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.     

Effect of bath concentration and curing time on the structure of non-functional thin organosilane layers on aluminium

The corrosion resistance of aluminium alloys can be improved by different surface treatments such as painting. A pre-treatment based on chromate is the current method used to increase the corrosion resistance and the adhesion of the organic layer. Silane films seem to be an interesting alternative system to replace the toxic chromates. In this paper, the characterisation of bis-1,2-(triethoxysilyl)ethane (BTSE) thin layers has been evaluated by coupling optical techniques like spectroscopic ellipsometry (SE) and infra-red spectroscopic ellipsometry (IRSE) along with electrochemical methods (electrochemical impedance spectroscopy (EIS)). This approach has been chosen to have a better understanding of the protection provided by these organosilane thin films. It will be demonstrated that the BTSE bath concentration modifies the thickness of the layers and that the curing of this thin film can also improve the barrier properties by forming a denser layer.     

Silane solution stability and film morphology of water-based bis-1,2-(triethoxysilyl)ethane for thin-film deposition on aluminium

Thin-film silane coating (<1 μm) has been introduced as a chromium free multi-metal surface pre-treatment for corrosion protection, adhesion promotion and surface passivation of metals such as aluminium, steel, zinc, magnesium and others. Bis-1,2-(triethoxysilyl)ethane (BTSE) has received much attention as it is, after hydrolysis, highly reactive towards (covalent) metal/film bonding and cross-link formation for the creation of barrier properties. Much of the past work on BTSE was performed on methanol-based laboratory solutions due to the low solubility of BTSE in water. For industrial applications these solutions are not considered suitable anymore because of the high process cost as well as ecological and health issues associated with methanol and the high monomer content of such solutions. For industrial practice water-based silane solutions are being considered. In the present work water-based BTSE solution is compared to a reference methanol-based solution. The silane solution is analysed using ²⁹Si NMR spectroscopy, and the deposited silane films are morphologically characterised using infra-red spectroscopic ellipsometry and field-emission gun-scanning electron microscopy.     

Electrochemical properties of Al and Al alloys relevant to corrosion protection in seawater environments

A series of electrochemical experiments on Al alloys were undertaken to determine their optimum protection potentials in seawater. With 1050 and 5456 alloys, passive films form during anodic polarization but are destroyed by the Cl in seawater, only to regrow as a result of the self-healing capacity of aluminum. The current density of 5456 Al alloy proved to be lower than that of 1050 as a whole. Any shift to more anodic or cathodic conditions in the potential range of-1.5∼-0.68 V resulted in a sudden increase in current density. Current densities in the high-strength 7075 Al alloy showed the greatest values. In contrast, the current densities of 5456 alloy, known to have excellent corrosion resistance in seawater, were the lowest in the range of -0.70∼-1.3 V, and we concluded that this potential range offered optimal protection.     

Cathodic breakdown of anodic oxide film on Al and Al–Sn alloys in NaCl solution

The effect of cathodic polarisation on stability of defined oxide films on Al and Al–Sn alloys (with up to 0.40% Sn) has been investigated in a 0.5 M NaCl solution using the potentiostatic pulse method. The dependence of the cathodic current on time (in the period of 1, 10 and 100 s) was recorded on Al and Al–Sn alloys when subjected to a potential pulse from E_OCP to different negative values (up to −2.0 V). Anodic current responses to the return to the E_OCP were also recorded at three different time scales (1, 10 and 100 s). It has been established that the cathodic polarisation of passivated Al and Al–Sn alloys in a chloride solution is characterized by two regions of potentials with distinctly different phenomena: the range of low and high cathodic potentials (LCP and HCP). In the LCP range, the oxide film retains its properties, while in the HCP range cathodic breakdown and hydration of the oxide take place. The boundary between these two potential ranges shifts towards more negative potential values when the percentage of Sn in the alloy increases. The longer the duration of the cathodic pulse, the more positive the potentials at which the oxide film breakdown takes place. This shift is more marked with alloys containing higher percentage of Sn. Cathodic polarisation (duration of 100 s) activates alloys with 0.20% and 0.40% Sn for anodic dissolution.     

