Sol-gel coatings for corrosion protection of 1050 aluminium alloy

The formation of organic-modified sol-gel coatings on 1050 aluminium alloy has been examined. The coatings were prepared by the combination of a completely hydrolysable precursor of tetra-n-propoxyzirconium (TPOZ), with a partly hydrolysable precursor of glycidoxypropyltrimethoxysilane (GPTMS). Influences of GPTMS/TPOZ ratio, withdrawal speed and curing temperature on the barrier-type corrosion protection of the alloy were examined. By control of process parameters and ratios of organic and inorganic moieties, crack-free sol-gel coatings with improved corrosion protection were produced on the alloy surface. Optimum protection was achieved by a coating formed with a GPTMS/TPOZ ratio of 2.7, a withdrawal speed of 180 mm/min and curing at 110 °C. The coating formed under the optimum conditions revealed good adhesion to the etched and desmutted alloy substrate. The pitting potential of the coated alloy increased significantly, indicative of an improved corrosion protection of the alloy.     

铝合金铈盐转化膜的研究

提出了一种铝合金铈盐化学转化成膜工艺,利用电化学方法研究了铝合金铈盐化学转化膜的成膜过程及耐蚀性能,结果表明,成膜促进剂的加入能有效提高铈盐转化膜的成膜速度,所得转化膜对铝合金的点蚀有较好的抑制作用。     

On the corrosion protection properties of fluoropolymer coatings

Fluoropolymers have attained great importance as coating materials because of their excellent resistance to high temperature, chemicals and organic solvents. Electrochemical impedance spectroscopy was used to study the corrosion protection properties both on undamaged fluoropolymer coatings and on samples with an artificial defect. The experimental results were interpreted on the basis of a proposed equivalent electrical circuit which best fits the experimental impedance spectra. In this way we observed the high quality of the fluorinated coatings compared with that of the corresponding hydrogenated polymer used as a reference. The protective properties of the fluoropolymer coatings studied can be related to chemical (fluorine and chlorine functional groups) and physical (good order of the polymeric chains) characteristics of the resins.     

Protective ability of hybrid nano-composite coatings with cerium sulphate as inhibitor against corrosion of AA2024 aluminium alloy

The corrosion protective ability of hybrid oxy silane nano-composite coatings deposited on AA2024 by sol-gel technique was studied. The coatings are developed as an environmentally friendly alternative of the toxic chromium containing coatings on aluminium. A cerium salt, Ce₂(SO₄)₃, was used as inhibitor of the corrosion process. Two methods were applied to introduce the salt in the hybrid matrix: directly in the matrix, or by porous Al₂O₃ nano-particles preliminary loaded by the salt. Atomic force microscopy (AFM) was used to evaluate the superficial morphology of the coatings, while their layer structure was studied by means of scanning electron microscopy (SEM). Linear voltammetry (LVA) and electrochemical impedance spectroscopy (EIS) were used for assessment of the barrier ability. The hybrid matrix was found to possess remarkable barrier ability which was preserved even after prolonged exposure of the coatings to a model corrosive medium of 0.05 M NaCl. In all cases, the cerium salt involved either directly or by Al₂O₃ nano-particles proved to deteriorate the protective properties of the coatings and to accelerate pitting nucleation. The experimental results have shown that cerium sulphate, introduced in the by the both manners in the hybrid matrix did not efficiently inhibit the corrosion of AA2024, unlike the reported inhibiting properties of other cerium salts.     

Effective corrosion protection of AA6061 aluminum alloy by sputtered Al-Ce coatings

Al-Ce coatings were deposited on silicon and AA6061 aluminum alloy substrates by DC magnetron sputtering using aluminum in combination with pure cerium targets. The materials were characterized by X-ray diffraction (XRD), atomic force microscopy (AFM), scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM) and electrochemical impedance spectroscopy (EIS) in order to consider their application as high corrosion resistance coatings. The corrosion behavior of the films was studied using a NaCl aqueous solution (3.5 wt%). As for the characterization results, an apparent amorphous phase of aluminum oxide with small cerium compounds embedded in the matrix was detected by the X-ray diffraction patterns and HRTEM on the deposited films at 200 W and 4 Pa. At these conditions, AFM and SEM images evidenced crack-free coatings with low-roughness nanometric structures and columnar growth. EIS and Tafel results converged to indicate an inhibition of the corrosion reactions. The film displayed good stability in the aggressive medium and after 1 day of exposure underwent very little degradation. The variations in the impedance and Tafel characteristics were found to occur as a function of cerium content, which provokes important changes in the film protective properties.     

