Investigating the effect of solution pH on speciation of La(4OH-cin)3 with correlation to electrochemical behaviour and corrosion efficiency

The nature of the species in solution plays a major role on the effectiveness of the corrosion inhibitor on a steel substrate. The speciation of lanthanum 4-hydroxy cinnamate (La(4OHCin) ₃) in solution has been evaluated using experimental techniques composed of potentiodynamic polarisation, immersion tests, nuclear magnetic spectroscopy and mass spectroscopy. It is evident that the species in solution are dependent on pH and this impacts the corrosion inhibition mechanism and the efficiency. It was found that at a neutral pH of 5.5 the La(4OH-Cin)₃ behaves as a strong anodic inhibitor. Whereas, when the pH shifts to low (pH2.5) and/or high (pH8) the corrosion mechanism changes.

Effect of La (40H-Cin)3 on filiform corrosion of coated steel

The potential of lanthanum 4-hydroxy cinnamate to inhibit filiform corrosion on coated mild steel (AS1020) was investigated. The effectiveness and behaviour of this rare-earth inhibitor was examined with filiform corrosion scribe tests and Potentiodynamic Polarisation. The filiform scribe tests showed that lanthanum 4-hydroxy cinnamate, as a pigment in a coating, inhibited the initiation and propagation of both delamination and filiform corrosion on coated steel. The polarisation tests demonstrated that at pH 3, no significant inhibition was observed but a secondary passivation effect was present. At pH 9, inhibition on coated steel was found to be greater than that of the inhibition found at pH 6. The models of filament initiation and growth proposed by previous authors are also discussed.