Bruce Hinton

The corrosion inhibition of zinc with cerous chloride

Abstract The corrosion of zinc and zinc-coated steel in tap water or 0.1 M NaCl solution is inhibited by small additions of cerous chloride. Polarization studies and surface analysis show that corrosion protection is due to the formation of a complex film of cerium-rich oxide which causes cathodic reaction rates to be substantially reduced.

Chromate Conversion Coatings on 2024 Al Alloy

X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and potentiodynamic measurements have been made on chromate conversion-coated Al 2024-T3 alloy. X-ray photoelectron spectroscopy measurements indicated that the conversion coating had a surface of CrOOH and Cr(VI), enriched in ferricyanide. The bulk of the coating was an equal mixture of CrOOH and Cr2O3 with significant levels of F− and Fe, the latter implying the presence of ferricyanide throughout the coating. Copper(II) ion was present at the interface between the conversion coating and the alloy, as well as Al3+. During ageing experiments, potentiodynamic measurements indicated that the corrosion current (icorr) decreased from ∽0.4 to ∽0.04 μA cm-2 during the first 40 h after preparation but thereafter slowly increased. No significant changes were observed in the chemistry of the coating by XPS for ageing times longer than 40 h, although morphological changes were observed with SEM. As the coating aged, a network of microcracks developed across the surface. It is believed that Cr6+ is consumed in the process in which plugs of hydrated chromium oxide form at the base of these cracks.

Cerium Dibutylphosphate as a Corrosion Inhibitor for AA2024-T3 Aluminum Alloys

The suitability of cerium dibutylphosphate [Ce(dbp) 3 ] as a corrosion inhibitor for AA2024-T3 aluminum alloy in sodium chloride aqueous solutions has been investigated. Weight loss tests combined with electrochemical assessment have been used to evaluate the degree of protection and determine the inhibition characteristics of this compound. It was found that Ce(dbp) 3 offers superior protection when compared to CeCl 3 , with no discernable corrosion products, significant pitting, or evidence of replated copper on the surface. Cathodic polarization indicated inhibited oxygen reduction reaction kinetics consistent with reduced Cu replating. Corrosion protection seems to be enhanced at higher Cl - concentrations, suggesting the inhibiting film is more readily deposited when some corrosion takes place. X-ray photoelectron spectroscopy analysis of the surface confirmed the presence of both Ce(III) and Ce(IV). Focused ion beam secondary-ion mass spectroscopy (SIMS) analysis clearly indicated the presence of a 500 nm thick cerium-containing layer on the surface of the alloy after 10 days immersion in the inhibited solution. A strong phosphorus signal was also detected in the SIMS experiment. Toxicity testing using the EC-50 test suggested that cerium dibutylphosphate is able to fulfil the basic requirements for consideration as an environmentally friendly corrosion inhibitor.

New Insights into the Fundamental Chemical Nature of Ionic Liquid Film Formation on Magnesium Alloy Surfaces

Ionic liquids (ILs) based on trihexyltetradecylphosphonium coupled with either diphenylphosphate or bis(trifluoromethanesulfonyl)amide have been shown to react with magnesium alloy surfaces, leading to the formation a surface film that can improve the corrosion resistance of the alloy. The morphology and microstructure of the magnesium surface seems critical in determining the nature of the interphase, with grain boundary phases and intermetallics within the grain, rich in zirconium and zinc, showing almost no interaction with the IL and thereby resulting in a heterogeneous surface film. This has been explained, on the basis of solid-state NMR evidence, as being due to the extremely low reactivity of the native oxide films on the intermetallics (ZrO2 and ZnO) with the IL as compared with the magnesium-rich matrix where a magnesium hydroxide and/or carbonate inorganic surface is likely. Solid-state NMR characterization of the ZE41 alloy surface treated with the IL based on (Tf)2N(-) indicates that this anion reacts to form a metal fluoride rich surface in addition to an organic component. The diphenylphosphate anion also seems to undergo an additional chemical process on the metal surface, indicating that film formation on the metal is not a simple chemical interaction between the components of the IL and the substrate but may involve electrochemical processes.

Inhibition of Corrosion on AA2024-T3 by New Environmentally Friendly Rare Earth Organophosphate Compounds

Abstract The combination of rare earth metals (REM), such as Ce, La, Pr, or the less-refined mischmetal (consisting of a combination of REM) with dibutyl phosphate or diphenyl phosphate, provides a complex that shows excellent corrosion inhibition for AA2024-T3 (UNS A92024) in aqueous chloride environments. In some instances, it is equal to the inhibition provided by the chromate compounds, which are still used by the aircraft industry. The addition of these compounds as an inhibiting pigment in an epoxy coating also shows efficient suppression of filiform corrosion from scribes in the coating, under both alternate immersion and high-humidity conditions. Surface characterization of AA2024-T3 specimens immersed in solutions of these complex salts suggests that a film containing both the organic and rare earth components deposits on the alloy surface. The electrochemical data indicate that this film is acting as a mixed inhibitor, suppressing both the anodic and cathodic processes on the surface.

