A rapid screening multi-electrode method for the evaluation of corrosion inhibitors

A method is described for the rapid screening of water-soluble corrosion inhibitors, which involves a fixed potential being applied between identical electrodes while immersed in an inhibitor solution. The current flowing between the electrodes is measured and the electrochemical response of the anodic and cathodic reactions, with and without the inhibitor, is then compared. The incorporation of nine pairs of metallic wires (Al, AA2024, AA7075, Fe, mild steel, SS316, Mg, Mg-AZ31 and Zn) into a single assembly enables the effect of inhibitors on different metals to be assessed rapidly. The technique is sensitive to both anodic and cathodic inhibition mechanisms and can provide information on the concentration dependency of inhibitors.This paper provides an electrochemical analysis of the method and demonstrates its application using several soluble chemical species (potassium dichromate, cerium nitrate, sodium vanadate, sodium diphosphate, sodium octanoate) on AA2024-T3 and mild steel at 10⁻² and 10⁻⁴ M and correlates the results with those obtained by conventional potentiodynamic polarization analysis. The errors in the approach were higher for poor inhibitors, however, efficient corrosion inhibitors have decreased experimental errors, thus, ensuring that promising inhibitor candidates are not overlooked.     

Influence of the doping agent on the corrosion protection properties of polypyrrole grown on aluminium alloy 2024-T3

The corrosion protection properties of polypyrrole (PPy) electrodeposited onto aluminium alloy 2024-T3 substrates were investigated as a function of the doping agent. We used camphor sulfonic acid (CSA), para toluene sulfonic acid (p-TSA), phenylphosphonic acid (PPA), oxalic acid (OA) and cerium nitrate salt (Ce(NO₃)₃) as doping agents. The resulting coatings have been evaluated towards corrosion protection of aluminium alloy 2024-T3 using electrochemical impedance spectroscopy (EIS). Complementary, scanning electron microscopy (SEM) provided images on the morphology and the thickness of the coatings. The results showed that coatings formed using Ce(NO₃)₃ solution protect the substrate more efficiently compared to the other coatings.     

The characterisation and performance of Ce(dbp)3-inhibited epoxy coatings

Cerium dibutyl phosphate (Ce(dbp)₃) was tested as a replacement for chromate corrosion inhibitors in a two-component epoxy applied to AA2024-T3 aluminium alloy. The interaction of the Ce(dbp)₃ with the epoxy-amine functionalities were investigated using FTIR. Water uptake and Ce release were studied using weight gain and ICP-AES analyses, respectively. Corrosion resistance was assessed using filiform testing. It was found that the filiform corrosion resistance increased with increasing Ce(dbp)₃ loading, and the best performance was obtained for the highest loading. The primer also showed improved wet adhesion. It was found using SEM and TEM that the corrosion protection offered by the Ce(dbp)₃-inhibited epoxy and the improved wet adhesion were due to the development of an interfacial oxide at the metal/primer interface.     

Electrochemical and morphological assessments of inhibition level of 8-hydroxylquinoline for AA2024-T4 alloy in 3.5% NaCl solution

The corrosion inhibition of AA2024-T4 in 3.5% NaCl solution by 8-hydroxylquinoline (8-HQ) was investigated by potentiodynamic polarisation (PDP), electrochemical impedance spectroscopy and dynamic electrochemical impedance spectroscopy. Experimental results were supported with scanning electron microscopy (SEM), atomic force microscopy and Fourier transform-infrared (FTIR) spectroscopy analysis. It was found that 8-HQ molecules adsorbed on the alloy surface and protected it against corrosion. SEM, energy dispersive spectroscopy, and FTIR results confirm the adsorption of 8-HQ molecules on AA2024-T4. The inhibition efficiency of 8-HQ is found to increase with increase in concentration and the highest concentration studied (0.05 M) offered corrosion inhibition efficiency of 84%. PDP results show that 8-HQ acts as mixed type inhibitor in the studied medium.     

Comportement electrochimique des alliages d'aluminium 2024 ET 7075 dans un milieu salin

Résumé Le comportement des alliages d'aluminium 2024 à l'état T3 et T4 7075-T6 a été par voie électrochimique dans la solution à 3% de NaCl et pour differents pH 1, 6 et 12. Les études potentiocinétiques ont permis de déterminer les potentiels de corrosion et de piqûration. L'état stationnaire pour certains potentiels a été examiné par voie potentiostatique. Un comportement actif-passif a été observé lors des études potentiodynamiques seulement à pH 6. ainsi, les études potentiostatiques ont été effectuées à ce même pH. L'examen de surfaces des électrodes après les essais électrochimiques ont été effectuées par microscopie électronique à balayage et par micro-analyse par dispersion des rayons-X (EDAX). Pour l'alliage 2024, l'analyse d'image a permis de relier la distribution et la taille des précipités (ou inclusions) aux différents traitements thermiques (T3 et T4) et aux phénomènes de corrosion.