Active corrosion protection and corrosion sensing in chromate-free organic coatings

In this study it is shown that anion-exchanging hydrotalcite compounds dispersed as a particulate additive in organic resins leads to potent corrosion inhibition of an underlying aluminum alloy substrate. The use of this additive also imparts the ability to detect environmental changes in the coating that are a prelude to substrate corrosion. Corrosion inhibition is derived from release of a decavanadate from crystalline Al–Zn hydroxide-based hydrotalcite particles into electrolyte that has permeated the pore space of the coating. Decavanadate release is accompanied by uptake of chloride ion in an exchange reaction. The exchange of the large decavanadate anion for the smaller chloride ion in the hydrotalcite structure results in a predictable change in crystal structure, which can be detected by X-ray diffraction. The occurrence of the decavanadate-chloride exchange reaction indicates that aggressive electrolyte has invaded the coating and that corrosion may be imminent. In this paper, methods for synthesizing an Al–Zn-decavanadate hydrotalcite particulate suitable for dispersion in an epoxy resin are described. Results from exposure and electrochemical tests illustrating corrosion protection by the hydrotalcite pigmented coatings are presented. Additionally, the characteristic changes in the X-ray diffraction pattern of hydrotalcite associated with the decavanadate-chloride exchange are presented. Diffraction patterns collected from coated Al substrates are also presented, indicating that diffraction-based interrogation of coatings is possible.     

Electropolymerised polyaniline films on AA 7075 alloy and its corrosion protection performance

Polyaniline has been electrodeposited on AA 7075 alloy and its corrosion protection ability has been studied by Tafel and impedance techniques in 1% NaCl. Pure polyaniline film is not found to protect the aluminium alloy due to galvanic interaction of polyaniline and aluminium surface exposed through pinholes and cracks. However, it is found that the corrosion resistance property of the polyaniline film can be substantially increased by post-treatment in cerium salt solution.     

Electropolymerized polyaniline coatings on aluminum alloy 3004 and their corrosion protection performance

Homogeneous and adherent polyaniline coatings were electrosynthesized on aluminum (Al) alloy 3004 (AA 3004) from an aqueous solution containing aniline and oxalic acid by using the galvanostatic polarization method. A higher applied current density in the polymerization stage proved to be the best condition to adopt for the synthesis of more compact and strongly adherent polyaniline coatings on Al. The corrosion performances of polyaniline coatings were investigated in 3.5% NaCl solution by the potentiodynamic polarization technique and electrochemical impedance spectroscopy (EIS).Potentiodynamic polarization and electrochemical impedance spectroscopy studies reveal that the polyaniline acts as a protective layer on Al against corrosion in 3.5% NaCl solution. The current corrosion decreases significantly from 6.55 μA cm⁻² for uncoated Al to 0.158 μA cm⁻² for polyaniline-coated Al. The corrosion rate of the polyaniline-coated Al is found to be 5.17 × 10⁻⁴ mm year⁻¹, which is ∼40 times lower than that observed for bare Al. The potential corrosion increases from −1.015 V versus SCE for uncoated Al to ∼−0.9 V versus SCE for polyaniline-coated Al electrodes. The positive shift of ∼0.11 V in potential corrosion indicates the protection of the Al surface by the polyaniline coatings.The synthesized coatings were characterized by UV-visible absorption spectrometry, Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM). Optical absorption spectroscopy reveals the formation of the emeraldine form of polyaniline. The results of this study clearly ascertain that the polyaniline has outstanding potential to protect the AA 3004 alloy against corrosion in a chloride environment.     

Effect of SO4 on the corrosion behavior of cerium-based conversion coatings on galvanized steel

The cerium-based conversion coatings on galvanized steel were investigated and the influence of SO₄²⁻ on the corrosion behavior was evaluated. The coatings were prepared by a simple immersion of galvanized steels in an aqueous solution composed of cerium nitrate and sodium sulfate. The corrosion behavior was studied by means of potentiodynamic polarization measurements and neutral salt spray tests. The addition of SO₄²⁻ to the coating solution considerably improved the corrosion resistance of the conversion coatings. Atomic force microscope observation shows that deposits prepared from the solution with SO₄²⁻ have smaller crystalline size than those prepared without SO₄²⁻. The results of X-ray photoelectron spectroscopy and titration curves demonstrate that SO₄²⁻ ions are incorporated in the coating during the conversion process. This indicates that SO₄²⁻ acts as a grain refiner and/or growth inhibitor, thus enhancing the corrosion resistance.     

