© 2006 by Taylor & Francis Group, LLC

30 Corrosion Control Through Organic Coatings
2.3.2.2.3 Cathodic Inhibition Theory
In the previously described work, low levels of lead were found in the rust layer
near the paint-rust interface, within 30 µm of the rust-paint interface. Thomas
suggests that because lead salts do not appear to reach the metal substrate to inhibit
the anodic reaction, it is possible that lead acts within the rust layer to slow down
atmospheric corrosion by interfering with the cathodic reaction (i.e., by inhibiting
the cathodic reduction of existing rust [principally FeOOH to magnetite]) [33]. This
presumably would suppress the anodic dissolution of iron because that reaction
ought to be balanced by the cathodic reaction. No conclusive proof for or against
this theory has been offered.
2.3.2.2.4 Lead Soap/Lead Azelate Theory
Thomas looked for lead (as a constituent of lead azelate) at the steel-rust interface
in an attempt to confirm this theory. Samples coated with lead-based paint were
exposed for three years and then cross-sections were examined in a LAMMS;
however, lead was not detected at the interface. As Thomas points out, this finding
does not eliminate the mechanism as a possibility; lead could still be present but in
levels below the 100 ppm detection limit of the LAMMS [30,31]. Appleby and
Mayne have shown that 5 to 20 ppm of lead azelate is enough to protect pure iron
[25]. The levels needed to protect rusted steel would not be expected to be so low,
because the critical concentration required for anodic inhibitors is higher when
chloride or sulphate ions are present than when used on new or clean steel [35].
Possibly, a level between 20 and 100 ppm of lead azelate is sufficient to protect the
steel. Another point worth considering is that the amounts of lead that would exist
in the passive film formed by complex azelates, suggested by Appleby and Mayne,
has not been determined. The lead soaps/lead azelate theory appears to be the most
likely mechanism to explain how red-lead paints protect rusted steel.
2.3.2.3 Summary of Mechanism Studies
Formation of lead soaps appears to be the mechanism by which lead-based paints
inhibit corrosion of clean steel. When formulated with linseed oil, lead reacts with
components of the oil to form soaps in the cured film; in the presence of water and
oxygen, these soaps degrade to, among other things, salts of a variety of mono- and
di-basic aliphatic acids. The lead salts of azelaic, suberic, and pelargonic acid act
as corrosion inhibitors; lead azelate is of particular importance in LBP. These acids
may inhibit corrosion by bringing about the formation of insoluble ferric salts, which
reinforce the metal’s oxide film until it becomes impermeable to ferrous ions, thus
suppressing the corrosion mechanism.
The formation of lead soaps is believed to be the critical corrosion-protection
step for both new (clean) steel and rusted steel.
2.3.2.4 Lead-Based Paint and Cathodic Potential
Chen et al. tested red lead in an alkyd binder in both open circuit conditions and under
cathodic protection. They found that this coating gave excellent service in open circuit
conditions, with almost no corrosion and minimal blistering. At –1000 mV Standard
© 2006 by Taylor & Francis Group, LLC