The corrosion inhibition mechanism of new rare earth cinnamate compounds — electrochemical studies

A combination of linear polarisation resistance (LPR) and cyclic potentiodynamic polarisation (CPP) measurements demonstrated that the lanthanum-4 hydroxy cinnamate compound could inhibit both the cathodic and anodic corrosion reactions on mild steel surfaces exposed to 0.01 M NaCl solutions. However, the dominating response was shown to vary with inhibitor concentration. At the concentrations for which the highest level of protection was achieved, both REM-4 hydroxy cinnamate (REM being lanthanum and mischmetal) displayed a strong anodic behaviour for mild steel and their inhibition performance, including their resistance against localised attack, improved with time.

Electrochemical impedance spectroscopy (EIS) measurements and modelling were carried out so as to propose a simple electrical model and correlate the extracted parameters to the inhibition mechanism put forward for REM-cinnamate based compounds. The results supported the high corrosion inhibition performance of the compounds as well as the build-up of a protective film with time. Based on a two-layer model the results suggested that the upper layer of the inhibitor film seemed to offer less resistance to the diffusion of electrochemically active species than the highly resistive inner layer at the film/metal interface.

Corrosion mitigation of mild steel by new rare earth cinnamate compounds

Corrosion rate measurements based on weight loss (i.e., mild steel immersed for seven days in 0.01 M NaCl) and linear polarization resistance (LPR) techniques have shown that even low concentrations (200 ppm) of cerium and lanthanum cinnamates are able to significantly inhibit corrosion. Of all the compounds investigated in this work Ce(4-methoxycinnamate)3· 2 H2O and La(4-methoxycinnamate)3· 2 H2O compounds exhibited the greatest inhibition and, in comparison with the component inhibitors, a synergy was clearly observed. The mechanism of corrosion inhibition was investigated using cyclic potentiodynamic polarization (CPP) measurements. The results suggest that La(4-nitrocinnamate)3· 2 H2O and Ce(4-methoxycinnamate)3· 2 H2O behave as mixed inhibitors and improve the resistance of steel against localized attack.

Thermal and spectroscopic study of the 3,4-(methylenedioxy) cinnamate compounds of transition metals in the solid state

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Recent developments in corrosion inhibitors based on rare earth metal compounds

Rare earth organic compounds can provide an environmentally safe and non-toxic alternative to chromates as corrosion inhibitors for some steel and aluminium applications. For steel lanthanum 4-hydroxy cinnamate offers corrosion protection and reduces the susceptibility to hydrogen embrittlement. Recent work has also indicated that it inhibits the corrosion of steel in environments containing high levels of carbon dioxide. For aluminium alloys, cerium diphenyl phosphate provides excellent corrosion inhibition in chloride environments, and reduces susceptibly to stress corrosion cracking. Furthermore, for both steel and aluminium alloys filiform corrosion can be suppressed when rare earth inhibitor compounds are added as pigments to polymer coatings. The levels of inhibition observed are thought to be due to synergistic effects between the rare earth and organic parts of these novel compounds, and are related to the various species that may be present in the complex chemical conditions that develop in solution close to a metal surface. This paper reviews some of the published research conducted by the group at Deakin University over recent years.©2014 Institute of Materials, Minerals and Mining.

Liquid crystal alignment generated by linearly polarized UV light on photoactive low molecular mass compounds

Alignment films prepared from low molar mass photo-crosslinkable materials containing the cinnamate group can be used for aligning LCs after irradiating the films with linearly polarized UV light. The high contrast observed in the polarizing optical microscope between dark and bright images indicates that the alignment is quite uniform. As the photoreaction progresses. the average roughness of the films is increased. All the aggregate structures, 'lamellar crystals'. produced by the photo-crosslinking reaction are of a square shape.