Novel hybrid sol-gel coatings for corrosion protection of AZ31B magnesium alloy

This work aims to develop and study new anticorrosion films for AZ31B magnesium alloy based on the sol-gel coating approach.Hybrid organic-inorganic sols were synthesized by copolymerization of epoxy-siloxane and titanium or zirconium alkoxides. Tris(trimethylsilyl) phosphate was also used as additive to confer additional corrosion protection to magnesium-based alloy. A sol-gel coating, about 5-μm thick, shows good adhesion to the metal substrate and prevents corrosion attack in 0.005 M NaCl solution for 2 weeks. The sol-gel coating system doped with tris(trimethylsilyl)-phosphate revealed improved corrosion protection of the magnesium alloy due to formation of hydrolytically stable Mg-O-P chemical bonds.The structure and the thickness of the sol-gel film were characterized by transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The corrosion behaviour of AZ31B substrates pre-treated with the sol-gel derived hybrid coatings was tested by electrochemical impedance spectroscopy (EIS). The chemical composition of the silylphosphate-containing sol-gel film at different depths was investigated by X-ray photoelectron spectroscopy (XPS) with depth profiling.     

Effect of silica/PVC composite coatings on steel-substrate corrosion protection

In this paper we focus on the preparation of thin polymer coatings synthetized from 30-nm and 600-nm silica particles dispersed in polyvinyl chloride (PVC) and deposited on two different steel substrates: duplex DSS 2205 and austenitic AISI 316L steel. We show that a silica surface modification with silane IO₇T₇(OH)₃ (trisilanol isooctyl polyhedral oligomeric silsesquioxane, POSS) significantly improves its dispersion properties when mixed with PVC. For comparison, the surface morphology and surface roughness of PVC coatings filled with both silanated and as-received (non-silanated) silica fillers were analyzed with scanning electron microscopy (SEM) and atomic force microscopy (AFM) when sprayed on the steel surface. The effect of the silica silanization is later on reflected in a decreased average surface roughness in the silanated, compared to non-silanated, silica/PVC-coatings. The wetting properties of the silanated and non-silanated silica/PVC-coatings on DSS 2205 and AISI 316L were investigated using contact-angle and surface-energy measurements, indicating an increased surface hydrophilicity in terms of a decreased static water contact angle and an increased total surface energy compared to the uncoated specimens. Finally, the beneficial corrosion resistance of the silica/PVC coatings was confirmed with potentiodynamic polarization spectroscopy in a 3.5% NaCl solution.     

Capped inhibitor-loaded halloysite nanoclay-based self-healing silica coatings for corrosion protection of mild steel

The effect of polymeric nanocapsule capping in benzotriazole encapsulated into halloysite nanoclay (HNTs) dispersed into hybrid silica coatings was investigated for corrosion protection of mild steel. Optimization of the amount of inhibitor-loaded halloysite nanotubes with and without capping in the coating sol was carried out. The prepared formulations were dip-coated on mild steel substrates using dip-coater and then cured at 130 °C for 1 h. Surface morphology and elemental analysis of the nanoclay were studied using scanning electron microscopy and energy dispersive X-ray spectroscopy. X-ray diffraction and Fourier Transform Infrared spectroscopy analyses were carried out to confirm the encapsulation and capping of the halloysite nanoclay. The anti-corrosion and autonomic-healing properties of bare and coated substrates in 3.5 wt% NaCl solution were studied using electrochemical impedance spectroscopy, potentiodynamic polarization measurements and scanning vibrating electrode technique for varying exposure times. The coatings generated from the capped inhibitor-loaded HNTs dispersed sol-gel matrix was seen to provide higher corrosion resistance when compared to uncapped HNT based silica coatings. Electrochemical studies carried out for capped inhibitor-loaded HNT based coatings have shown an increase in charge transfer resistance to 10⁸ Ω cm² from 10⁶ Ω cm² of uncapped inhibitor-loaded HNT based coatings.     

