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.     

Improvement of the Durability of Zinc-Coated Steel/Epoxy Bonded Joints

The durability properties of bonded lap shear joints made from an epoxy/dicyandiamide adhesive and hot-dipped galvanized (G2F) or electroplated-phosphated (EZ2) steel have been investigated. The degradation mechanisms have been studied after three accelerated ageing tests: the “cataplasme humide” (“C.H.T.”), immersion (“I.T.”), and salt spray (“S.S.T.”) tests. X-ray photoelectron spectroscopy (XPS) analysis of fracture surfaces after ageing have shown that anodic dissolution of the zinc-coating is responsible for debonding in all cases and that intergranular corrosion phenomena account for poorer performances of the hot-dipped galvanized substrate during “C.H.T.” and “I.T.” Silane coupling agents were successfully used as primers on both substrates to increase the hydrolytic stability of the metal/adhesive interface. XPS results indicate that both the interfacial dissolution of the phosphate coating of EZ2 and intergranular corrosion of G2F are delayed for silane-primed specimens. The observed improvements do not appear to depend on the nature of the silane coupling agents. Alkylsilanes have been found to perform as well as silanes having a group capable of reacting with the epoxy/dicyandiamide system.

Additional tests were carried out in view of the possible application of organosilane reagents as additives in corrosion-protective oils. Good durability properties have been obtained by priming the metal coupons with a standard oil/silane mixture prior to bonding.

When corrosion was the controlling degradation mechanism as is the case during the salt spray test, silane treated specimens did not generally perform better than control specimens.     

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.     

黄铜表面硅烷自组装膜在氯化钠溶液中的耐蚀性

采用自组装技术在黄铜表面分别制备γ-巯基丙基三甲氧基硅烷(PropS-SH)、十二烷基三甲氧基硅烷(DTMS)、氨基丙基三甲氧基硅烷(APS)和γ-氯丙基三甲氧基硅烷(CPTMS)自组装膜,并分别采用电化学极化法、接触角测试和傅里叶红外光谱研究硅烷膜的耐蚀性、疏水生和结构.结果表明:经PropS-SH、DTMS和CPT...     

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.     

Corrosion resistance properties of Ormosil coatings on 2024-T3 aluminum alloy

The corrosion resistance behavior of organically modified silane (Ormosil) thin films on 2024-T3 aluminum alloy substrates was investigated using electrochemical impedance spectroscopy (EIS) and accelerated salt spray analysis techniques. Coatings were prepared containing 0–16.6 vol.% alkyl-modified silane, X_n---Si(OR)_4−n, where X=methyl, dimethyl, n-propyl, n-butyl, i-butyl, n-hexyl, n-octyl, or i-octyl. Coating thicknesses were measured to be in the 6–16 μm range, with the thickest coatings being observed for the highest concentrations of alkyl-modified silane. Contact angle measurements showed an enhancement in hydrophobicity of the Ormosil film imparted by increasing size and concentration of the alkyl-modifiers in the coating. In general, corrosion resistance characteristics, as determined using EIS and salt spray techniques, were found to increase with increasing alkyl-modified silane concentration and alkyl chain length. The best overall corrosion resistance was observed for coating systems containing ≥10.4 vol.% alkyl-modified silane; the hexyl-modified films exhibited corrosion resistance properties superior to the other Ormosil coatings. Immersion studies conducted in 0.5 M K₂SO₄ indicated that coating degradation occurs via hydration of the dense linear chain silicate network leading to the formation of porous cyclic structures.     

Low temperature decomposition of PCDD/PCDF, chlorobenzenes and PAHs by TiO2-based V2O5–WO3 catalysts

The oxidation of representative congeners of polychlorinated dibenzo-p-dioxins (PCDDs), polychlorinated dibenzofurans (PCDFs), polychlorinated chlorobenzenes (PCBzs), and of polyaromatic hydrocarbons (PAHs) was investigated on two commercial V₂O₅–WO₃/TiO₂-based catalysts, optimized for the combined reduction of nitrogen oxides and decomposition of dioxins.

The non-chlorinated polyaromatic compounds (including non-chlorinated dibenzodioxin and dibenzofuran) are destroyed at temperatures as low as 150°C with an efficiency of more than 95%. PCDD and PCDF were also removed from the gas phase with an efficiency of >98%. However, at 150°C they remained mainly unchanged (up to 75%) adsorbed on the catalyst.

A decrease in the oxidation rate with increasing chlorine substitution was found for the PCDD/PCDF. This could be explained by an increasing “redox potential” with increasing chlorine substitution due to the electron withdrawing effect of the chlorine.

