Pyrrole-based silane primer for corrosion protection of commercial Al alloys. Part II. Corrosion performance in neutral NaCl solution

A chromate-free, direct-to-metal treatment using pyrrole-based silane (PySi) was developed for protection against corrosion of as-received commercial Al alloys, following the typical procedure for silane deposition. The protection performance of composite PPySi films, containing polysiloxane linkages and polypyrrole units, was evaluated in near neutral NaCl solution by simple corrosion tests such as single-cycle anodic polarization, corrosion potential monitoring and long-term immersion experiments. Control coatings of polymethylsiloxane (PMeSi) and electrochemically synthesized polypyrrole (Ppy) were also studied. The superior performance of PPySi with respect to PMeSi and Ppy was attributed to highly crosslinked, well-packed and adherent composite films of thickness of the order of microns, manifesting both barrier action and active protection. The use of pyrrole-based silane for corrosion protection of Al alloys constitutes a promising approach for effective replacement of chromium-based treatments in practical applications. Further investigation from the fundamental point of view is deserved.     

Evidence for formation of metallo-siloxane bonds by comparison of dip-coated and electrodeposited silane films

It has been amply demonstrated that thin films of organofunctional silanes deposited by dipping or spraying on metals, such as aluminum, can provide protection against various forms of corrosion. In this paper we show that denser films with higher pore resistance and better corrosion protection performance can be obtained if the silane film is produced by electrodeposition rather than by dipping. In such a process the silane reacts with the metal oxide in a different way, and in the case of aluminum, aluminate ions seem to be incorporated into the silane film. The resulting films can protect aluminum alloys against uniform and pitting forms of corrosion in a salt solution for more than 1000 h, which is comparable to the standard chromate-based treatments. This paper discusses the protection mechanism in some detail. An important aspect of this work is that the use of XPS and TOFSIMS to analyze electrodeposited silane films provided direct evidence for the presence of O₂Al(O–Si–O) and OAl(O–Si–O)₂ groups at the silane–aluminum oxide interface. Such bonds have been suggested, but they have largely remained elusive over the years.     

Aniline-based silane as a primer for corrosion inhibition of aluminium

Hybrid films based on polyaniline and polysiloxane moieties were successfully deposited on commercial aluminium alloy using N-(3-trimethoxysilylpropyl)aniline (AnSi) as primer. The spectroscopic studies conducted on both the silane solution and the resulting films, jointly with several corrosion tests in NaCl, strongly support the protection performance of the hybrid films. The overall study demonstrates that typical silane-based treatments with principally barrier action can gain in active properties if the silane compound contains monomers of conducting polymers as a functional group.     

Bonding and corrosion protection mechanisms of γ-APS and BTSE silane films on aluminum substrates

Films of γ-aminopropyltriethoxysilane (γ-APS), 1,2-bis[triethoxysilyl] ethane (BTSE) and their mixtures adsorbed onto pure aluminum from aqueous solutions were characterized by means of ellipsometry, infrared spectroscopy (IR) and X-ray photoelectron spectroscopy (XPS). It was found that after hydrolysis in water the silanes were readily adsorbed onto aluminum oxide surfaces initially forming hydrogen bonds. Upon curing, such bonds are replaced by metallosiloxane bonds, Si - O - Al. The remaining silanol groups in the film condense and form Si - O - Si bonds. As the Si - O - Al bonds are known to hydrolyze, the corrosion protection is related to the hydrophobicity of the siloxane films formed on the metal substrate. BTSE films are acidic as they contain free silanol groups, therefore these are compatible with some paints but not with others. Electrochemical impedance spectroscopy (EIS) results, salt spray test results and filiform corrosion test results showed that some silane treatments, such as two-step γ-APS/BTSE and BTSE only, provided better corrosion protection on aluminum substrates as compared with a chromate treatment. Mechanisms of adhesion and corrosion protection of these silane films on aluminum substrates are proposed.     

Modification of bis-silane solutions with rare-earth cations for improved corrosion protection of galvanized steel substrates

Chemical formulations based on silane solutions are currently used for the pre-treatment of metallic substrates. These pre-treatments provide corrosion protection of the metallic substrates due to the good barrier properties of the silane films that form on the surface. The corrosion protection of silane-based pre-treatments can be improved by adding dopants to the silane solutions. The dopants must present corrosion inhibition properties and must keep or improve the barrier properties of the silane film without modification of surface functionality and bulk properties.     

