Interference by cerium cations during the multi-step zinc dissolution process in a chloride-containing electrolyte

The influence of cerium ions in the dissolution mechanism of zinc has been evaluated by means of electrochemical impedance spectroscopy in a sodium chloride media. The multi-step dissolution process of zinc is modified during the early stages of immersion. The initial chloride ions adsorption on the surface is hindered by the adsorption of cerium ions which act as a precursor of the rich cerium layer formed with time. This was revealed by the appearance of two constant times at high and intermediate frequencies related to the charge transfer resistance and the adsorption process. At longer times, 24 h, the second arc evolves toward a diffusion tail. As time increases the behaviour of the galvanised steel depends on the Ce concentration. For 500 and 1000 ppm CeCl₃ · 7H₂O there was no evidence of the constant time associated to the porous corrosion products of zinc at the high frequency range.     

Corrosion protection of aluminium alloy 2024-T3 in 0.05 M NaCl by cerium cinnamate

The corrosion protection of AA 2024-T3 in 0.05 M NaCl by cerium cinnamate has been studied. Polarization measurements demonstrate that cerium cinnamate is markedly effective for suppressing anodic process of the alloy corrosion during the initial 72 h of immersion. The protection mechanism during the immersion period appears to involve two stages: the deposition of cerium cinnamate, and then hydrolysis of cerium ions forming a cerium oxide/hydroxide, prevailing over the foregoing deposition. This study also elucidates the inhibition effect of cerium cinnamate on the early corrosion attack occurred from the second phase particles by X-ray photoelectron spectroscopy and electron-probe microanalysis.     

Electrodeposition of ceria-based layers on zinc electroplated steel

CeO₂ layers were electrodeposited on a Zn-electroplated steel from relatively high concentrations of cerium nitrate. Increasing the current density resulted in a better coverage of the surface but cracks also appeared. Partial dissolution of the substrate occurred, but no defined conversion layer was observed by SEM. XRD, Raman and FT-IR indicated however the formation of zinc hydroxides and hydrozincite as corrosion products. The addition of polyethylene glycol (PEG) limited such dissolution. DSC results showed that PEG was trapped in the coating. This limited the reduction of water, hence to decrease the number of cracks. The coatings microstructure remained however similar.     

New contributions to the understanding of rust layer formation in steels exposed to a total immersion test

Adherent (AR), differentiated as scraped and hit, and non-adherent (NAR) rusts formed on carbon and weathering steels exposed to chloride solutions in total immersion tests were examined by means of different techniques. The NAR for both steels is composed of lepidocrocite, the most abundant, akaganeite, and goethite, whereas the AR in both steels contains additionally a spinel phase (magnetite/maghemite), which is the most abundant. Differences in crystallographic, relative amount, physical and chemical properties of the compounds in the different rust layers on both steels are reported. For both steels around 21% of the corroded iron converts completely into adherent rust.     

Zinc corrosion protection with 1,5-diphosphono-pentane

A set of identical measurement was performed to characterize the corrosion behaviour in Na₂SO₄ solution and surface structure of layers formed in aqueous and ethanol solutions of 1,5-diphosphonic acid on mechanically polished zinc substrates in order to understand the inhibition mechanism. Although, the protective layers formed in ethanol and in aqueous solution show substantial differences, their protection efficiency tends to the same value after about 4 h in corrosion tests. Based on EIS, XPS and GD-OES data, a simple oxide–hydroxide/diphosphonate model of the interface is proposed.     

