Page 41 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

LOCALIZED CORROSION 19

dissolution of the metal decreases as the solubility of the oxygen increases. The metal-
lic ions oxidize and form compounds or corrosion products. The zones are called
tails.
Filiform corrosion of AZ91 magnesium alloy involves a corrosion mechanism
different from the conventional mechanism. In this case, dissolved oxygen is not nec-
essary, and the filiform corrosion propagation is fueled by hydrogen evolution at the
filament head and is controlled by mass transfer because of the salt film on the tip of
the filament (20, 21).

1.4.5 Breakdown of Passivation
Pitting and crevice corrosion are usually associated with the breakdown of passivity.
During pitting corrosion of passive metals and alloys, local metal dissolution leads to
cavities within passivated surface area. Pitting corrosion usually occurs in the pres-
ence of chlorides. Pitting may also occur in pure water as in the case of carbon steel
at high temperatures or aluminum in nitrate solutions at high potentials. In all these
forms of localized corrosion, both active and passive states are stable on the same
metal surface over an extended period of time so that local pits can grow to a larger
size (22).
It is necessary to exceed the critical anodic potential (23) E for the electrochem-
bd
ical breakdown of passivation by pitting and consists of these factors: (i) presence
of halides at the interface; (ii) induction time for the initiation of the breakdown pro-
cess, leading to localized conditions that may increase the localized corrosion current
density; (iii) development of favorable conditions inside the pits for propagation when
the local sites become immobile and localized at certain sites. Electrochemical break-
down of some metal oxides is possible in the case of copper, lead, and tin cathodically
to metal while ferric oxide is reduced to the ferrous ion in aqueous solutions. Zinc
and aluminum oxides are not cathodically reducible and in these cases hydrogen is
reduced. The vigorous evolution of hydrogen assisted by electron conducting zinc
oxide can accelerate the breakdown of passivity.
Among metals there are differences in composition and stoichiometry of the oxide
films. Halides such as chlorides play an important role in the growth and breakdown
of passive films. Borates help stabilize the oxide film. Chloride ions cause severe
localized corrosion such as pitting. Well-developed pits have high chloride ion con-
centration and low pH. Pitting can be random and amenable to stochastic (statistical)
theory and very sensitive to experimental parameters such as induction time and elec-
trochemical properties, which are difficult to reproduce. Electrochemical noise (EN)
can clarify the initial conditions for pit initiation (24).
The tendency of halides to form metal halide complex is very important in under-
standing the stabilization of corrosion pit by prevention of the repassivation of a defect
site within the passive layer. Among the halides, fluoride forms strong complexes with
metals. The resistance of chromium to localized corrosion is because of slow disso-
lution kinetics of Cr(III) salts. Higher-valence oxides are the best passivators (films)
because of their slow rates of dissolution.