The 1,2,4-triazole, 3-amino-1,2,4-triazole, benzotriazole and 2-mercaptobenzothiazole were evaluated in the present work as corrosion inhibitors for protection of the 2024 aluminium alloy in neutral ...chloride solutions. The corrosion protection performance was investigated by means of DC polarization and electrochemical impedance spectroscopy (EIS). Scanning Kelvin probe force microscopy (SKPFM) and atomic force microscopy (AFM) were used to study the evolution of the Volta potential distribution and the surface topography during corrosion tests.
The results show that all inhibitors under study confer corrosion protection to the AA2024 alloy forming a thin organic layer on the substrate surface. Benzotriazole and 2-mercaptobenzothiazole offer better corrosion protection in comparison with the other two. The inhibitors studied act decreasing the rate of both the anodic and cathodic processes. In the latter case the dealloying of the copper-reach particles is hindered, slowing down the oxygen reduction.
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•A sandwich coating consisting of a LDH conversion layer and a sol-gel coating was produced.•The sandwich protective system offers long term active corrosion protection to ...AA2024.•Localized measurements demonstrate efficient self-healing in artificially made defects in the coating.•The main mechanism of inhibition involves release of inhibiting vanadate anions from LDH.
In this work active corrosion protection of AA2024 by a bilayer system consisting of a LDH (layered double hydroxides) conversion layer coated by a sol-gel film was studied. Impedance and Scanning vibrating electrode technique (SVET) studies demonstrated an efficient active corrosion protection and self-healing abilities of the systems containing LDH layer loaded with vanadate corrosion inhibitor. Microstructural and chemical analysis of the surface after corrosion testing showed accumulation of chlorine signal in LDH coatings. The scavenging of chlorides and release of vanadate inhibitor from LDH can explain high efficiency of corrosion inhibition of AA2024 by the novel protective system.
•Carboxylates modify kinetics of Mg(OH)2 film growth on Mg during aqueous corrosion.•Mg(OH)2 nano-crystals formed in inhibitors solutions limit Cl− access to Mg surface.•Salicylate adsorbs chemically ...on Mg(OH)2, dissolves Fe-containing inclusions.•2,5-pyridinedicarboxylate precipitates in the form of coordination polymer.•Inhibition by fumarate seems to be due to adsorption via carboxylate group.
Mechanisms of inhibition of Mg aqueous corrosion in presence of chloride by sodium salicylate (Sal), 2,5-pyridinedicarboxylate (PDC) and fumarate (Fum) were studied by in situ Raman spectroscopy, ATR-FTIR, GD-OES and hydrogen collection. In situ detected surface films were composed by Mg(OH)2 nano-crystals and included inhibitors. All carboxylates significantly modified Mg(OH)2 growth kinetics as well as pevented chloride incorporation in the film. Vibrational spectra of the surface films demonstrated specific interactions between the carboxylates and the surface: adsorption of Sal and Fum on the oxide/hydroxide, precipitation of coordination polymer by PDC, dissolution of iron inclusions via formation of iron-Sal soluble complexes.
•The combined addition of calcium and yttrium significantly improved the corrosion resistance of the die-cast AZ91D alloys.•The formation of the Al-Mn-Y phase instead of the Al-Mn-Fe phase ...significantly reduced the VPD values, which decreased micro-galvanic corrosion.•The stability of the surface film of the AZXW911 alloy were improved by the adding of a small amount of Y.
The corrosion behaviours of commercial AZ91D alloy and Ca–Y-containing AZ91 alloy, fabricated by casting, were evaluated, and their microstructural changes and surface properties were analysed. The Ca–Y-containing AZ91 alloy showed superior corrosion resistance compared with the AZ91D alloy mainly because of the decrease in the Volta potential difference between the matrix and the secondary phases formed in the Ca–Y-containing alloy, such that micro-galvanic corrosion was significantly suppressed. Furthermore, passivity and stability of the surface film improved with the addition of Y to the AZ91 alloy. Thus, the overall corrosion resistance was significantly improved in the Ca–Y-containing AZ91 alloy.
Amorphous cerium molybdate nanowires (CMN) have been used to impart active corrosion protective properties to hybrid sol–gel coatings on AA2024. Inhibitive ability of the novel sol–gel coatings ...against corrosion has been studied in detail by electrochemical impedance spectroscopy (EIS) and scanning vibrating electrode technique (SVET). Localized electrochemical measurements demonstrate significant suppression of corrosion activity in micro-scale defects made in sol–gel coatings loaded with CMN when compared to blank systems. The mechanism of active protection involves release of cerium and molybdate ions from the sol–gel coating during immersion in NaCl electrolyte and their inhibiting action in corroding defects.