Long-term atmospheric corrosion of mild steel

A great deal of information is available on the atmospheric corrosion of mild steel in the short, mid and even long term, but studies of the structure and morphology of corrosion layers are less abundant and generally deal with those formed in just a few years. The present study assesses the structure and morphology of corrosion product layers formed on mild steel after 13 years of exposure in five Spanish atmospheres of different types: rural, urban, industrial and marine (mild and severe). The corrosion layers have been characterised by X-ray diffraction (XRD) and scanning electron microscopy/energy dispersive X-ray spectroscopy (SEM/EDS). Long-term corrosion is seen to be more severe in the industrial and marine atmospheres, and less so in the rural and urban atmospheres. In all cases the corrosion rate is seen to decrease with exposure time, stabilising after the first 4–6 years of exposure. The most relevant aspects to be noted are (a) the great compaction of the rust layers formed in the rural and urban atmospheres, (b) the formation of hematite and ferrihydrite phases (not commonly found) in the industrial and marine atmospheres, respectively and (c) identification of the typical morphological structures of lepidocrocite (sandy crystals and flowery plates), goethite (cotton balls structures) and akaganeite (cotton balls structures and cigar-shaped crystals).     

Anodic oxide nanotube layers on Ti-Ta alloys: Substrate composition, microstructure and self-organization on two-size scales

The present work reports the self-ordered formation of nanotubular oxide layers on Ti-Ta alloys of different compositions (Ti-13Ta, Ti-25Ta, Ti-50Ta, and Ti-80Ta) by anodization in 1 M H₂SO₄ + 0.15 wt.% HF. Depending on the alloy composition, nanoporous or highly ordered nanotube structures can be formed. The homogeneity of the nanotubular surface layers is strongly affected by the microstructure of the alloys. Over a wide composition range the alloys exhibit a two-phase structure, and this results in two distinct tube morphologies formed on the different phases. In particular, on the chemically less stable phase, the top of nanotube layers is etched, which results in a disordered top morphology of the tubes on Ti-13Ta and Ti-25Ta, whereas this effect yields a difference in tube length on the two phases on Ti-50Ta. For the Ti-50Ta alloy, a bimodal self-organization can be observed, i.e. ordered patterns of oxide tubes with two distinct diameters are formed where larger tubes are surrounded by smaller tubes with a regularity that depends on the chemical composition of the two phases.     

Corrosion behaviour of zinc–cerium alloys: Role of intermetallic phases

New zinc–cerium alloys (up to 5 wt.%) was synthesized and their corrosion behaviour was studied in a reference corrosive media. Metallographic analyses show that cerium is exclusively present in a Zn₁₁Ce intermetallic phase, homogeneously dispersed in the zinc matrix. With an optimal concentration at around 1.5 wt.%, the Zn₁₁Ce phase acts as a tank of cerium to form a protective Ce-enriched passive layer. Nevertheless, at high Ce content, the benefit in terms of corrosion resistance obtained by the incorporation of Ce in the corrosion layer is shaded by the galvanic coupling between the intermetallic phase and the matrix.     

The evolution of the corrosion of iron in hydraulic binders analysed from 46- and 260-year-old buildings

Corrosion patterns on samples taken on binder and rebars from two buildings, respectively, aged of 46 and 260 years old have been characterised by coupling different analytical methods at microscopic scale. Different corrosion patterns have been observed. The first one is constituted by the initial mill scale made of wustite, magnetite and hematite. The second one contains remains of this mill scale embedded in phases formed after aqueous corrosion: oxyhydroxides as goethite or lepidocrocite containing marblings of ferrihydrite, maghemite and magnetite. On the thicker and older layers, marblings were only constituted of magnetite and maghemite. It is proposed that the structural evolution of the pattern and their marbling is linked to wet/dry cycles and/or pH condition evolution during the corrosion processes.     

Corrosion and electrochemical behavior of aluminium treated with high-temperature pulsed plasma in CsCl–NaCl–NaNO3 melt

Dense protective layers of aluminium corrosion products, whose composition depends on the oxidation temperature, are formed on the surface of aluminium treated with high-temperature pulsed plasma (HTPP) without visible remelting and then held in a chloride–nitrate melt in conditions of anodic polarization. Modification of aluminium treated with HTPP changes the properties of 20 μm layer under its surface and the oxide layer formed by such treatment has different morphology: it consists of smaller crystals and so has the other protective properties as compared with aluminium untreated by plasma.     