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The electrochemical behaviour of aluminium alloys, 2024 in conditions T3 and T4 and also of 7075-T6, in 3% NaCl solution was studied. The corrosion and the pitting potentials were determined by potentiodynamic techniques. The potentiostatic method was also used to study the steady state at specific potentials. Among the three pH levels (1, 6 and 12), polarization studies showed active-passive behaviour only at pH 6. Thus, pitting potential was imposed at pH 6 for further corrosion behaviour. Scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS), were used to characterize the surfaces of electrodes after they have been subjected to electrochemical testing. In the case of alloy 2024, image analysis permitted to relate the distribution and the size of the precipitates to different types of heat treatment (T3 and T4) and to the corrosion phenomena.

     

Studies on the influence of chloride ion and pH on the electrochemical behaviour of aluminium alloys 8090 and 2014

The influence of chloride ion concentration and pH (2.0, 6.0 and 11.0) on the corrosion behaviour of 8090 (Al-Li-Cu-Mg-Zr) and 2014 (Al-4.4%Cu) alloys has been studied in NaCl solution using a potentiodynamic polarization technique. The corrosion rate for both the alloys was high at pH values of 2.0 and 11.0 as compared to that at pH 6.0, and the rate increased in chloride ion concentration at all pH levels. A similar result was found for the passive current density. Increase in pH changed the slope of the cathodic polarization curve by changing the cathodic reaction. Increasing the chloride ion concentration decreased the cathodic reaction rate. On the other hand, the anodic reaction rate increased with increase in chloride ion concentration. The open circuit corrosion potential and the pitting potential shifted in the active (negative) direction with increasing pH and chloride ion concentration. The i _p values for 8090-T851 were slightly lower than those for 2014-T6.     

Release of cerium dibutylphosphate corrosion inhibitors from highly filled epoxy coating systems

Carcinogenic chromates are phased out as corrosion inhibitors in organic coatings, and are replaced by benign alternatives. Cerium-based compounds are excellent corrosion inhibitors in an aqueous environment. However, whether they are effective as corrosion inhibitor in an organic coating also depends on their interaction with the coating matrix, which should result in a sufficient release of inhibitors over a desired period of time. In this work we report on the leaching of cerium dibutylphosphate, Ce(dbp)₃, from an epoxy coating system as a first step toward a fundamental understanding of the leaching behavior. To this end, coating systems containing various levels of inhibitor loading, ranging from 1 up to 50 wt%, were fabricated. These coatings were characterized using Fourier transform spectroscopy (FTIR), and Scanning Electron Microscopy (SEM). Leaching experiments were performed by exposing the coating films to water, while the concentration of dissolved Ce(dbp)₃ in this water was monitored in situ using ultraviolet–visible (UV–vis) spectroscopy. The leaching of inhibitors showed a clear dependence on the initial inhibitor loading of the coating system. For highly loaded coatings (>25 wt%) all inhibitors leach out, whereas for a low loading level (<10 wt%) no leaching was observed. The difference between the high and low loading levels might be the formation of a porous network in the highly loaded coatings due to the dissolution of large Ce(dbp)₃ clusters. Using Magnetic Resonance Imaging (MRI) we visualized the penetration of a water front into this porous network during the leaching experiments. This water-filled porous network provides an excellent pathway for transport of Ce(dbp)₃. For low levels of inhibitor loadings such a network is absent, and the transport of Ce(dbp)₃ is blocked. The samples containing moderate levels of inhibitor loading showed an increasing amount of leaching for decreasing values of pH. We believe that this pH dependence is the result of the concurrent leaching of significant amounts of Bisphenol-based coatings residues at low pH since the removal of these residues will also result in an increased porosity of the coating.     