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.     

Corrosion protection of Nd–Fe–B magnets by silanization

Nd–Fe–B type permanent magnets present excellent magnetic properties. However, they are highly sensitive to the attack of corrosive environment. The aim of the present work is to describe the improvement of corrosion resistance of such magnets by dodecyl trimethoxysilane (DTMS) treatment. Electrochemical impedance spectroscopy (EIS) tests showed that, after treated by either DTMS, phosphate or chromate precursor, the protectiveness of Nd–Fe–B alloys was apparently increased. Corrosion performance of magnet treated with silane only was slightly inferior to those of phosphatized or chromatized ones. However, significant improvement in corrosion protection was achieved after two-step treatments, i.e. by top-coating silane films on phosphate or chromate under-layer. The depth profiles of element concentrations on surfaces measured by secondary ion mass spectroscopy (SIMS) indicate a two-layer surface structure for two-step treated samples. Co-existing of silane and phosphate or chromate was detected in the top-layer, suggesting the interaction between the two components. This results in the formation of more stable structure of hybrid organ–inorganic coatings against the attack of corrosive species.     

Multinuclear 1D- and 2D-NMR study of the hydrolysis and condensation of bis-1,2-(triethoxysilyl)ethane

This paper focuses on the study of the effect of aging on the composition of the solutions used for deposition of silane films onto metals. Such films are used for corrosion protection and adhesion applications. The silane films interact with metallic surfaces by reactions between the OH groups of both materials with elimination of water. Therefore, prior to the immersion of the metal into the silane solution, the silane has to be hydrolysed in order to generate a maximum of silanol (SiOH) groups, while minimizing condensation leading to siloxane (SiOSi) groups. One of the main factors influencing the amounts of silanol and siloxane groups produced is the solution aging. This paper demonstrates by NMR spectroscopy that in the case of a bridged silane, namely bis-1,2-(triethoxysilyl)ethane, a stable plateau offering a good compromise between hydrolysis and condensation in the solution is obtained between 24 h and a few days of aging.     

铝合金表面新型硅烷膜的制备及表面形貌研究

通过正交试验确定了铝合金表面硅烷膜技术的最佳工艺参数。在基础配方上制备了硅烷膜,利用硫酸铜点滴试验评价了工艺参数,结果表明,水解温度、硅烷浓度、醇水比及pH主要影响了硅烷溶液的水解与缩合反应。分析了硅烷膜的表面形貌、初步探讨了硅烷膜的耐腐蚀性能,结果表明,硅烷膜均匀、致密,具有比铬酸盐钝化更优的防护作用。     

Electrochemical investigation of high-performance silane sol–gel films containing clay nanoparticles

Silane sol–gel coatings are widely used as adhesion promoters between inorganic substrates, such as metals, and organic coatings. The aim of these pre-treatments is to enhance the corrosion protection performance of the organic coating improving the adhesion to the substrate and acting as a barrier against water and aggressive ions diffusion. It is a matter of fact that the silane sol–gel pre-treatments do not provide an active protection against corrosion processes except for the partial inhibition of the cathodic reaction. Inorganic pigments can improve the barrier properties of the silane sol–gel film, enhancing the resistance against corrosion. In this study, different amounts of montmorillonite nanoparticles were added to a water based silanes mixture in order to improve the barrier properties of the sol–gel coating. Hot dip galvanized steel was used as substrate. The sol–gel film consists of a combination of three different silanes, GPS, TEOS and MTES. The clay nanoparticles used in this study were mainly neat montmorillonite. The proper concentration of filler inside the sol–gel films was determined comparing the corrosion resistance of silane layers with different nanoparticles contents. Additionally, the effect of CeO₂ and Ce₂O₃ enriched montmorillonite particles. The EIS analysis and the polarization measurements demonstrated that the optimal amount of neat montmorillonite nanoparticles is about 1000 ppm. The same electrochemical techniques highlighted the limited effect of the cerium oxides grafted to the clay nanoparticles on the corrosion resistance of the silane sol–gel film. The TEM analysis proved the presence of a nano-crystalline structure inside the silane sol–gel film due to the formation of crystalline silica domains.     