Deposition of cerium oxide thin films by reactive magnetron sputtering for the development of corrosion protective coatings

As an alternative to chromate based coatings cerium oxide thin films were deposited by magnetron sputtering on Al clad 2024 alloy in order to evaluate their corrosion protection potential. Varying plasma properties by modifying O₂ content during deposition allowed to affect the chemistry and microstructure of the synthesized films. The electrochemical characterization showed that specific self-healing mechanisms are suspected for the samples deposited with the highest amount of oxygen. Those mechanisms, leading to the reinforcement of barrier properties versus immersion time in NaCl solution, are associated with the interaction between alumina and cerium oxide film in the presence of electrolyte.     

Lanthanide salts as thermal conversion coatings on silicon carbide reinforced AA5A06 aluminium metal matrix composite

In this study, cerium and lanthanum chloride binary mixtures conversion coatings were investigated as corrosion inhibitors for silicon carbide particulates reinforced AA5A06 aluminium composite and AA5A06 aluminium alloy. Electrochemical impedance spectroscopy and potentiodynamic polarization tests revealed that cerium (III) chloride additions markedly improved the pitting corrosion resistance as compared to lanthanum (III) chloride additions, with maximum increase noted for samples treated with 750 ppm cerium (III) + 250 ppm lanthanum (III) chlorides. Scanning electron microscopy, energy dispersion spectroscopy and X‐ray photoelectron spectroscopy examinations revealed the existence of a heterogeneous layer formed by precipitation of cerium/lanthanum oxide/hydroxide on the cathodic intermetallics and an aluminium oxide film on the rest of the metal matrix. Improved corrosion resistance was attained for SiC particulates reinforced aluminium composite by means of the cerium/lanthanum conversion treatment, due to the presence of mainly cerium (IV) species as a result of oxidation of cerium (III), which provides auto‐protection properties.     

Assessment of protective function of steel rust layers by N2 adsorption

The specific surface area (SA) of the rusts formed by exposing various kinds of steels in different situations was determined by N₂ adsorption. The SA values of the rusts increased with the increase of corrosion rate, implying that the rust layers with large SA exhibit a high resistance to corrosion. The suppression of rusting by compact rust layers was interpreted by the blockage of pores in rust layers by the adsorption and capillary condensation of water. The SA values clearly reflect the corrosion levels estimated by the external observation. It was convinced that the SA measurement is a universal quantitative technique to appraisal the protective function of rust layers.     

Development of nano cerium oxide incorporated aluminium alloy sacrificial anode for marine applications

Aluminium-zinc alloy sacrificial anodes are extensively used for cathodic protection. The performance of the sacrificial anodes can be significantly improved by incorporation of microalloying elements in the aluminium matrix. In the present work nano cerium oxide particles of different concentrations, ranging from 0 to 1 wt% were incorporated for activating and improving the performance of the anode. The electrochemical test results revealed the increased efficiency of the anode. The electrochemical impedance spectroscopy revealed the information that the presence of nano cerium oxide in the anode matrix caused effective destruction of the passive alumina film, which facilitated enhancement of galvanic performance of the anode. Moreover, the biocidal activity of cerium oxide prevented the bio accumulation considerably which enables the anodes to be used in aggressive marine conditions.     

The influence of sulphate-reducing bacteria biofilm on the corrosion of stainless steel AISI 316

This work investigates microbially-influenced corrosion (MIC) of stainless steel AISI 316 by two sulphate-reducing bacteria, Desulfovibrio desulfuricans and a local marine isolate. The biofilm and pit morphology that developed with time were analyzed using atomic force microscopy (AFM). Electrochemical impedance spectroscopy (EIS) results were interpreted with an equivalent circuit to model the physicoelectric characteristics of the electrode/biofilm/solution interface. D. desulfuricans formed one biofilm layer on the metal surface, while the marine isolate formed two layers: a biofilm layer and a ferrous sulfide deposit layer. AFM images corroborated results from the EIS modeling which showed biofilm attachment and subsequent detachment over time.     

