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. 

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  

In the previously described work, low levels of lead were found in the rust layer near the paint-rust interface, within 30 tm 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. 

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. 

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