Hydrophobic benzoxazine-cured epoxy coatings for corrosion protection

A hydrophobic benzoxazine-cured epoxy coating (EPB) was prepared by a dip coating and thermal curing method using benzoxazine monomer (B-TMOS) as curing agent. Fourier transform infrared (FTIR) analyses confirmed the presence of thermal curing reactions and hydrogen-bonding interactions in the epoxy/polybenzoxazine system. The hydrophobicity of epoxy coatings induced by the incorporation of B-TMOS was enhanced significantly, and the water contact angles of resultant EPB coatings were higher than 98°. The corrosion protection ability of epoxy coatings was investigated by open-circuit potentials, potentiodynamic polarization curves and electrochemical impedance spectroscopy (EIS) methods. The results showed that the charge transfer resistance (R_ct) of EPB coatings was increased by about three orders of magnitude compared with bare mild steel, and the protection efficiency values of all EPB samples were more than 98%. This increased corrosion protection property could be attributed to the high hydrophobic performance of EPB coatings.     

Fabrication of active corrosion protection waterborne polyurethane coatings using cerium modified palygorskite nanocontainers

The development of eco-efficient and environmental friendly active anti-corrosion coatings for metallic substrates represents a fundamental milestone in many engineering applications. Herein, a new type of active corrosion protection coating was successfully synthesized simply based on waterborne polyurethane (WPU) and nanocontainer of Ce³⁺ modified palygorskite (Ce-PAL). Particularly, the nanocontainer of Ce-PAL was achieved via a facile and green process by cation exchange in water. Inductively coupled plasma atomic emission spectrometry (ICP) revealed that Ce³⁺ had successfully incorporated into PAL nanofibers with a loading amount of 10.334 g/kg. The salt immersion test, electrochemical potentiodynamic measurements and electrochemical impedance spectroscopy (EIS) demonstrated that the incorporation of Ce-PAL only with 2 wt% into WPU led to a significant improvement of their corrosion protection properties. Finally, according to the results of scanning electron microscopy with energy dispersive X-ray spectroscopy, the active anti-corrosion mechanism of Ce-PAL/WPU coating was proposed.     

A novel approach to heal the sol-gel coating system on magnesium alloy for corrosion protection

Sol-gel-based coatings exhibit high potentiality to be as an alternative to toxic chromate coatings for surface pre-treatment of metals and alloys. However, as soon as even small defects appear in the coating, the coating cannot stop the development of corrosion process. Present work demonstrates the possibility to use zinc nitrate as healing agent to repair the organic silane coatings in NaCl solution. The zinc nitrate was added to the 0.005 M NaCl solution where AZ91D magnesium alloy coated with organic silane coating was immersed. The healing process and the healing mechanism were investigated by electrochemical measurements and scanning electron microcopy coupled with energy dispersive spectroscopy. The results demonstrated the introduction of zinc nitrate to the electrolyte could stop the development of corrosion process of the coating system and a remarkable recovery on corrosion resistance could be obtained. This effect may be attributed to the formation of zinc oxide/hydroxide on the defective areas, hindering the corrosion activities.     

Sol–gel coatings on metals for corrosion protection

Sol–gel protective coatings have shown excellent chemical stability, oxidation control and enhanced corrosion resistance for metal substrates. Further, the sol–gel method is an environmentally friendly technique of surface protection and had showed the potential for the replacement of toxic pretreatments and coatings which have traditionally been used for increasing corrosion resistance of metals. This review covers the recent developments and applications of sol–gel protective coatings on different metal substrates, such as steel, aluminum, copper, magnesium and their alloys. The challenges for industrial productions and future research on sol–gel corrosion protective coatings are also briefly discussed.     

Enhanced corrosion protection for MAO coating on magnesium alloy by the synergism of LDH doping with deposition of 8HQ inhibitor film

The inherent micropores of micro-arc oxidation (MAO) coatings on magnesium alloy often cause poor long-term corrosion resistance. Herein, a low-porosity and high corrosion-resistant 8HQ/LDH/MAO composite coating, comprising a layered double hydroxide (LDH)-doped MAO primer and a top layer of 8-hydroxyquinoline (8HQ) inhibitor film, was deposited onto the magnesium alloy surface. Characterizations such as high-resolution field-emission transmission electron microscopy were performed to observe the synthesized nanoparticles’ morphology, size, composition, and structure. Results confirmed the successful synthesis of nitrate ion intercalated MgAl-LDH nanosheets and demonstrated the increase in thickness and denseness of the MAO coating after LDH doping. Based on electrochemical impedance spectroscopy and Tafel curves, the corrosion current density (j_corr) of the newly fabricated 8HQ/LDH/MAO composite coating was reduced by about three orders of magnitude. The low-frequency impedance modulus (|Z|_ƒ=0.1 Hz) increased by nearly four orders of magnitude compared with that of bare Mg alloy. After 14 days of exposure to the corrosive electrolyte, the composite coating maintained a low j_corr, showing significantly enhanced corrosion resistance compared with single MAO coating. These findings demonstrated the synergism of LDH doping with 8HQ sealing to enhance the corrosion protection of MAO coating on magnesium alloy.     