Chemical composition and corrosion protection of silane films modified with CeO2 nanoparticles

The present work aims at understanding the role of CeO₂ nanoparticles (with and without activation in cerium(III) solutions) used as fillers for hybrid silane coatings applied on galvanized steel substrates.The work reports the improved corrosion protection performance of the modified silane films and discusses the chemistry of the cerium-activated nanoparticles, the mechanisms involved in the formation of the surface coatings and its corrosion inhibition ability.The anti-corrosion performance was investigated using electrochemical impedance spectroscopy (EIS), the scanning vibrating electrode technique (SVET) and d.c. potentiodynamic polarization. The chemical composition of silanised nanoparticles and the chemical changes of the silane solutions due to the presence of additives were studied using X-ray photoelectron spectroscopy (XPS) and nuclear magnetic resonance spectroscopy (NMR), respectively.The NMR and XPS data revealed that the modified silane solutions and respective coatings have enhanced cross-linking and that silane-cerium bonds are likely to occur.Electrochemical impedance spectroscopy showed that the modified coatings have improved barrier properties and the SVET measurements highlight the corrosion inhibition effect of ceria nanoparticles activated with Ce(III) ions. Potentiodynamic polarization curves demonstrate an enhanced passive domain for zinc, in the presence of nanoparticles, in solutions simulating the cathodic environment.     

氨丙基甲基二甲氧基硅烷防腐保护镀锌钢板的研究

本次实验采用氨丙基甲基二甲氧基硅烷在镀锌钢板表面制备抗腐蚀保护膜,采用正交实验得出硅烷钝化最优工艺条件：硅烷浓度V（SCA）=2mL,钝化时间t1=10s,固化温度T=100℃,固化时间t2=5h。采用醋酸铅点滴和交流阻抗研究试样的耐腐蚀性能,实验结果表明试样经硅烷钝化后耐腐蚀性得到大幅度提高,耐黑变时间达到35s,阻抗值达到1600n。初步讨论了硅烷的成膜机理和各因素对成膜的影响。     

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.     

Electrodeposition of silane films on aluminum alloys for corrosion protection

In this paper, three types of protective silane films, methyltrimethoxysilane (MTMS), vinyltrimethoxysilane (VTMS) and dodecyltrimethoxysilane (DTMS) were prepared on aluminum alloys AA 2024-T3 by electrodeposition technique. The Reflection-Absorption Fourier Transform IR (FTRA-IR) measurements showed that, the silane films were successfully deposited through chemical bonding between silane agents and Al alloys. Electrochemical impedance spectroscopy (EIS) tests indicated that in comparison with those by conventional “dip-coating” method, silane films electrochemically prepared at cathodic potentials exhibited obviously higher corrosion resistances. “Critical potential” was all observed for each silane system. Silane films prepared at this potential performed the highest corrosion resistance. The scanning electron microscopy (SEM) images indicated a potential dependence of surface morphology of silane films. The highest compactness was obtained at the “critical potential”. Due to the presence of long hydrophobic dodecyl chain in bone structure, DTMS films displayed the highest barrier properties.     

Effects of electrodeposition potential on the corrosion properties of bis-1,2-[triethoxysilyl] ethane films on aluminum alloy

Bis-1,2-[triethoxysilyl] ethane (BTSE) films were prepared on 2024-T3 alloys by using potentiostatic method for corrosion protection. This work mainly investigated the effects of electrodeposition potential on the corrosion properties of silane films. Films prepared at cathodic potentials display an improvement in corrosion inhibition properties, while those prepared at anodic potentials present the deterioration of protectiveness. In the case of cathodic deposition, when the potential shifts negatively from the open-circuit potential (OCP), corrosion protection of the obtained films initially increases and then decreases, with the optimal deposition potential at −0.8 V/SCE. As indicated in scanning electron microscopy (SEM) images, films deposited at the optimum potential present the most uniform and compact morphologies. In addition, steady-state polarization and current-time curves have been also recorded on Al alloys in BTSE solutions during the deposition, respectively.     