A silanol-based nanocomposite coating for protection of AA-2024 aluminium alloy

A new hybrid sol–gel type film, composed of tetraethylorthosilicate (TEOS) and tetraocthylorthosilicate (TEOCS), and modified with different nanoparticle systems, has been investigated as a coating for protection of AA-2024-T3 aluminium alloy. The nanoparticle systems considered were either ZrO₂ or CeO₂ or their combination_. The zirconia nanoparticles were prepared from a Zr (IV) propoxide sol (TPOZ), using an organic stabilizer, and the CeO₂ nanoparticles were developed spontaneously after adding cerium nitrate solution to the hybrid sol. The chemical composition and the structure of the hybrid sol–gel films were examined by X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and atomic force microscopy (AFM). The corrosion resistance of the coated AA-2024 alloy was examined by potentiodynamic polarization. The results revealed that, for short exposure times in the electrolyte, incorporation of ZrO₂ or CeO₂ nanoparticles in the hybrid film does not provide an increase in the corrosion resistance of the coated AA-2024 alloy. Further, the resistance was significantly reduced by increasing the nanoparticle content. Conversely, by incorporating both nanoparticles (ZrO₂ and CeO₂), the corrosion resistance of the resulting hybrid films increased slightly. The behavior changed significantly when the coated alloy was exposed to the electrolyte for 5 days. The corrosion resistance of the coatings, unmodified and modified with CeO₂ or ZrO₂ nanoparticles, decreased by two or three orders of magnitude, while the film modified with both nanoparticles (CrO₂ and ZrO₂) showed a relatively high corrosion resistance and responsiveness to activation processes during anodic polarization.     

Nanoporous titania interlayer as reservoir of corrosion inhibitors for coatings with self-healing ability

Active corrosion protection of AA2024-T3 alloy has been provided by an environmental-friendly, well adhering pre-treatment system consisting of an inhibitor-loaded titanium oxide porous layer and a sol–gel based thin hybrid film. A novel approach aimed at developing a nanoporous reservoir for storing of corrosion inhibitors on the metal/coating interface has been proposed. The nanostructured porous TiO₂ interlayer was prepared on the aluminium alloy surface by controllable hydrolysis of titanium alkoxide in the presence of template agent. The morphology and the structure of the TiO₂ film were characterized with TEM, EDS, SEM, and AFM techniques. Different ways of loading of the inhibitor in the pre-treatment coating were discussed. In contrast to direct embedding of the inhibitors into the sol–gel matrix, the use of the porous reservoir eliminates the negative effect of the inhibitor on the stability of the hybrid sol–gel matrix. TiO₂/inhibitor/sol–gel systems show enhanced corrosion protection and self-healing ability confirmed by EIS and SVET measurements.     

Corrosion protection of AA2024-T3 using rare earth diphenyl phosphates

Existing corrosion protection technologies for aluminium alloys utilising chromates are environmentally damaging and extremely toxic. This paper presents a preliminary investigation into rare earth diphenyl phosphates as new environmentally benign corrosion inhibitors. Full immersion weight loss experiments, cyclic potentiodynamic polarisation measurements and Raman spectroscopy were used in this study. Results show cerium diphenyl phosphate (Ce(dpp)₃) acts as a cathodic inhibitor, decreasing cathodic current density and E_corr by passivating cathodic intermetallic particles on the alloy surface. Mischmetal diphenyl phosphate (Mm(dpp)₃) acts a mixed inhibitor, shifting E_corr to more noble values, decreasing cathodic current density, increasing the breakdown potential and suppressing pitting.     

Galvanic corrosion of aluminum alloy (Al2024) and copper in 1.0M hydrochloric acid solution

The corrosion of an aluminum alloy (Al2024) and copper in 1.0M HCl solution was investigated at 30, 40, 50 and 60 °C using open circuit potential (OCP), electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization (PDP) measurements. The galvanic corrosion of Al2024 and copper was studied using the zero resistance ammeter (ZRA) method. Galvanic current densities (I _g ) and galvanic potential (E _g ) were measured at 30 °C in 1.0M HCl solution. Thermodynamic parameters, such as activation energy (E _a ), enthalpy of activation (??H _a ) and entropy of activation (??S _a ), were calculated and discussed. The results indicated that the corrosion rates of both Al2024 and copper increased with temperature. The ZRA results demonstrated that Al2024 is a sacrificial anode in 1.0M HCl solution when coupled with copper.     