For the more volatile monoaromatic PCBz, however, the effect of lowering the volatility with increasing chlorine substitution (resulting in longer residence time on the catalyst) over-compensates the effect of the increasing “redox potential” with higher degree of chlorination.     

Surface enhanced Raman spectroelectrochemical studies of the corrosion films on iron and chromium in aqueous solution environments

“In situ” laser Raman spectra of the corrosion films on iron have been observed in aerated 5 M KOH and 0.15 M NaCl solutions via surface enhancement by the electrodeposition of a silver overlayer. Essentially the same spectra are observed in the two solutions as a function of applied potential in spite of a breakdown of passivity on iron in the chloride solution. Fe(OH)₂ and Fe₃O₄ are found in the prepassive potential region while FeOOH is present in the passive region. A film which is very difficult to reduce appears to be always present on the electrode surface even at hydrogen evolution potentials; this film is believed to be -FeOOH. Surface enhanced Raman spectra of the corrosion films on chromium have also been obtained in NaCl solution for the first time. The passive film has a composition that corresponds most closely to an amorphous form of Cr₂O₃, with some Cr(OH)₃ also present. The film is converted in the transpassive region to a higher oxide form, presumably CrO²⁻₄. Reversible reduction of this species to Cr₂O₃ is indicated.     

The Mechanical Behaviour of Polyimide/Copper Laminates Part 2: Peel Energy Measurements

The mechanical peel behaviour of laminates consisting of polyimide films adhered to copper foil using a modified acrylic adhesive has been studied over a wide range of test rates and temperatures. The laminates were prepared from polyimide films which had been subjected to either a “high-thermal history” or a “low-thermal history” treatment during the production of the film. The measured peel energies of the laminates could be superimposed to give a master curve of peel energy versus the reduced rate of peel test, Ra_T, where R is the rate of peel test and a_T is the time-temperature shift factor. The appropriate shift factors were a function of the test temperature and were mainly deduced from tensile tests conducted on the bulk adhesive. The “high-thermal history” laminates gave higher peel energies and the locus of failure of the laminates was mainly by cohesive fracture through the adhesive layer. At low values of log₁₀ Ra_T, i.e. Low rates of peel and high test temperatures, the “low-thermal history” laminates also failed in the adhesive layer and possessed similar peel energies to those measured for the “high-thermal history” laminates. However, at high log₁₀ Ra_T values, the peel energies measured for the “low-thermal history” laminates were lower and showed a wider scatter. These arose from a different locus of failure occurring in these “low-thermal history” laminates when tested under these conditions. Namely, it was found that most of these laminates failed in a weak boundary layer in the outer regions of the “low-thermal history” polyimide film.     

Effect of the Molecular Length of n-Alkylsilanes on the Adhesion at a Rubber/Glass Interface

The effect on the peeling energy, G, of glass/styrene-butadiene rubber (SBR) assemblies of the length, N, of the alkyl chain, ranging from 4 to 30 carbon atoms, of silane coupling agents is determined. Experimentally, it is shown that G strongly increases with N. Therefore, considering that the rheological model of adhesion (or model of multiplying factors) is valid, G is assumed equal to the product of three terms: the reversible adhesion energy, W. at the interface, the viscoelastic dissipation factor, φ, of SBR and a “molecular factor” to be determined. Finally, it appears that this latter factor depends linearly on N. Such a result should be consistent with an extraction (“suction”) process of the silane alkyl links from bulk SBR during peeling experiments.     

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.     

Synthesis and Characteristics of Novel Poly(Imide Siloxane) Segmented Copolymers

New poly(imide siloxane) copolymers for possible use as tough environmentally stable structural matrix resins and structure adhesives have been prepared. Thus, 3,3'-4,4'-benzophenone tertracarboxylic dianhydride was reacted with various Mn aminopropyl-terminated polydimethylsiloxane oligomers and a meta-substituted diamine “chain-extender” such as 3,3'-diaminodiphenyl sulfone or 3,3'-diaminobenzophenone to produce the siloxane-modified poly(amic acid). Thin films were cast from the reaction mixtures and subsequent thermal dehydration produced the poly(imide siloxane) block or segmented copolymers. Upper “cure” temperatures of 300°C were used to insure complete imidization. By varying the amount and molecular weight of the siloxane oligomer, a variety of novel copolymers of controlled composition have been synthesized. Tough, transparent, flexible soluble films were produced by this method. The thermal and bulk properties of films having low to moderate siloxane content closely resemble those of the unmodified polyimide controls. However, toughness and surface behavior can be enhanced.     

