Somers A.E., Deacon G.B., Hinton B.R.W., MacFarlane D.R., Junk P.C., Tan M.Y.J., Forsyth M.

In 2002, our group and collaborators began initial investigations on the use of rare-earth carboxylates as non-toxic and environment-friendly corrosion inhibitors for mild steel. This was followed by a more comprehensive study, reported in 2004 by Blin et al., in which a range of such carboxylate compounds were investigated. This study identified lanthanum 4-hydroxycinnamate, La(4-OHcin)3 as a promising compound. In the review presented here our more recent investigations on mild steel corrosion inhibitors with structures closely related to La(4-OHcin)3 are presented.

In another study, Lee investigated the effect on corrosion of subtle changes to the La(4-OHcin)3 structure. Seter et al. found that small structural changes could have a major effect on the inhibition performance.

Nam et al. investigated cerium, lanthanum and praseodymium 4-hydroxycinnamate as corrosion inhibitors for mild steel in carbon dioxide atmospheres in sodium chloride solution. In this particular situation, Pr(4-OHcin)3 led to the largest reduction in corrosion current.

A totally organic complex, imidazolinium 4-hydroxycinnamate (Imn 4-OHcin) has been investigated with the aim of developing a compound that can inhibit both corrosion and microbial growth. This compound was found to inhibit mild steel corrosion across a wide pH range and was particularly effective at a pH of 2.

We have also been investigating a rare-earth compound with an alternative carboxylate structure to the cinnamate; 3-(4-methylbenzoyl)propionate(mbp). This ligand differs from 4-hydroxycinnamate by having a carbonyl group present, which may give an extra anchor point to a metal surface when forming a barrier coating. A range of rare-earth mbp complexes was investigated, with Nd(mbp)3 resulting in the largest reduction in corrosion current density at a concentration of 0.125 mM.

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