Montmorillonite nanoclay-based self-healing coatings on AA 2024-T4

Cation-exchanged montmorillonite (MMT), which is an aluminum-pillared clay, was dispersed into an organic–inorganic hybrid silica matrix sol. Cationic inhibitors Ce³⁺/Zr⁴⁺ were introduced into montmorillonite by two methods, namely (1) physical mixing of MMT with the inhibitor solutions and (2) physical mixing for short durations and evacuation of the mixture of MMT and inhibitor solutions. Coatings using these sols were deposited on aluminum alloy AA 2024-T4 substrates, by the dip coating technique followed by thermal curing at 130°C for 1 h. TEM, BET, and XRD analyses were carried out for the inhibitor-modified montmorillonite powders. Potentiodynamic polarization and electrochemical impedance spectroscopic analyses were carried out on uncoated and coated substrates to evaluate the corrosion protection property after exposure to 3.5 wt% NaCl for varying time durations, up to 120 h. Self-healing ability was evaluated using SVET analysis by creating an artificial scribe on the surface and exposure to 3.5 wt% NaCl. Elemental mapping was carried out on scribed substrates before and after exposure to 5 wt% salt fog. Sols containing montmorillonite intercalated with cationic inhibitors when deposited on AA 2024-T4 substrates exhibited low corrosion currents after a prolonged duration of exposure to corrosion medium.     

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.     

Biomaterials for corrosion protection: evaluation of mustard seed extract as eco-friendly corrosion inhibitor for X60 steel in acid media

The toxic nature of most organic and inorganic corrosion inhibitors has necessitated the search for corrosion inhibitors with an excellent environmental profile. The present work is focused on the widening utilization of plant extracts for metallic corrosion control and reports on the corrosion inhibition effect of mustard seed extract (MSE) for typical X60 pipeline steel in 2 M HCl and 1 M H₂SO₄ solutions. Gravimetric and electrochemical (electrochemical impedance spectroscopy, linear polarization resistance and potentiodynamic polarization) methods were employed. The effect of immersion time and temperature on the corrosion inhibition effect of the plant extract was also studied. Results obtained show that MSE inhibited the corrosion of steel in both media which was more pronounced in H₂SO₄ than in HCl environment. Inhibition efficiency increased with increase in the concentration of the extract but decreased with increase in temperature. The potentiodynamic polarization studies revealed that MSE functions as a mixed-type inhibitor. The corrosion inhibition is assumed to occur via adsorption of the components of the extract on the steel surface which was found to obey Langmuir adsorption isotherm model. The morphology of the corroding steel surface in the absence and presence of the MSE was visualized using scanning electron microscopy.     

Deposition of hybrid 3-GPTMS's film on AA2024-T3: Dependence of film morphology and protectiveness performance on coating conditions

This paper describes the elaboration of 3-glycidoxypropyltrimethoxysilane (3-GPTMS) films onto AA2024-T3 aluminum alloy for corrosion protection.The dependence of sol–gel morphology on both precipitation under cathodic polarization and nitrate incorporation was investigated via scanning electron microscopy (SEM).Once added into silanization solution, sodium nitrate promoted the reaction of silane condensation and enhanced the film compactness.Electrochemical impedance spectroscopy results (EIS) indicated that doping silane film with NaNO₃ ameliorated its barrier property and protectiveness. Silane films applied onto AA2024-T3 surface by using potentiostatic method, exhibited obviously higher corrosion resistance than those obtained by conventional “dip-coating” method. The resistance of coating is accentuated when not very negative potential was applied.     

A comparative study on the corrosion resistance of AA2024-T3 substrates pre-treated with different silane solutions: Composition of the films formed

This work reports a comparative study on the corrosion resistance of AA2024-T3 pre-treated with three different silane solutions. The silanes used for the pre-treatments of the AA2024-T3 panels were: 1,2-bis(triethoxysilyl)ethane (BTSE), bis-[triethoxysilylpropyl]tetrasulfide (BTESPT) and γ-mercaptopropyltrimethoxysilane (γ-MPS). The analytical characterisation of the silane films was performed by Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS). The corrosion performance of the pre-treated substrates was evaluated by electrochemical impedance spectroscopy (EIS). The results show that the pre-treatments based on silanes provide good corrosion protection of unpainted AA2024-T3. Painted substrates, previously pre-treated with the silane solutions also revealed improved corrosion resistance and good adhesion properties. Fatigue tests show that the silane pre-treatments do not affect the fatigue behaviour of the AA2024-T3. The work also discusses the formation of the protective silane films.     

A study of the mechanisms of corrosion inhibition of AA2024-T3 by vanadates using the split cell technique

The mechanisms of corrosion inhibition of AA2024-T3 by vanadates were studied in this work using the split cell technique and polarization curves. The electrochemical behavior of clear solutions containing metavanadates and orange solutions containing decavanadates was clearly distinctive. Injection of metavanadates to the cathode side of the different split cell setups greatly reduced the galvanic current, indicating a potent inhibition of the oxygen reduction kinetics. The galvanic current never exhibited a transient current peak, suggesting that metavanadates inhibit AA2024-T3 corrosion by a mechanism that does not involve electrochemical reduction. Injection of metavanadate to the anode side of the different split cells had no effect on the galvanic current. Injection of orange decavanadate to the cathode side of the AA2024-T3 split cell resulted in a large current peak, associated with the electrochemical reduction of decavanadate. However, decavanadates did not impart significant corrosion protection.     

