A series of rare earth organic compounds pioneered by our group have been shown to provide a viable alternative to the use of chromates as corrosion inhibitors for some steel and aluminium applications. For example we have shown that the lanthanum 4-hydroxy cinnamate offers excellent corrosion mitigation for mild steel in aqueous environments while rare earth diphenyl phosphates offer the best protection in the case of aluminium alloys. In both cases the protection appears to be related to the formation of a nanometre thick interphase occurring on the surface that reduces the electrochemical processes leading to metal loss or pitting. Very recent work has indicated that we may even be able to address the challenging issue of stress corrosion cracking of high strength steels. Furthermore, filiform corrosion can be suppressed when selected rare earth inhibitor compounds are added as pigments to a polymer coating. There is little doubt from the work thus far that a synergy exists between the rare earth and organic inhibitor components in these novel compounds. This paper reviews some of the published research conducted by the senior author and colleagues over the past 10 years in this developing field of green corrosion inhibitors.

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We describe a series of novel compounds designed to combat the bacterial growth that leads to microbially induced corrosion on steel in the marine environment. A synergistic effect of the ionic components in these dual active organic salts is demonstrated.

Speciation of the inhibitor lanthanum 4-hydroxy cinnamate in solution as a function of pH and concentration has been evaluated and correlated with corrosion inhibition efficiency for AS1020 steel in an aqueous chloride solution. A combination of nuclear magnetic resonance (NMR) spectroscopy, electrospray mass spectrometry (ESMS), potentiodynamic polarization, scanning electron microscopy (SEM) and profilometric techniques were found to be useful in the characterization of this system. Potentiodynamic polarization experiments established that lanthanum 4-hydroxy cinnamate inhibits well over a range of pH and concentration conditions, but more interestingly the mechanism of inhibition is dependent on the environment. It is also evident from NMR and ESMS that the species in solution are dependent on pH, and this impacts the corrosion inhibition efficiency and mechanism. These data give an insight into the types of species present in a given environment.

Corrosion protection by lanthanum hydroxy cinnamate (La(4OH-cin) 3 ) in a polyurethane based varnish coating for mild steel has been investigated. Filiform scribe tests, energy-dispersive X-ray spectroscopy (EDXS) and potentiodynamic polarisation (PP) techniques have been powerful tools to better understand the corrosion process at defects and under the coating. Filiform scribe tests showed that La(4OH-cin) 3 , as a pigment in a coating, inhibited the initiation and propagation of both delamination and filiform corrosion (FFC) on coated steel. The PP experiments provided an insight into the fundamental mechanism of FFC. The results suggest that La(4OH-cin) 3 behaves as a mixed inhibitor and stifles the initiation and propagation of FFC. In this paper, the theory of delamination leading to FFC and the likely mechanism of inhibition by the La(4OH-cin) 3 will be discussed.