The influence of organic structure and rare earth metal cation on the corrosion efficiency observed on AS1020 steel compared with La(4OHCin)3

Marianne Seter,Glen Deacon,Bruce Hinton,Winnie W Lee,Peter Junk,Maria Forsyth,Gaetan M A Girard

doi:10.3934/matersci.2015.1.1

Abstract

Whilst the corrosion protection of steel in aqueous chloride environments by the rare earth inhibitor lanthanum 4-hydroxycinnamate is well known, the influence of the structural variation of the organic component as well as the nature of the metal centre has not previously been addressed. Herein we show that praseodymium 4-hydroxy cinnamate is comparable to its lanthanum counterpart in aqueous solution. On the other hand, cerium 4-hydroxycinnamate and lanthanum 2-hydroxycinnamate show poor corrosion protection performance while lanthanum 3-hydroxycinnamate provides a level of inhibition between these. These differences are shown to be related to the speciation in solution and are postulated to be linked to steric influences which are likely to affect the bonding environment within the rare earth compound itself, as well as its bonding with the steel substrate.

Highlights

In order to ameliorate the deterioration of metal substrates from the effects of harsh environments, environmentally friendly inhibitors are an ideal alternative to the carcinogenic and toxic chromates [1−4]
The observation that slight cathodic inhibition was more evident under acidic conditions, and that this correlated with an excess of the LaCl4- species, strongly suggests that the speciation of La(4OHCin)3 is critical to the anodic nature and high efficiency of the inhibition and confirms the synergy of the organic and REM components previously suggested by Forsyth et al [17]
We have shown that the nature of the metal – ligand complexes in solution are critical for the corrosion inhibition efficiency; in particular the presence of species such as [LnL4]- and [LnL3Cl]

Summary

Introduction

In order to ameliorate the deterioration of metal substrates from the effects of harsh environments, environmentally friendly inhibitors are an ideal alternative to the carcinogenic and toxic chromates [1−4]. Lanthanum 4-hydroxycinnamate in solution has shown remarkable corrosion inhibition properties with mild steel substrates [15] This compound performs better as an inhibitor of steel at higher concentrations and under alkaline conditions. Under acidic pH conditions dissociation of the lanthanum 4-hydroxycinnamate, (La(4OHCin), occurs where the predominant species are lanthanum chloride [LaCl4]- and 4-hydroxylcinnamic acid as separate entities in solution. This leads to a rather low mixed inhibition effect compared with the strong anodic inhibition observed at higher pH values when the complex [LaL4]- is present [15]. The observation that slight cathodic inhibition was more evident under acidic conditions (except for high concentrations, i.e 1000 ppm where the inhibition was via both anodic and cathodic processes), and that this correlated with an excess of the LaCl4- species, strongly suggests that the speciation of La(4OHCin) is critical to the anodic nature and high efficiency of the inhibition and confirms the synergy of the organic and REM components previously suggested by Forsyth et al [17]

Methods

Results

Conclusion