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。     

Chemical composition and bond structure of aerosol particles of amorphous hydrogenated silicon forming from thermal decomposition of silane

Aerosol particles of amorphous hydrogenated silicon resulting from thermal decomposition of silane were investigated by hydrogen evolution, IR-, EPR-, NMR spectroscopy, and transmission electron microscopy.

The experimental data show that aerosol particles contain to a various extent {SiH₂}_n polymer structures and two types of monohydride groups SiH- “clustered” and “dilute” monohydride groups. The hydrogen atoms of the “clustered” monohydride groups are located close to each other. The “clustered” monohydride groups are inaccessible to the ambient because they are embedded in the amorphous network. The “dilute” monohydride groups are relatively isolated from each other. The majority of “dilute” monohydride groups are open to the ambient. They are located on the surface of preferentially interconnected microchannels and microvoids.

Interaction between the “dilute” SiH groups and atmospheric oxygen results in formation of OSiH groups in which hydrogen and oxygen are bonded to a common silicon atom. Evidently, the interaction occurs throw the oxygen reaction with weak bonds associated with “dilute” monohydride groups. There is no interaction between oxygen and both “clustered” SiH groups and {SiH₂}_n chain because the former are inaccessible to atmospheric oxygen and the latter has presumably no weak bonds in the chains.     

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。     

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.     

The synergistic combination of bis-silane and CeO2·ZrO2 nanoparticles on the electrochemical behaviour of galvanised steel in NaCl solutions

Bis-1,2-[triethoxysilylpropyl]tetrasulfide silane films containing CeO₂·ZrO₂ nanoparticles were deposited by dip-coating on galvanised steel substrates. The morphological features of the coated substrates were evaluated by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The anti-corrosion performance of the modified silane film applied on galvanised steel substrates was studied by electrochemical impedance spectroscopy (EIS). The ability of nanoparticles to mitigate localized corrosion activity at artificially induced defects was investigated via the scanning vibrating electrode technique (SVET) and by the scanning ion-selective electrode technique (SIET). The results showed that the addition of nanoparticles provides good corrosion protection of the galvanised steel substrates pre-treated with the modified silane solutions. The corrosion activity was reduced by more than one order of magnitude. Complementary d.c. experiments, using zinc electrodes exposed to NaCl solutions containing the nanoparticles were also performed in order to better understand the role of the nanoparticles. An ennoblement of the corrosion potential and polarisation of the anodic reactions could be detected.     

The adsorption of coumarin at solid metal electrodes

The effect of coumarin on the “deposition potential” of a number of metals at solid metal surfaces has been compared with that at a dropping mercury electrode. These results have been used to interpret the adsorption of coumarin on different metal surfaces and to suggest that Tl⁺ and Pb²⁺ ions are specifically adsorbed on mercury and that Tl⁺, Co²⁺, Fe²⁺, Ni²⁺ and Zn²⁺ are specifically adsorbed on their parent metals.     

CRACK GROWTH OF STRUCTURAL ADHESIVE JOINTS IN HUMID ENVIRONMENTS

The adhesive fracture energy, G_c, of aluminiumalloy and steel joints bonded with a rubber-toughened epoxy adhesive has been measured using monotonicallyloaded tests. Such tests have been conducted at different levels of relative humidity, and two surface pretreatments have been employed for the substrates prior to bonding: a simple grit-blast and degrease (GBD) pretreatment or a silane primer (GBS) pretreatment. When G_c was plotted against the crack velocity, three regions of fracture behaviour could be distinguished. At low rates of displacement the crack grew in a stable manner, visually along the interface, and relatively low crack velocities could be readily measured. This was termed “Region I”, and here the value of the adhesive fracture energy was relatively low and decreased steadily as the relative humidity was increased. On the other hand, at relatively high rates of displacement the crack grew in a stick-slip manner mainly cohesively in the adhesive layer at approximately 20 km/min. This was termed “Region III”, and here the value of G_c was relatively high and independent of the relative humidity. In this region the crack was considered to grow faster than the water molecules were able to reach the crack tip, which explains the independence of G_c upon the test environment. In between Region I and Region III a transition region was observed, which was designated “Region II”. The major effect of the GBS pretreatment, compared to which the GBD pretreatment, was to increase the value of G_c both in Regions I and III, although the presence of the silane primer had the greater effect in Region I.     

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.