Corrosion protection of Mg alloys by cathodic electrodeposition coating pretreated with silane

The corrosion resistance characteristics of three coatings on magnesium alloy AZ31—conventional paint with phosphate film, cathodic electrodeposition coating (E-coating), and E-coating pretreated with silane (Mg/silane/E-coating)—have been studied by means of electrochemical impedance spectroscopy (EIS) in a 3.5 wt% NaCl neutral aqueous solution and salt spray test using ASTM B117. Silane film was obtained by dipping AZ31 specimens in diluted hydroalcoholic silanic solutions and successively curing. It was found that the corrosion resistance of the Mg alloy with E-coating was superior to conventional paint and could be further enhanced with silane pretreatment as an interfacial film. The results of water volume fraction (Φ_saturation) and diffusion coefficient (D) also indicated that the Mg/silane/E-coating possessed excellent compactness and corrosion resistance. A model of the corrosion mechanism for Mg/silane/E-coating has been presented through EIS analysis.     

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.     

Adhesion of Injection Molded PVC to Steel Substrates

Interactions occurring at the interface between injection-molded poly (vinyl chloride) (PVC) and steel substrates that were coated with thin films of aminosilanes were investigated by X-ray photoelectron spectroscopy (XPS) and Fourier-transform infrared spectroscopy (FTIR). The silane films were formed by adsorption of γ-aminopropyltriethoxysilane (γ-APS) or N-(2-aminoethyl-3-aminopropyl)trimethoxysilane (γ-AEAPS) from 2% aqueous solutions onto polished steel substrates. PVC was injection molded onto the silane-primed steel substrates and annealed at temperatures up to 170°C for times as long as 30 min. PVC was peeled off of the primed steel substrates using a 90° peel test and the substrate failure surfaces were thoroughly rinsed with tetrahydrofuran (THF) and distilled water to remove PVC and other compounds that were not strongly bonded to the substrates. The PVC failure surfaces were characterized by attenuated total reflection infrared spectroscopy (ATR) and PVC rinsed off of the substrate failure surfaces was characterized by transmission infrared spectroscopy. The resulting transmission and ATR spectra showed an absorption band near 1650 cm^-1 that was attributed to unsaturation in PVC. The substrate failure surfaces were characterized by XPS; curve-fitting of N(1s) and Cl(2p) high-resolution spectra showed the formation of amine hydrochloride complexes by protonation of amino groups of the silanes with HCl that was liberated from PVC during the onset of thermal dehydrochlorination. Furthermore, quaternization or nucleophilic substitution of labile pendent allylic chloride groups by amino groups on the silanes took place, thus grafting PVC onto the aminosilanes. It was determined that PVC that had β-chloroallyl groupings along its chains showed better adhesion with steel primed with aminosilanes and that generation of allylic chloride groups in PVC chains was the rate-limiting step in the reaction between PVC and aminosilane. Moreover, the effect of crosslinking of silane films on adhesion between PVC and aminosilane primed steel was investigated and it was concluded that interdiffusion of the polymer phase and the silane phase was also critical in obtaining good adhesion.     

Epoxysilane as adhesion promoter in duplex system

Coatings are one of the most used protection methods for metals. Metallic coatings, such as zinc and its alloys, are used to protect steel in mild corrosive environments. In aggressive environments, on the other hand, organic coatings must be employed in the so-called duplex systems. However, the galvanized steel/organic coating adhesion is a problem and many attempts had been done to solve it with the incorporation of a chromate-based or phosphate-based interlayer. Nowadays, the use of these compounds is questioned due to their environmental impact and new adhesion promoters, like silanes, are being investigated. The aim of this paper was to study the adhesion and the anticorrosive behavior of a duplex system with a layer of glycidoxypropyltrimethoxysilane (γ-GPS) between the zinc and the coating. Polarization tests and corrosion potential measurements were done on the γ-GPS/galvanized steel to select the better anticorrosive pretreatment conditions for the application of an organic traditional paint. Dried and wet adhesion of the coating to the pretreated substrate was studied by the standard tape test. Salt spray test and electrochemical noise technique were employed to study the corrosion behavior of the duplex systems. Results showed that the films of γ-GPS formed on galvanized steel diminished the corrosion current of the metal, but they do not protect the substrate by a barrier effect. The incorporation of the pretreatment in the duplex system increased the adhesion of the paint, especially when the pretreated substrate was cured 1?h at 200?°C.     