铝合金表面GPTMS硅烷膜的制备及耐蚀性能研究

为提高铝合金耐腐蚀力,运用正交试验法研究在铝合金表面制备 γ-（2,3-环氧丙氧）丙基三甲氧基硅烷（GPTMS）自组装膜最佳工艺条件,利用极化曲线和扫描电子显微镜研究该硅烷膜在铝合金表面的耐腐蚀性能。研究表明：最佳工艺条件为 100 mL溶液中, pH＝4. 5,V（GPTMS）∶V（EtOH）∶V（H₂O）= 2∶7∶91,T_{1（水解温度）}＝25 ℃,t_{1（水解时间）}＝7 h,t_{2（浸涂时间）}＝10 min,t_{3（固化时间）}＝90 min,T_{2（固化温度）}＝120 ℃,该工艺条件下制备的硅烷膜具有优异的耐腐蚀性能。     

Improvement of corrosion protection offered to galvanized steel by incorporation of lanthanide modified nanoclays in silane layer

Silane sol–gel based films are very promising alternatives to the traditional chromate pre-treatments. However, the protection offered by the silane films strongly decreases when the coating is damaged. Some previous studies showed that the barrier properties of the silane layer can be improved by incorporation of clay nanoparticles. Moreover, inhibitive metallic cations can be incorporated in the nanoclays by ion exchange, providing a way to prepare cheap corrosion inhibitors nanoreservoirs offering self-healing properties. Rare earth (RE) metal salts have been shown to be effective corrosion inhibitors on a wide range of metals, including hot dip galvanized (HDG) steel. For this study, montmorillonite clay is modified to obtain a Ce(III) montmorillonite clay (Ce-MMT). The amount of incorporated Ce(III) is characterized by means of XRF measurements. X-ray diffraction showed that the Ce(III) is located in the interlayer space.     

Methacryloxypropyltrimethoxysilane films on aluminium: Electrochemical characteristics, adhesion and morphology

The electrochemical characteristics, adhesion and morphology of methacryloxypropyltrimethoxysilane (MAPT) films on aluminium were investigated during exposure to 3 wt.% NaCl. The MAPT films were deposited on aluminium surface from 2 to 5 vol.% methacryloxypropyltrimethoxysilane solutions, with the aim to investigate the influence of deposition parameters (silane solution concentration and curing time) on electrochemical characteristics, adhesion and morphology of MAPT films on aluminium.Using electrochemical impedance spectroscopy (EIS), potential–time measurements, adhesion measurements and optical microscopy coupled with image analysis, it was shown that films deposited from 5 vol.% solution exhibited better corrosion stability and adhesion, as well as lower porosity comparing to 2 vol.% solution and improved the corrosion protection of aluminium substrate, while the curing time had no influence on these characteristics.     

Electro-assisted preparation of dodecyltrimethoxysilane/TiO2 composite films for corrosion protection of AA2024-T3 (aluminum alloy)

Thin films of organosilanes have been successfully used as the alternative to toxic chromate coatings for surface pretreatment of metals and alloys. To further improve their corrosion performance, in the present work nano-scaled TiO₂ particles were added to the dodecyltrimethoxysilane (DTMS) films coated onto AA2024-T3 substrates, by using either the dip-coating or the cathodically electro-assisted deposition process. The obtained composite films were investigated by scanning electron microscopy (SEM), atomic force microscopy (AFM), water contact angle measurements, Fourier transform reflection-absorption IR (FTRA-IR) and electrochemical impedance spectroscopy (EIS). The results show that these two techniques (nanoparticles incorporation and the electro-assisted deposition) both facilitate the deposition process of silane films, giving thicker deposit and higher coverage surface along with higher roughness and hydrophobicity, and thereby improve their corrosion resistance. Moreover, the corrosion performance of silane films is further improved by the combined use of nanoparticles modification and electro-assisted deposition.     

防腐涂料用苯丙乳液的新型改性偶联剂的选择

采用新型二烷氧基型硅烷偶联剂及传统三烷氧基型硅烷偶联剂对苯丙乳液进行改性,通过电化学阻抗方法对改性前后涂膜的防腐性能进行研究。结果表明,改性涂膜的防腐性能优于未改性涂膜,而新型二烷氧基型硅烷偶联剂KH-578改性苯丙乳液的防腐性能最佳。