Oxidation of ultra low carbon and silicon bearing steels

Oxidation tests were carried out in samples from an ultra low carbon and two silicon bearing steels to determine the distribution and morphology of the oxide species present. The ultra low carbon steel was oxidized for short periods of time within a chamber designed to obtain thin oxide layers by controlling the atmosphere, and for longer times in an electric furnace; the silicon steels were reheated only in the electric furnace. The chamber was constructed to study the behaviour encountered during the short period of time between descaling and rolling in modern continuous mills. It was found that the oxide layers formed on the samples reheated in the electric furnace were made of different oxide species. The specimens treated in the chamber had layers made almost exclusively of wustite. Selected oxide samples were studied by scanning electron microscopy to obtain electron backscattered diffraction patterns, which were used to identify the oxide species in the layer.     

Native oxidation of sputter deposited polycrystalline copper thin films during short and long exposure times: Comparative investigation by specular and non-specular grazing incidence X-ray absorption spectroscopy

Thin copper-oxide layers formed on Cu-metal due to the exposure to ambient air at short and long timescales were investigated by means of reflection mode grazing incidence X-ray absorption spectroscopy under specular and non-specular conditions. The quantitative evaluation of near edge X-ray absorption, as well as specular and non-specular EXAFS data measured for various different grazing angles show that the oxide film is best described by a model structure consisting of a duplex type oxide layer with an outer layer of CuO in contact with the gas atmosphere and an inner Cu₂O layer at the interface to the underlying Cu-metal.     

Synergistic inhibition effect of rare earth cerium(IV) ion and anionic surfactant on the corrosion of cold rolled steel in H2SO4 solution

The synergistic inhibition effect of rare earth cerium(IV) ion and anionic surfactant of sodium oleate (C₁₇H₃₃-COONa, SO) on the corrosion of cold rolled steel (CRS) in H₂SO₄ solution was first investigated by weight loss and potentiodynamic polarization methods. The results revealed that SO had a moderate inhibitive effect, and the adsorption of SO obeyed the Freundlich adsorption isotherm. For the cerium(IV) ion, it had a negligible effect. However, incorporation of Ce⁴⁺ with SO significantly improved the inhibition performance, and produced strong synergistic inhibition effect. Depending on the results, the synergism mechanism was proposed.     

Synergistic inhibition effect of rare earth cerium(IV) ion and 3,4-dihydroxybenzaldehye on the corrosion of cold rolled steel in H2SO4 solution

The synergistic inhibition effect of rare earth cerium(IV) ion and 3,4-dihydroxybenzaldehye (DHBA) on corrosion of cold rolled steel (CRS) in H₂SO₄ solution was first investigated by weight loss and potentiodynamic polarization methods. Effects of inhibitor concentration, temperature, immersion time and acid concentration on synergism are discussed in detail. The results reveal that DHBA has moderate inhibitive effect and its adsorption obeys Temkin adsorption isotherm. For the cerium(IV) ion, it has negligible effect. However, incorporation of Ce⁴⁺ with DHBA improves the inhibition performance significantly, and produces strong synergistic inhibition effect.     

Influence of cerium addition on the resistance to oxidation of AM50 alloy prepared by rapid solidification

The influence of various cerium (Ce) additions and starting temperature on the resistance to oxidation of AM50 alloy prepared by rapid solidification (RS, RS-AM50 alloy) has been investigated. The Ce addition has two opposite effects on the oxide scale. To the RS-alloy, the beneficial effect outweighs the detrimental one. However, when the alloy was prepared by slow solidification at the normal cooling rate (SS, SS-AM50 alloy), the detrimental effect was dominant. The improvement of oxide scale is closely related to the adherence of metal/oxide and tightness of oxide scale of the alloy. In addition, the reaction rate at the surface increased with starting temperature, which finally results in the formation of a protective oxide scale.