Adhesion of Injection Molded PVC to Steel Substrates

Interactions occurring at the interface between injection-molded poly (vinyl chloride) (PVC) and steel substrates that were coated with thin films of aminosilanes were investigated by X-ray photoelectron spectroscopy (XPS) and Fourier-transform infrared spectroscopy (FTIR). The silane films were formed by adsorption of γ-aminopropyltriethoxysilane (γ-APS) or N-(2-aminoethyl-3-aminopropyl)trimethoxysilane (γ-AEAPS) from 2% aqueous solutions onto polished steel substrates. PVC was injection molded onto the silane-primed steel substrates and annealed at temperatures up to 170°C for times as long as 30 min. PVC was peeled off of the primed steel substrates using a 90° peel test and the substrate failure surfaces were thoroughly rinsed with tetrahydrofuran (THF) and distilled water to remove PVC and other compounds that were not strongly bonded to the substrates. The PVC failure surfaces were characterized by attenuated total reflection infrared spectroscopy (ATR) and PVC rinsed off of the substrate failure surfaces was characterized by transmission infrared spectroscopy. The resulting transmission and ATR spectra showed an absorption band near 1650 cm^?1 that was attributed to unsaturation in PVC. The substrate failure surfaces were characterized by XPS; curve-fitting of N(1s) and Cl(2p) high-resolution spectra showed the formation of amine hydrochloride complexes by protonation of amino groups of the silanes with HCl that was liberated from PVC during the onset of thermal dehydrochlorination. Furthermore, quaternization or nucleophilic substitution of labile pendent allylic chloride groups by amino groups on the silanes took place, thus grafting PVC onto the aminosilanes. It was determined that PVC that had β-chloroallyl groupings along its chains showed better adhesion with steel primed with aminosilanes and that generation of allylic chloride groups in PVC chains was the rate-limiting step in the reaction between PVC and aminosilane. Moreover, the effect of crosslinking of silane films on adhesion between PVC and aminosilane primed steel was investigated and it was concluded that interdiffusion of the polymer phase and the silane phase was also critical in obtaining good adhesion.     

Enhanced corrosion resistance of AA 2024-T3 and hot-dip galvanized steel using a mixture of bis-[triethoxysilylpropyl]tetrasulfide and bis-[trimethoxysilylpropyl]amine

The corrosion resistance of AA 2024-T3 and hot-dip galvanized steel (HDG) was studied after treatment with bis-[3-(triethoxysilyl)propyl]tetrasulfide (bis-sulfur silane), bis-[trimethoxysilylpropyl]amine (bis-amino silane), and their mixture. Electrochemical tests in neutral 0.6 M NaCl as well as scanning electronic microscopy (SEM)/energy-dispersive X-ray spectroscopy (EDX) were performed. The results showed that: (1) hydrophilic bis-amino silane did not offer good corrosion protection on either of the metals. This is probably because the bis-amino silane film tends to be positively charged. This promotes ingress of anions like Cl⁻ ions as well as water into the film by electrostatic attraction. As a result, corrosion readily proceeds at the interface. (2) Hydrophobic bis-sulfur silane performed very well on AA 2024-T3, but failed on HDG. The failure here stems from non-uniform film coverage on HDG owing to an insufficient wetting of bis-sulfur silane solution on the Zn oxide on HDG. Local corrosion initiates at defective sites which are poorly covered by the silane film. (3) A bis-sulfur/bis-amino mixture at the ratio of 3/1 greatly enhanced the corrosion resistance of both AA 2024-T3 and HDG. This substantial improvement is achieved by selectively overcoming the major shortcomings of the individual silanes.     

Corrosion protection behavior of AZ31 magnesium alloy with cathodic electrophoretic coating pretreated by silane

As an alternative process to phosphate and chromate conversion coatings, silane pretreatment was used to improve the performance of cathodic electrophoretic coating (E-coat) on AZ31 Mg alloy in this study. The galvanic corrosion behavior of AZ31 Mg alloy with E-coat coupled with Q235 steel was investigated. Compared to bare Mg alloy and Mg alloy with conventional painting, the corrosion properties of the AZ31 Mg alloy pretreated with silane and subsequently E-coated were studied during salt solution immersion and salt spray testing. The surface morphologies of the Mg alloy were examined in detail after immersion in NaCl solution for different times using digital photography and scanning electron microscopy (SEM). The corrosion current density of the specimens was characterized by DC polarization tests. It was found that silane pretreatment of AZ31 Mg alloy followed by subsequent E-coat led to much better corrosion protection than that without silane treatment. The silane pretreatment and E-coat delayed the galvanic corrosion of Mg alloy coupled with 235 steel bolts.     