The inhibiting effect of several organic compounds on the corrosion of 2024 aluminium alloy in neutral chloride solution was investigated in the present work. The candidates were selected based on ...the assumption that effective inhibitors should form highly insoluble complexes with components of AA2024. Along with organic complexing agents, the salts of rare-earth elements were included into screening electrochemical impedance spectroscopy (EIS) test for getting comparative data. Results of EIS analysis revealed three most effective organic inhibitors: salicylaldoxime, 8-hydroxyquinoline and quinaldic acid. Their anti-corrosion performance was additionally investigated via dc polarization, as well as localized techniques: scanning electron microscopy coupled with energy dispersive spectroscopy (SEM/EDS), X-ray photoelectron spectroscopy (XPS) and atomic force microscopy coupled with scanning Kelvin probe (SKPFM). Localized measurements at exactly the same microdimensional zones of the alloy before and after immersion into 0.05
M sodium chloride solution allowed tracing the evolution of the Volta potential, chemical composition, surface topography and formation of corrosion products on the surface and intermetallic inclusions during the corrosion tests. The results show that the quinaldic acid, salicylaldoxime and 8-hydroxyquinoline provide anti-corrosion protection for AA2024 forming a thin organic layer of insoluble complexes on the surface of the alloy. Inhibiting action is the consequence of suppression of dissolution of Mg, Al and Cu from the corrosion active intermetallic zones.
Nanostructured hybrid sol-gel coatings doped with cerium ions were investigated in the present work as pre-treatments for the AA2024-T3 alloy. The sol-gel films have been synthesized from ...tetraethylorthosilicate (TEOS) and 3-glycidoxypropyltrimethoxysilane (GPTMS) precursors. Additionally the hybrid sol was doped with zirconia nanoparticles prepared from hydrolyzed tetra-n-propoxyzirconium (TPOZ). Cerium nitrate, as corrosion inhibitor, was added into the hybrid matrix or into the oxide nanoparticles.
Corrosion protection and surface properties of Magnesium (Mg) containing 51 ppm Fe (HP-Mg) and 341 ppm Fe (CP-Mg) were assessed by EIS, AFM/SKPFM, and photoluminescence spectroscopy in NaCl solutions ...with/without fumarate (Fum), 2,5-pyridinedicarboxylate (PDCA) and salicylate (Sal) carboxylates. The PDCA was effective in corrosion inhibition of HP-Mg and CP-Mg, while the Sal was efficient in inhibition of CP-Mg only. Volta potential (VPD) changes on Mg were evaluated considering the contribution of molecular dipoles and chemical dipoles due to interaction of carboxylate groups present in carboxylates with Mg. VPD of CP-Mg increased due to adsorption of Fe(III)-salicylate complexes and salicylate molecules.
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•2,5-pyridinedicarboxylate inhibits corrosion of Mg with high and low iron levels.•Salicylate is an efficient corrosion inhibitor for Mg with high iron levels only.•Adsorption of salicylate (Sal) and FeIII-Sal complexes increase Volta potential of Mg.•Volta potential of pure Mg decreases after immersion in NaCl solution.
•Model anodised aluminium samples were coated with sol-gel and characterized by EIS.•The contribution of various interface phenomena to the EIS spectra was discussed.•The barrier oxide film was ...always visible as a time constant on the EIS spectra.•Degradation of the sol-gel/oxide interface was assessed via oxide capacitance.
A proper assignment of time constants is important for the correct interpretation of the electrochemical impedance spectra of coated substrates. In this work, several model aluminium samples with different anodic oxide thicknesses were prepared and characterised. The aluminium samples were coated by a hybrid sol-gel formulation and studied by electrochemical impedance spectroscopy (EIS) during immersion in NaCl solution. A hierarchical model containing three R-C elements adequately described the impedance spectra of the coated samples. The R-C elements described the properties of sol-gel coatings, aluminium oxide layers and corrosion process. The contribution of the aluminium oxide layer to impedance was unambiguously proven to be at middle frequencies on the spectra. Furthermore, the “real” capacitances of oxide layers were obtained from two capacitance–CPE equations and their applicability was scrutinized. The obtained results presented an opportunity to determine the adhesion behaviour of the sol-gel coatings to the oxide layers, which was studied by analysing the ratio of the oxide layer capacitance of the sol-gel coated samples to the capacitance of the uncoated ones. It was demonstrated that the exposed surface area of the oxide to the electrolyte was about 60–68% and was not changing during immersion. Based on these findings, a degradation model of the sol-gel coating on anodised aluminium was proposed.
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The present work explores effects of shelf-life, temperature and the presence of cerium nitrate additive on the protective performance of sol–gel coatings applied on AA2024. These are important ...parameters when industrial applicability of coating formulations is considered. The studied sol–gel system is based on a combination of metalorganic precursor titanium(IV) propoxide and hybrid epoxy silane 3-glycidoxypropyltrimethoxysilane. The synthesized sol–gel formulations without or with addition of 0.007M Ce(NO3)3 were aged during different times at temperatures 25°C or −6°C. FTIR results show that the intensities of the bands associated to SiOSi increase with respect to SiOH groups during the aging process as a result of extended condensation. However, such transformations occur less rapidly at low temperature. The electrochemical impedance spectroscopy measurements clearly show that sol–gel coatings corresponding to the sol–gel aged at low temperature exhibit similar corrosion protective capabilities compared to the coating obtained without sol–gel aging. It is also observed that the protection performance of the sol–gel coatings with cerium nitrate obtained after two weeks of sol–gel aging is lower compared to the coatings without cerium additive. This effect is attributed to the enhanced reactivity of the sol–gel due to cerium(III) presence.
•Influence of sol-gel ageing on the corrosion performance of sol-gel coatings has been studied.•Sol-gel ageing at room temperature creates microdefects in the coating matrix thus decreasing protective effectiveness.•Cerium nitrate chemically interacts with sol-gel during long term ageing at room temperature.•Ageing at low temperature improves sol-gel shelf-life and retains protection effectiveness of the sol-gel coatings.
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