© 2006 by Taylor & Francis Group, LLC

28 Corrosion Control Through Organic Coatings
2.3.2.1 Mechanism on Clean (New) Steel
Appleby and Mayne [24,25] have shown that formation of lead soaps is the mechanism
used for protecting clean (or new) steel. When formulated with linseed oil, lead reacts
with components of the oil to form soaps in the dry film; these soaps degrade to,
among other things, the water-soluble salts lead of a variety of mono- and di-basic
aliphatic acids [26,27]. Mayne and van Rooyen also showed that the lead salts of
azelaic, suberic, and pelargonic acid were inhibitors of the iron corrosion. Appleby
and Mayne have suggested that these acids inhibit corrosion by bringing about the
formation of insoluble ferric salts, which reinforce the air-formed oxide film until it
becomes impermeable to ferrous ions. This finding was based on experiments in which
pure iron was immersed in a lead azelate solution, with the thickness of the oxide film
measured before and after immersion. They found that the oxide film increased 7%
to 17% in thickness upon immersion [25,28].
The lead salt of azelaic acid dissociates in water into a lead ion and an azelate ion.
To determine which element was the key in corrosion inhibition, Appleby and Mayne
also repeated the experiment with calcium azelate and sodium azelate [24,132]. Interestingly,
they did not see a similar thickening of the oxide film when iron was immersed
in calcium azelate and sodium azelate solutions, demonstrating that lead itself — not
just the organic acid — plays a role in protecting the iron. The authors note that 5 to
20 ppm lead azelate in water is enough to prevent attack of pure iron immersed in the
solution. They note that, at this low concentration, inhibition cannot be caused by the
repair of the air-formed oxide film by the formation of a complex azelate, as is the case
in more concentrated solutions; rather, it appears to be associated with the thickening
of the air-formed oxide film. ‘‘It seems possible that, initially, lead ions in solution may
provide an alternative cathodic reaction to oxygen reduction, and then, on being reduced
to metallic lead at the cathodic areas on the iron surface, depolarize the oxygen reduction
reaction, thus keeping the current density sufficiently high to maintain ferric film
formation. In addition any hydrogen peroxide so produced may assist in keeping the
iron ions in the oxide film in the ferric condition, so that thickening of the air-formed
film takes place until it becomes impervious to iron ions” [25].
2.3.2.2 Mechanism on Rusted Steel
Protecting rusted steel, rather than clean or new steel, may demand of a paint a
different corrosion mechanism, simply because the paint is not applied directly to
the steel that must be protected but rather to the rust on top of it. Inhibitive pigments
in the paint that require intimate contact with the metallic surface in order to protect
it may therefore not perform well when a layer of rust prevents that immediate
contact. Red-lead paint, however, does perform well on rusted steel. Several theories
about the protective mechanism of red-lead paint on rusted steel exist.
2.3.2.2.1 Rust Impregnation Theory
According to this theory, the low viscosity of the vehicle used in LBP allows it to
penetrate the surface texture of rust. This would have several advantages:
• Impregnation of the rust means that it is isolated and thereby inhibited in
its corroding effect.
© 2006 by Taylor & Francis Group, LLC