Formulation and testing of a waterborne primer containing chestnut tannin

The development of a water-based pretreatment system containing chestnut tannin and phosphoric acid and the assessment of its anticorrosive properties through different conventional tests are discussed in this work. Treated steel panels coated with different paint systems were subjected to standardized (salt spray, humidity chamber, adhesion, and flexibility) and electrochemical (corrosion potential, EIS) tests. Electrochemical tests were performed by employing only panels coated with the wash primer to determine its protective effect. The binder employed in this research was prepared in the laboratory by emulsion polymerization of acrylic monomers. It was found that the tested formulation protected steel against corrosion by forming ferric tannate, which prevented oxide formation. After performing the salt spray tests, it was observed that no oxide spots developed on the scratch mark, although some blisters were detected. The good stability of the binder in low pH media (pH: 2.0–2.5) and the binder/substrate interaction are decisive factors in the performance of this aqueous pretreatment system. Av. 52 entre 121 y 122, (1900) La Plata, Argentina; Tel. 54-221-4831141/44. Fax 54-221-4271537.     

Rubber-to-metal bonding by silanes

This paper deals with the bonding of sulfur-vulcanized rubber compounds to metals. It had previously been reported that bis-(triethoxysilyl)ethane and vinyltriethoxysilane were found to work for bonding peroxide-cured rubber compounds to metals. These silanes were found not to work with sulfur-cured rubber compounds. In this case, a mixture of bis-(trimethoxysilylpropyl)amine and bis-(triethoxysilylpropyl) tetrasulfide was found to work and results are presented of experiments in which brass, steel, and electrogalvanized steel were bonded to a typical tire cord skim compound with and without a cobalt additive. This new silane treatment was found to be as effective as or better than brass adhesion to cobalt-containing rubber compounds. The superior corrosion protection offered by the silanes was also demonstrated using DC corrosion and polarization resistance measurements. The silane process reported in this paper behaves almost similarly with all metal substrates. In this paper, the structure of the silane film on a metal substrate is studied and a model is proposed for a possible mechanism of the adhesion of rubber compounds with silane-coated metals.     

Silane sol–gel film as pretreatment for improvement of barrier properties and filiform corrosion resistance of 6016 aluminium alloy covered by cataphoretic coating

The aim of this study is to develop a newly silane sol–gel pretreatment on the barrier properties and filiform corrosion resistance of 6016 aluminium alloy covered by cataphoretic coating. The sol–gel coatings are used as coupling agent between aluminium substrate and cataphoretic paint. The pretreatment is an aqueous solution of three different silane compounds (glycidyloxypropyltrimethoxysilane (GPS), tetraethoxysilane (TEOS) and methyltriethoxysilane (MTES)) hydrolysed at two different pH (2 and 3.5). A system without pretreatment was studied as reference. The electrocoatings were cured between 155 °C and 195 °C in order to modify their mechanical properties.Polarisation curves, EIS and FT-IR measurements were used in order to characterize the silane layers. EIS measurements were used to follow the barrier properties and the water uptake evolution on intact coatings.The filiform corrosion protection of the coating was also evaluated by a normalized filiform corrosion test.     

The effect of surface modification on the cathodic disbondment rate of epoxy and alkyd coatings

Surface modification of carbon steel substrates using appropriate functionalised silanes was carried out to investigate their effect on the dry and wet adhesion strength, and the cathodic disbondment rate, of coating binders based on epoxy and alkyd chemistries. Results show that pre-treatment of the steel substrate with 3-glycidoxypropyltrimethoxy silane (3-GPS) enhanced the dry and wet adhesion of an epoxy-based coating. Similarly, pre-treatment with 3-aminopropyltriethoxy silane (3-APS) enhanced the dry and wet adhesion of alkyd-based systems. However, although pre-treatment with 3-GPS reduced the cathodic disbondment rate for epoxy by a factor of 3, no effect on the disbondment rate for alkyd-based binders on substrates pre-treated with 3-APS was found. This strongly suggests that cathodic disbondment of epoxy proceeds by disruption of interfacial bonds (i.e. at the binder/substrate interface) but that disbondment of alkyds proceeds by direct degradation of the binder and that the interface plays little part in the process.     