Potentiostatic control of ionic liquid surface film formation on ZE41 magnesium alloy

The generation of potentially corrosion-resistant films on light metal alloys of magnesium have been investigated. Magnesium alloy, ZE41 [Mg-Zn-Rare Earth (RE)-Zr, nominal composition approximately 4 wt % Zn, approximately 1.7 wt % RE (Ce), approximately 0.6 wt % Zr, remaining balance, Mg], was exposed under potentiostatic control to the ionic liquid trihexyl(tetradecyl)phosphonium diphenylphosphate, denoted [P(6,6,6,14)][DPP]. During exposure to this IL, a bias potential, shifted from open circuit, was applied to the ZE41 surface. Electrochemical impedance spectroscopy (EIS) and chronoamperometry (CA) were used to monitor the evolution of film formation on the metal surface during exposure. The EIS data indicate that, of the four bias potentials examined, applying a potential of -200 mV versus OCP during the exposure period resulted in surface films of greatest resistance. Both EIS measurements and scanning electron microscopy (SEM) imaging indicate that these surfaces are substantially different to those formed without potential bias. Time of flight-secondary ion mass spectrometry (ToF-SIMS) elemental mapping of the films was utilized to ascertain the distribution of the ionic liquid cationic and anionic species relative to the microstructural surface features of ZE41 and indicated a more uniform distribution compared with the surface following exposure in the absence of a bias potential. Immersion of the treated ZE41 specimens in a chloride contaminated salt solution clearly indicated that the ionic liquid generated surface films offered significant protection against pitting corrosion, although the intermetallics were still insufficiently protected by the IL and hence favored intergranular corrosion processes.

Laser cladding as a potential repair technology for damaged aircraft components

Purpose – The purpose of this paper is to demonstrate the preliminary work on using laser cladding technology for the restoration of structural integrity.Design/methodology/approach – The primary methodology used in this research is to develop a laser cladding‐based metal deposition technique to articulate restoration of structural geometry affected by corrosion damages. Following from this method, it is planned to undertake further work to use the laser cladding process to restore geometry and the associated static/fatigue strength.Findings – This work has found that it is possible to use laser cladding as a repair technology to improve structural integrity in aluminium alloy aircraft structures in terms of corrosion reduction and geometrical restoration. Initial results have indicated a reduction of static and fatigue resistance with respect to substrate. But more recent works (yet to be published) have revealed improved fatigue strength as measured in comparison to the substrate structural properties.O...

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 the use of chromates as corrosion inhibitors for some steel and aluminium applications. For example we have shown that the lanthanum 4-hydroxy cinnamate offers excellent corrosion mitigation for mild steel in aqueous environments while rare earth diphenyl phosphates offer the best protection in the case of aluminium alloys. In both cases the protection appears to be related to the formation of a nanometre thick interphase occurring on the surface that reduces the electrochemical processes leading to metal loss or pitting. Very recent work has indicated that we may even be able to address the challenging issue of stress corrosion cracking of high strength steels. Furthermore, filiform corrosion can be suppressed when selected rare earth inhibitor compounds are added as pigments to a polymer coating. There is little doubt from the work thus far that a synergy exists between the rare earth and organic inhibitor components in these novel compounds. This paper reviews some of the published research conducted by the senior author and colleagues over the past 10 years in this developing field of green corrosion inhibitors.

Towards replacement of chromate inhibitors by rare earth systems

Rare earths as inhibitors have garnered increasing interest in recent years as replacements for Chromate in deoxidizers, conversion coatings, Chromate inhibited primers and repair processes.They have demonstrated performance as aqueous inhibitors for ferrous metals and aluminium alloys, as conversion coatings, deoxidizers repair processes and have been incorporated into paint systems. This paper looks at the prospects of developing a complete rare earth alternative process for aircraft applications to Chromates from metal finishing to painting.

Characterisation of aluminium alloys after HNO3/HF–NaOH–HNO3/HF pretreatment

Abstract The effect of a pretreatment sequence on several aluminium alloys was examined by X-ray photoelectron spectroscopy and SEM. The pretreatment consisted of vapour degreasing followed by a three step sequence comprising 1 min immersions in (a) nitric acid–hydrofluoric acid solution, (b) sodium hydroxide solution and (c) a 1 or 5 min immersion in nitric acid–hydrofluoric acid as in step (a). Following each pretreatment step the alloy panels were rinsed in tap water for 1 min. The surfaces of aluminium alloys 2024-T3, 6061-T6, and 7075-T6 were examined at each stage in the pretreatment process. The neutral salt spray resistance of specimens deoxidised with this pretreatment and chromate conversion coated were compared to those deoxidised with a conventional chromate based deoxidiser before conversion coating.