An electrochemical impedance study of ionic liquid film formation and aqueous corrosion of magnesium alloy ZE41

Ionic liquid surface treatments are proposed as a method of controlling corrosion processes on magnesium alloys. An important magnesium alloy, ZE41 (nominally 4% Zn and 1% rare earth), was treated with the ionic liquid trihexyl(tetradecyl)phosphonium diphenylphosphate (P₆₆₆₁₄DPP). Impedance spectra were acquired at intervals during the treatment, indicating the development of a film and allowing a measure of the film formation process to be obtained over time. Mechanically polished and electro-polished surfaces were prepared; these surfaces, treated and untreated, were subsequently exposed to 0.1 M NaCl aqueous solutions. The corrosion behavior of the prepared surfaces were assessed using impedance spectroscopy and optical microscopy. The results indicated a significant role for the method of surface preparation used and, in both cases, the ionic liquid treatment produced a more corrosion-resistant surface.

Anodising AA5083 aluminium alloy using ionic liquids

Aluminium, as the current collector in lithium batteries, has shown reduced corrosion susceptibility in room temperature molten salts (1, 2). Moreover, previous studies have established that corrosion mitigation is achieved on magnesium alloys using ionic liquids pretreatments (3, 4). This paper investigated the anodisation of AA5083 aluminium alloy in Trihexyl(tetradecyl)phosphonium bis(trifluoromethylsulfony) ([P6,6,6,14][NTf2]) ionic liquid by applying a constant current followed by holding at the maximum potential for a period of time. Potentiodynamic polarisation results show that the treated surfaces were more corrosion resistant in 0.1 M sodium chloride solution compared with the control specimen. The anodising treatment was effective both in shifting the free corrosion potential to more noble values and in suppressing the corrosion current. Optical microscope and optical profilometry images indicated that an anodising film was deposited onto the alloy surface, which is thought to have inhibited corrosion in chloride environment. Further characterisation of the anodising film will be carried out in future work.

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.

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

Chromate replacement in coatings for corrosion protection of aerospace aluminium alloys

Mixed rare earth organophosphates have been investigated as potential corrosion inhibitors for AA2024-T3 with the aim of replacing chromate-based technologies. Cerium diphenyl phosphate (Ce(dpp) 3) and mischmetal diphenyl phosphate (Mm(dpp) 3) were added to epoxy coatings applied to AA2024-T3 panels and they were effective in reducing the amount and rate of filiform corrosion in high humidity conditions. Ce(dpp) 3 was the most effective and characterisation of the coating formulations showed approximately a factor of 5 reduction in both the number of corrosion filaments initiated as well as the length of these. Mm(dpp) 3 appeared to reduce the corrosion growth rate by a factor of 2 although it was the more effective inhibitor in solution studies. Spectroscopic characterisation of the coatings indicated that the cerium based inhibitor may disrupt network formation in the epoxy thus resulting in a coating that absorbed more water and allowed greater solubilisation of the corrosion inhibiting compound.

Corrosion of heat treated magnesium alloy ZE41

This paper investigates the effect of heat treatment upon the corrosion morphology and mechanism of ZE41 alloy. The results of optical and scanning electron microscopy (SEM) together with potentiodynamic polarisation reveal the importance of the microstructure in the initiation and propagation of corrosion in an aqueous environment. The corrosion of the heat-treated alloy is significantly altered due to changes in the microstructure, specifically the Zr-rich regions and the grain boundary T-phase.

Effect of grain size on corrosion of high purity aluminium

A complete understanding of how grain refinement, grain size, and processing affect the corrosion resistance of different alloys has not yet been fully developed. Determining a definitive 'grain size-corrosion resistance' relationship, if one exists, is inherently complex as the processing needed to achieve grain refinement also imparts other changes to the microstructure (such as texture, internal stress, and impurity segregation). This work evaluates how variation in grain size and processing impact the corrosion resistance of high purity aluminium. Aluminium samples with a range of grain sizes, from ∼100 μm to ∼2000 μm, were produced using different processing routes, including cold rolling, cryo rolling, equal channel angular pressing, and surface mechanical attrition treatment. Evaluation of all the samples studied revealed a tendency for corrosion rate to decrease as grain size decreases. This suggests that a Hall-Petch type relationship may exist for corrosion rate and grain size. This phenomenon, discussed in the context of grain refinement and processing, reveals several interesting and fundamental relationships.

Effects of corrosion inhibiting pigment lanthanum 4-hydroxy cinnamate on the filiform corrosion of coated steel

Corrosion protection by lanthanum hydroxy cinnamate (La(4OH-cin)3) in a polyurethane based varnish coating for mild steelhas 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 scribetests showed that La(4OH-cin)3, as a pigment in a coating, inhibited the initiation and propagation of both delamination and filiformcorrosion (FFC) on coated steel. The PP experiments provided an insight into the fundamental mechanism of FFC. The resultssuggest that La(4OH-cin)3 behaves as a mixed inhibitor and stifles the initiation and propagation of FFC. In this paper, the theory ofdelamination leading to FFC and the likely mechanism of inhibition by the La(4OH-cin)3 will be discussed.

The determination and characterisation of 2-naphthyloxycarbonyl chloride derivatised biogenic amines in pet foods

The need to monitor biogenic amines levels is essential for many areas of the food industry for two main reasons: the caustic nature and potential toxicity of these amines, and the potential to use amine levels as markers for freshness and quality in foodstuffs. Optimised analysis conditions used for the determination of biogenic amines derivatised with 2-napthyloxycarbonyl chloride has been applied to different pet food samples to assess the effectiveness of this method for complex sample matrices. Further to this, the use of high-resolution mass spectrometry has enabled the previously unconfirmed derivatised form of seven biogenic amines to be established. The derivatised forms identified include as mono substituted (tryptamine and histamine), bisubstituted (putrescine, cadaverine and tyramine), trisubstituted (spermidine) and tetrasubstituted (spermine). The methodology of biogenic amine determination was performed successfully to a range of pet food products highlighting the applicability to a variety of complex sample matrices.