Evolution of oxygen reduction current and biofilm on stainless steels cathodically polarised in natural aerated seawater

The aim of a series of works recently performed at ISMAR was to provide new useful information for a better understanding of the mechanisms by which bacteria settlement causes corrosion on Stainless Steels (SS) and similar active-passive alloys exposed to seawater. In this work, the evolutions of cathodic current, bacteria population, and electronic structure of the passive layer were investigated on SS samples polarised at fixed potentials during their exposure to natural seawater. It was found that, during the first phase of biofilm growth, cathodic current increase is proportional to the number of settled bacteria at each fixed potential. However, the proportionality factor between settled bacteria and cathodic current depends on imposed potential. In particular, the proportionality factor strongly decreases when the potential is increased above a critical value close to −150 mV Ag/AgCl. This effect seems to be correlated with the electronic structure of the passive layer. Indeed, the outer part of the passive layer on tested SS was found to behave like a conductor at potentials more active than −150 mV Ag/AgCl, and like an n-type semiconductor at more noble potentials.     

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.     

The structure and electronic properties of passive and prepassive films of iron in borate buffer

The films that form on pure iron during potentiodynamic anodic polarization in aqueous borate buffer were investigated by surface enhanced Raman spectroscopy (SERS), and by electrochemical impedance spectroscopy and Mott-Schottky analysis at selected potentials. According to SERS, the passive film is a bilayer film with an outer layer of an as yet undetermined Fe(III)oxide/hydroxide, identified by a strong Raman peak at 560 cm⁻¹. The inner layer was a spinel compound. The capacitances of passive iron were frequency dependent and a constant phase element (CPE) best described the frequency dispersion. Current increases in cathodic polarization scans confirmed the accuracy of flatband potentials calculated from Mott-Schottky tests at two different film formation potentials. Both films were found to be n-type and flatband potentials of −846 and −95 mV vs. SHE and carrier densities of 1.6 × 10²² and 8.3 × 10²⁰/cm³ were found for films grown at −500 and +1000 mV, respectively. The cathodic polarization curve of passivated iron exhibited a complex shape that was explained by the electronic properties of iron's passive and prepassive films. The reductive dissolution of the films abruptly began when the potential was lowered below their flatband potentials. It is suggested that the cathodic polarization behavior contributes to iron's susceptibility to localized corrosion.     

Comparison between wet deposition and plasma deposition of silane coatings on aluminium

Silane coatings are applied to metal surfaces for various purposes, e.g., to form a protective layer against corrosion or to act as a primer for subsequent coating. In this work bis-1,2-(triethoxysilyl) ethane (BTSE) was used as a precursor to deposit coatings on Al 99.99% substrates with three different techniques: dipcoating (water based solution), vacuum plasma and atmospheric plasma. Infrared reflection absorption spectroscopy (IRRAS), X-ray photoelectron spectroscopy (XPS) and field emission gun-scanning electron microscope (FE-SEM) were used to characterise the structure, composition and surface morphology of the silane coatings. The aim of this investigation is to compare the surface and bulk characteristics of the films prepared with the three different methods, in order to get information on how the BTSE molecule is modified by the deposition technique.     

Zinc–magnesium-pigment rich coatings for corrosion protection of aluminum alloys

Chromate (Cr(VI))-based pigments have been widely used for corrosion protective coatings because of their outstanding protection efficiency especially for aluminum alloy products. However, due to environmental issues associated with Cr VI, more and more requests are being made for alternative solutions. In the presented work zinc was modified by alloying with magnesium to achieve a combination of properties – cathodic protection and less reactivity during production, storage and application of the pigments. The magnesium content leads to a lowering of the electrochemical potential which allows the cathodic protection of aluminum alloys. zinc–magnesium pigments were prepared in different compositions with special attention to the intermetallic phases MgZn, Mg₂Zn₃, and MgZn₂. Pigments were produced and a zinc–magnesium rich coating was formulated and compounded. Pickled samples of AA 2024 unclad were coated and the corrosion behavior investigated. A durability of more than 10,000 h in salt spray test could be achieved.     