The nature of the surface film on steel treated with cerium and lanthanum cinnamate based corrosion inhibitors

The corrosion inhibition mechanisms of new cerium and lanthanum cinnamate based compounds have been investigated through the surface characterisation of the steel exposed to NaCl solution of neutral pH. Attenuated Total Reflectance-Fourier Transform Infrared (ATR-FTIR) spectroscopy was used to identify the nature of the deposits on the metal surface and demonstrated that after accelerated tests the corrosion product commonly observed on steel (i.e. lepidocrocite, γ-FeOOH) is absent. The cinnamate species were clearly present on the steel surface upon exposure to NaCl solution for short periods and appeared to coordinate through the iron. At longer times the Rare Earth Metal (REM) oxyhydroxide species are proposed to form as identified through the bands in the 1400–1500 cm⁻¹ region. These latter bands have been previously assigned to carbonate species adsorbed onto REM oxyhydroxide surfaces. The protection mechanism appears to involve the adsorption of the REM–cinnamate complex followed by the hydrolysis of the REM to form a barrier oxide on the steel surface.

Synthesis and characterisation of rare earth complexes supported by para-substituted Cinnamate Ligands

The preparations and characterisations of a range of lanthanoid 4-(R)-substituted (4-Rcinn, R = OH, OMe, NO2, Cl), known to have good anticorrosion properties, are reported. The crystal structure of [Ce(4-OHcinn)3(MeOH)2(H2O)]·MeOH is a polymer, in which the cerium atoms are nonacoordinate, and adjacent cerium atoms are bridged by either two bridging bidentate or two bridging tridentate carboxylate ligands. Each cerium atom also has one monodentate 4-hydroxycinnamate ligand, one aqua ligand, and two methanol ligands.

New 'green' corrosion inhibitors based on rare earth compounds

A series of rare earth organic compounds pioneered by our group have been shown to provide a viable alternative to theuse of chromates as corrosion inhibitors for some steel and aluminium applications. For example we have shown thatthe lanthanum 4-hydroxy cinnamate offers excellent corrosion mitigation for mild steel in aqueous environments whilerare earth diphenyl phosphates offer the best protection in the case of aluminium alloys. In both cases the protectionappears to be related to the formation of a nanometre thick interphase occurring on the surface that reduces theelectrochemical processes leading to metal loss or pitting. Very recent work has indicated that we may even be able toaddress the challenging issue of stress corrosion cracking of high strength steels. Furthermore, filiform corrosion can besuppressed when selected rare earth inhibitor compounds are added as pigments to a polymer coating. There is little doubtfrom the work thus far that a synergy exists between the rare earth and organic inhibitor components in these novelcompounds. This paper reviews some of the published research conducted by the senior author and colleagues over the past10 years in this developing field of green corrosion inhibitors

Recent developments in corrosion inhibitors based on rare earth metal compounds

Rare earth organic compounds can provide an environmentally safe and non-toxic alternative to chromates as corrosion inhibitors for some steel and aluminium applications. For steel lanthanum 4-hydroxy cinnamate offers corrosion protection and reduces the susceptibility to hydrogen embrittlement. Recent work has also indicated that it inhibits the corrosion of steel in environments containing high levels of carbon dioxide. For aluminium alloys, cerium diphenyl phosphate provides excellent corrosion inhibition in chloride environments, and reduces susceptibly to stress corrosion cracking. Furthermore, for both steel and aluminium alloys filiform corrosion can be suppressed when rare earth inhibitor compounds are added as pigments to polymer coatings. The levels of inhibition observed are thought to be due to synergistic effects between the rare earth and organic parts of these novel compounds, and are related to the various species that may be present in the complex chemical conditions that develop in solution close to a metal surface. This paper reviews some of the published research conducted by the group at Deakin University over recent years.

Rare earth 4-hydroxycinnamate compounds as carbon dioxide corrosion inhibitors for steel in sodium chloride solution

A series of rare earth 4-hydroxycinnamate compounds including Ce(4OHCin)3, La(4OHCin)3, and Pr(4OHCin)3 has been synthesized and evaluated as novel inhibitors for carbon dioxide corrosion of steel in CO2-saturated sodium chloride solutions. Electrochemical measurements and surface analysis have shown that these REM(4OHCin)3 compounds effectively inhibited CO2 corrosion by forming protective inhibiting deposits that shut down the active electrochemical corrosion sites on the steel surface. Inhibition efficiency was found to increase in the order Ce(4OHCin)3 < La(4OHCin)3 < Pr(4OHCin)3 and with increase in inhibitor concentration up to 0.63 mM. Detailed insights into corrosion inhibition mechanism of these compounds in carbon dioxide environment are also provided.