic part. The width of the pit may also increase with time,
but not to the extent to which the depth grows. Most often the pit opening remains
covered with the corrosion product, making it difficult to detect during inspection.
This, along with a negligible loss in weight or absence of apparent reduction in
the overall wall thickness, gives little idea about the extent of damage. Pitting
may thus lead to an unpredictable leakage in a pipeline or storage tank or the
rupture of a pressure pipe due to the reduction in wall thickness as a result of
extensive undercutting. Pits may also assist in brittle fracture, fatigue failure,
environment-assisted cracking like stress corrosion cracking, and corrosion fa-
tigue by providing sites of stress concentration.
3.4.1
Characteristic Features
1.
The attack is spread over small discrete areas. Pits are sometimes isolated
and sometimes close together, giving the area of attack a rough appearance.
2.
Pits usually initiate on the upper surface of the horizontally placed parts and
grow in the direction of gravity.
3.
Pitting usually requires an extended initiation period before visible pits ap-
pear.
4.
Pitting is autocatalytic in nature. The conditions prevailing inside the pit
makes it self-propagating without any external stimulus. Thus, once initiated,
the pit grows in an ever-increasing rate.