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.     

Characterization of films of organofunctional silanes by TOFSIMS and XPS

The structures of thin films formed by the silanes N-[2-(vinylbenzylamino)-ethyl]-3-aminopropyltrimethoxysilane (SAAPS) and γ-aminopropyltriethoxysilane (γ-APS) deposited onto mechanically polished zinc or mild steel from dilute aqueous solutions were determined using time-of-flight (TOF) SIMS and XPS. TOFSIMS gave structural information which was highly complementary to the XPS data. Aspects such as silane condensation and crosslinking, oxidation at elevated temperatures, the formation of metallosiloxane bonds, and incomplete hydrolysis were detected by TOFSIMS by virtue of its high mass resolution and unlimited mass range. The structures of the films were found to be strongly dependent on the nature of the substrate, the deposition conditions, and heat treatment of the films.     

普通冷轧钢板与镀锌板车身共线硅烷表面处理技术研究及应用

将以普通冷轧钢板为主材的车身导入镀锌板车身硅烷处理生产线上后,普通冷轧钢板车身出现了大面积的前处理、底漆质量问题。通过配方调整和工艺改进解决了普通冷轧钢板车身前处理后产生白色点状物及电泳漆膜上出现缩孔、爆孔、块状花斑等问题,成功实现了镀锌板车身和普通冷轧钢板车身在硅烷处理线上的混线生产。     

Structural characterization of bis-[triethoxysilylpropyl]tetrasulfide and bis-[trimethoxysilylpropyl]amine silanes by Fourier-transform infrared spectroscopy and electrochemical impedance spectroscopy

Bis-[triethoxysilylpropyl]tetrasulfide (or bis-sulfur silane) and bis-[trimethoxysilylpropyl] amine (or bis-amino silane) were deposited on 2024-T3 aluminum alloy (AA 2024-T3). The structures of the films were characterized using Fourier-transform infrared spectroscopy (FTIR) and electrochemical impedance spectroscopy (EIS) techniques. The results showed that: (1) The silane structures were affected significantly by the hydrolysis time of the silane solutions. A minimum hydrolysis time is required to obtain a crosslinked silane film. (2) Hydrolysis progressed more readily and faster in the bis-amino silane system than in the bis-sulfur silane system, probably due to the catalytic action of the amine of the bis-amino silane. (3) Both silane systems experienced significant crosslinking upon curing at 100°C, during which denser interfacial layers were formed via crosslinking in the interfacial regions. The interfacial layer contributes to corrosion protection of metals by silanes. (4) A new phase was observed in the fully cured bis-amino silane film after aging in the atmosphere. This new phase is likely to be carbamates and bicarbonates formed via a reaction between the secondary amino groups, carbon dioxide, and moisture absorbed from the atmosphere.     

Evaluation of steel/primer based on chestnut tannin/paint film systems by EIS

The influence of a tannin pretreatment applied to slightly rusted steel panels on the anticorrosive performance of alkyd paint systems has been investigated using electrochemical impedance spectroscopy (EIS) and open circuit potential measurements. From EIS and corrosion potential data analysis it was found that (1) there is not a significant influence of the tannin pretreatment on the anticorrosive protection level, which depends particularly on the barrier effect afforded by the applied paint system (however, as the corrosion process starts, tannins may cause the repassivation of steel); (2) the enhancement of the protection level afforded by the tannin primer was restricted to short immersion periods and corrosive media that were not very aggressive; and (3) water uptake was not significantly affected by the presence of the tannin primer. Centro de Investigación y Desarrollo en Tecnologia de Pinturas. (CIC-CONICET). Av. 52, 121 y 122. CPB1900AYB La Plata, Argentina.     

