Forms of Metallic Corrosion
41
5. The gaps along the periphery of tanks mounted on masonry platforms should be closed with tar or bitumen to avoid seepage of rainwater. Vessels and tanks should be designed to provide complete
drainage, thereby preventing the buildup of solid deposits in the
bottom.
6. Regular inspections and removal of deposits should be scheduled.
3.5 Pitting Corrosion
Pitting corrosion is in itself a corrosion mechanism but is also a form of corrosion often associated with other types of corrosion mechanisms. It is characterized by a highly localized loss of metal. In the extreme case, it appears as
a deep, tiny hole in an otherwise unaffected surface. The initiation of a pit is
associated with the breakdown of the protective film on the metal surface.
The depth of a pit eventually leads to a through penetration or a massive
undercut of the thickness of a metal part. The width of the pit may increase
with time but not to the extent to which the depth increases. 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 evidence as to the extent of the damage. Pitting may result in the perforation of
a water pipe, making it unusable even though a relatively small percentage
of the total metal has been lost due to rusting.
Pitting can also cause structural failure from localized weakening effects
even though there is considerable sound material remaining. Pits may also
assist in brittle failure, fatigue failure, environment-assisted cracking like
stress corrosion cracking (SCC), and corrosion fatigue by providing sites of
stress concentration.
The main factor that causes and accelerates pitting is electrical contact
between dissimilar metals, or between what are termed “concentration cells”
(areas of the same metal where oxygen or conductive salt concentrations in
water differ). These couples cause a difference of potential that results in an
electric current flowing through the water or across the moist steel, from the
metallic anode to the nearby cathode. The cathode may be brass or copper,
mill scale, or any other portion of the metal surface that is cathodic to the
more active metal areas. However, when the anodic area is relatively large
compared with the cathodic area, the damage is spread out and is usually
negligible. When the anodic area is relatively small, the metal loss is concentrated and may be serious. For example, it can be expected when large
areas of the surface are generally covered by mill scale, applied coatings, or
deposits of various kinds, but breaks exist in the continuity of the protective
41
5. The gaps along the periphery of tanks mounted on masonry platforms should be closed with tar or bitumen to avoid seepage of rainwater. Vessels and tanks should be designed to provide complete
drainage, thereby preventing the buildup of solid deposits in the
bottom.
6. Regular inspections and removal of deposits should be scheduled.
3.5 Pitting Corrosion
Pitting corrosion is in itself a corrosion mechanism but is also a form of corrosion often associated with other types of corrosion mechanisms. It is characterized by a highly localized loss of metal. In the extreme case, it appears as
a deep, tiny hole in an otherwise unaffected surface. The initiation of a pit is
associated with the breakdown of the protective film on the metal surface.
The depth of a pit eventually leads to a through penetration or a massive
undercut of the thickness of a metal part. The width of the pit may increase
with time but not to the extent to which the depth increases. 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 evidence as to the extent of the damage. Pitting may result in the perforation of
a water pipe, making it unusable even though a relatively small percentage
of the total metal has been lost due to rusting.
Pitting can also cause structural failure from localized weakening effects
even though there is considerable sound material remaining. Pits may also
assist in brittle failure, fatigue failure, environment-assisted cracking like
stress corrosion cracking (SCC), and corrosion fatigue by providing sites of
stress concentration.
The main factor that causes and accelerates pitting is electrical contact
between dissimilar metals, or between what are termed “concentration cells”
(areas of the same metal where oxygen or conductive salt concentrations in
water differ). These couples cause a difference of potential that results in an
electric current flowing through the water or across the moist steel, from the
metallic anode to the nearby cathode. The cathode may be brass or copper,
mill scale, or any other portion of the metal surface that is cathodic to the
more active metal areas. However, when the anodic area is relatively large
compared with the cathodic area, the damage is spread out and is usually
negligible. When the anodic area is relatively small, the metal loss is concentrated and may be serious. For example, it can be expected when large
areas of the surface are generally covered by mill scale, applied coatings, or
deposits of various kinds, but breaks exist in the continuity of the protective