Corrosion behaviors of Zn/Al-Mn alloy composite coatings deposited on magnesium alloy AZ31B (Mg-Al-Zn)

After being pre-plated a zinc layer, an amorphous Al-Mn alloy coating was applied onto the surface of AZ31B magnesium alloy with a bath of molten salts. Then the corrosion performance of the coated magnesium alloy was examined in 3.5% NaCl solution by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). The results showed that the single Zn layer was active in the test solution with a high corrosion rate while the Al-Mn alloy coating could effectively protect AZ31B magnesium alloy from corrosion in the solution. The high corrosion resistance of Al-Mn alloy coating was ascribed to an intact and stable passive film formed on the coating. The performances of the passive film on Al-Mn alloy were further investigated by Mott-Schottky curve and X-ray photoelectron spectroscopy (XPS) analysis. It was confirmed that the passive film exhibited n-type semiconducting behavior in 3.5% NaCl solution with a carrier density two orders of magnitude less than that formed on pure aluminum electrode. The XPS analysis indicated that the passive film was mainly composed of AlO(OH) after immersion for long time and the content of Mn was negligible in the outer part of the passive film. Based on the EIS measurement, electronic structure and composition analysis of the passive film, a double-layer structure, with a compact inner oxide and a porous outer layer, of the film was proposed for understanding the corrosion process of passive film, with which the experimental observations might be satisfactorily interpreted.     

Spectroscopic and corrosion resistance characterization of GLYMO–TEOS Ormosil coatings for aluminum alloy corrosion inhibition

Corrosion resistance properties of spray- and dip-coated 3-glycidoxypropyltrimethoxysilane (GLYMO)–tetraethoxysilane (TEOS) Ormosil films have been investigated using salt spray analysis. ¹H–¹³C and ¹H–²⁹Si CP/MAS NMR analyses indicate that organic content and hydrolysis water ratio affect the Ormosil structure, and in turn, the corrosion resistance properties of the Ormosil film. For spray-coated samples, films derived from the 11 mol% GLYMO Ormosil prepared using hydrolysis water content (W=2 and 4) provided good corrosion protection, as this combination of organic content, water ratio, and deposition method produced films of the appropriate thickness, hydrophobicity, and density for good corrosion protection. Films prepared from high organic contents (25 and 67 mol% GLYMO) and low hydrolysis water content values (W=1 and 2) exhibited the best corrosion resistance for films prepared by dip coating. Presumably, these films comprise a dense network structure with organic groups dispersed throughout the film, providing a hydrophobic barrier coating capable of repelling water and corrosion initiators.     

Heat exchangers corrosion protection by using organic coatings

Corrosion phenomena in aluminium heat exchangers represent a problem in terms of durability and efficiency of thermal exchange. This work evaluates the barrier properties of two coatings that represent the state of the art for the protection in the heat, ventilating, air conditioning/refrigerating field (HVAC/R): electrophoretic coating (E-coating) using epoxy resin and spraying of polyurethane coating with addition of metallic pigments. The ability of the coatings to cover the surface of heat exchanger has been evaluated by means of optical microscopy in order to highlight critical zones of the system for the application of the coatings. The electrochemical behaviour of coated heat exchangers has been studied by means of electrochemical impedance spectroscopy in 3.5% NaCl solution. The local electrochemical behaviour of coating defects has been investigated using the electrochemical micro-cell, which enables to perform potentiodynamic polarization measurements on single defects. E-coatings evidenced difficulties to provide uniform thickness of the coating at the extremities of the fins. Spraying of the polyurethane coating containing pigments require particular care in order to fully cover zones of the heat exchanger with difficult accessibility. The electrochemical behaviour of coated heat exchangers is affected by the existence of defects in the coatings.