Electrochemical effects of silane pretreatments containing cerium nitrate on cathodic disbonding properties of epoxy coated steel

In the recent years, silane materials, because of their environmental friendly nature and ease of application have been attended as an alternative for chromate conversion coatings. Different materials were searched for improvement of the efficiency of silane formulation. In this research, pretreatment of carbon steel substrates was carried out using γ-glycidoxypropyl-trimethoxysilane (γ-GPS) as functionalized silane. Cerium nitrate as a corrosion inhibitor material was introduced into the silane material and epoxy resin was applied on the pretreated steel substrates. Effects of the pretreatment on electrochemical properties, cathodic disbondment, dry and wet adhesion strength, and surface morphology of resultant epoxy coating were investigated. Results showed that pretreatment of steel substrate with γ-glycidoxypropyl-trimethoxysilane (γ-GPS) doped with cerium nitrate leads to improvement of cathodic disbondment and also dry and wet adhesion of epoxy coating. Furthermore, this type of pretreatment reduced the disruption of interfacial bonds at the binder/substrate interface. Addition of 2?wt% cerium nitrate into the silane formulation led to the maximum efficiency of resultant coating.     

Influence of silane-based treatment on adherence and wet durability of fusion bonded epoxy/steel joints

Three layers polyolefin coatings are widely used in Europe to protect pipelines against corrosion. Loss of adhesion at a fusion bonded epoxy (FBE)/steel interface has occasionally been observed even on pipelines without external defects. Silane-based surface pre-treatments are developed to improve adhesion with limited impact on the environment unlike usual chromate conversion pre-treatments; however the mechanisms involved in silane action need to be more deeply understood. The application process of silane surface treatment implies a series of key parameters of which optimization is imperative to form a silane layer with good properties at the interface. This paper studies the influence of the cure temperature and the pH of an aminosilane based pre-treatment on adhesion strength and durability via single lap shear tests. SEM/EDX, FT-IR and XPS analyses are used to characterize the silane and silane/epoxy structures. Silane pretreatment improved the FBE joints durability and appears as a relevant solution to replace the usual chromate conversion pretreatments. The joints pretreated at pH 10.6 had better durability than at pH 4.6. This was related to the formation of a bridge across the silane/epoxy interface at pH 10.6, whereas, at pH 4.6, formic acid reacts with amino groups of silanes, limiting the reaction between amino groups and epoxy.     

The Effect of the Silane Deposition Conditions on the Durability of Aluminium Joints Pretreated using 3-Aminopropyltrimethoxysilane

The interfacial adhesive fracture energies and durabilities of aluminium/polyurethane joints pretreated with 3-aminopropyltrimethoxysilane using a range of deposition conditions has been investigated using the blister test. For substrates that were rinsed in order to remove the excess silane, the initial adhesive fracture energy was found to increase markedly compared with the unpretreated joints, but was not affected by changes in the pretreatment solvent (water and toluene), time (five and twenty minutes), or, for substrates pretreated from aqueous solutions, pH (6.8 and 10.4). However, the durability of these rinsed silane pretreated joints varied considerably with the silane pretreatment conditions. The most durable joints were formed when the aluminium was pretreated from aqueous solutions at pH 6.8 for five minutes. XPS analysis of the failed fracture surfaces revealed that the failure was associated with the very thin silane film, occurring at the metal/silane/polymer interface. When the excess silane was not removed by rinsing, the initial adhesive fracture energy was approximately half that obtained from the rinsed joints and the durability was also very poor. XPS analysis of the fracture surfaces showed that failure had occurred within a thick silane film.     

The Effect of the Silane Deposition Conditions on the Durability of Aluminium Joints Pretreated using 3-Aminopropyltrimethoxysilane

The interfacial adhesive fracture energies and durabilities of aluminium/polyurethane joints pretreated with 3-aminopropyltrimethoxysilane using a range of deposition conditions has been investigated using the blister test. For substrates that were rinsed in order to remove the excess silane, the initial adhesive fracture energy was found to increase markedly compared with the unpretreated joints, but was not affected by changes in the pretreatment solvent (water and toluene), time (five and twenty minutes), or, for substrates pretreated from aqueous solutions, pH (6.8 and 10.4). However, the durability of these rinsed silane pretreated joints varied considerably with the silane pretreatment conditions. The most durable joints were formed when the aluminium was pretreated from aqueous solutions at pH 6.8 for five minutes. XPS analysis of the failed fracture surfaces revealed that the failure was associated with the very thin silane film, occurring at the metal/silane/polymer interface. When the excess silane was not removed by rinsing, the initial adhesive fracture energy was approximately half that obtained from the rinsed joints and the durability was also very poor. XPS analysis of the fracture surfaces showed that failure had occurred within a thick silane film.