Computational modeling of inhibitor release and transport from multifunctional organic coatings

A computational code that was originally designed to model crevice corrosion was extended to multifunctional coatings on Al alloys exposed to thin layers of electrolytes. The model is able to calculate the transient distributions of potential, current density, and all chemical species concentration, enabling the dynamic simulation of inhibitor release, inhibitor transport, and sacrificial cathodic protection. The model has been applied to both inhibitor release from and aggressive anion capture by hydrotalcites (HTs) pigments in epoxy primer coatings applied to AA2024-T3. Computational studies were carried out to investigate the effects of HT/vanadate (HT/V) epoxy coating system parameters including scratch size, inhibitor release rate, Cl⁻ gettering rate (GR), cathodic kinetics on the bare AA2024-T3, and solution layer thickness on system performance. The analyses of the computational results have quantified the important factors controlling successful corrosion inhibition by inhibitor release from coatings. The pH-dependence of the steady state inhibitor release rate was found to be the most important parameter controlling system performance. Cl⁻ gettering can also reduce the aggressiveness of solution at long times, especially when considered in conjunction with inhibitor release. However, the ion exchange capacity required poses stiff design challenges involving the loading of the ion exchanger into the resin and the service conditions. The effectiveness of inhibition decreased significantly for the larger scratch sizes. The cathodic kinetics within the scratch play an important role in determining the ability of a given inhibitor to function effectively. When the scratch is the cathode in the galvanic couple with the substrate under the coating, inhibition was more effective. For the conditions simulated here, the net effect of a decreased solution layer thickness is to increase the protection ability of the system. The increase in the inhibitor concentration overcomes the decrease in the pH at the anode.     

Adsorption and inhibitive properties of Apigenin derivatives as eco-friendly corrosion inhibitors for brass in nitric acid solution

Six apigenin derivatives were synthesized through Mannich reaction by using apigenin isolated from Hypericum perforatum as raw material. The inhibition performance of these apigenin derivatives were investigated as eco-friendly corrosion inhibitors for brass in 1.0 M HNO₃ solution by means of weight loss, potentiodynamic polarization, and electrochemical impedance spectroscopy (EIS) measurements. The obtained results show that the as-synthesized apigenin derivatives act as efficient inhibitors for brass in 1.0 M HNO₃. The inhibition efficiency increased with the increase in inhibitor concentration but decreased with a rise in temperature. The inhibition efficiency higher than 90% for these compounds was found even at a low concentration of 30?mg L^?1, which is superior to the commonly used synthetic organic corrosion inhibitors for brass in acidic media. Electrochemical studies indicate that the inhibitors are of mixed type but predominantly cathodic in HNO₃ solution. The inhibitor performance depends on the adsorption of the molecules on the metal surface. The thermodynamic parameters for inhibiting process were calculated according to the statistical model. The calculated thermodynamic parameters revealed that the adsorption of these inhibitors on brass surface was spontaneous, controlled by physiochemical processes. The adsorption behaviour of these apigenin derivatives on the surface of brass was analyzed utilizing SEM, AFM, XPS, and Raman spectroscopy measurements. The results confirmed that the apigenin derivatives prevented corrosion of brass by forming protective layer on its surface.     

Silane and epoxy coatings: A bilayer system to protect AA2024 alloy

This work has proved that a good combination of a simple and fast metal pre-treatment, followed by the deposition of a thin layer of an organic–inorganic silane coating and further layer of epoxy coatings, are able to protect the aluminium alloy AA2024-T3 against corrosion in high concentrations of NaCl solution. The alloy AA2024 is one of the most employed aluminium alloy in structural applications due to its good mechanical properties. However, AA2024 alloy series commonly presents galvanic corrosion due to the rich content of copper element. The influence of different surface pre-treatments, the presence of a silane layer as pre-coating treatment and the influence of phosphonic acids combined with the silane layer on the corrosion protection and adhesion to the aluminium alloy have been examined using accelerated corrosion tests. High roughness and the presence of a pre-coating film between the metal surface and the organic coating were essential for a good protection and resistance to blistering appearance in the surface of AA2024-T3.