Electrochemical study on the inhibitory effect of the underpotential deposition of zinc on Zn-Co alloy electrodeposition

Zn-Co alloy electrodeposition from chloride baths containing different Zn²⁺/Co²⁺ ratios was investigated by cyclic voltammetry and anodic linear sweep voltammetry using a Pt electrode. The peaks were attributed by means of EDX analysis, SEM and TEM observations performed on some alloys potentiostatically deposited. In the range of potential where zinc deposits underpotential, cyclic voltammetry showed a complex cathodic peak with one maximum and two shoulders, correlated with the deposition of different cobalt rich alloys. Up to four anodic peaks, two correlated with zinc oxidation from η and γ phases and two correlated with oxidation of solid solutions of zinc in cobalt, were observed. ALSV and TEM indicated that the remarkable increase in Zn content of the alloy, which occurs with a strong inhibition of the process at potentials more negative than that of the cathodic peak and more positive than the bulk deposition potential of zinc, is due to the deposition of γ phase. No inhibition of the alloy deposition process was observed with very low concentrations of zinc (<0.015 M) in the bath containing 0.19 M Co²⁺.     

Deposition temperature effect on sputtered hydroxyapatite coatings prepared on AZ31B alloy substrate

The corrosion of Mg alloys is a provocative topic and it is still a challenge to find a solution for the improvement of their degradation rate into solution found in human body (Simulated Body Fluid, SBF). The aim of the present paper is to coat AZ31B alloy by hydroxyapatite (HAp) as a possible solution in order to change its degradation behaviour for medical implants. Since the Mg alloy is sensible to temperature while the HAp properties depend on the deposition temperature, in this study, the effect of deposition temperature on the properties of the AZ31Balloy was evaluated. The HAp coatings were prepared using the RF magnetron sputtering technique, ranging the temperature from the room one to 400 °C. It was found that the grain size of the investigated Mg alloy increased more than 100% when the deposition temperature was increased. By increasing the temperature, the hardness level was reduced of about 15%. All HAp coatings revealed corrosion behaviour much better than the uncoated AZ31B alloy; in particular, the coating deposited at 200 °C exhibited the best corrosion behavior. Moreover, the best protection to the corrosive attack of SBF was found for the HAp coating deposited at 200 °C (97.3%), which was also characterized by the lowest porosity. To conclude, HAp coatings can be used to improve the properties of AZ31B alloys, but just up to 200 °C; beyond this temperature, the mechanical and the anticorrosion properties are lost.     

Effect of curing on the corrosion protective properties of thin organosilane films on aluminium

Film properties and corrosion protection given by thin organosilane films (bis-1,2-(triethoxysilyl)ethane: BTSE) on aluminium were investigated as a function of curing. The thickness was determined using spectroscopic ellipsometry (SE). Impedance spectra, modelled by an electrical equivalent circuit, show three time constants. The variation of the elements in these three time constants as a function of the immersion time in a NaCl solution indicate the structure of the film after curing. It can be concluded that the barrier properties of the organosilane films are influenced more by the curing temperature than by the curing time.     

Potential dependence of composition and structure of electrodeposited Fe-Pt films

Composition and structure of electrodeposited Fe-Pt-O films are investigated in dependence on deposition potential. At potentials positive to the hydrogen evolution, a solid solution of Pt with some Fe is present that can be explained by underpotential deposition of Fe. In the potential region where hydrogen evolution by proton reduction is a side reaction, oxygen is detected in the films. The O content scales with the Fe content and at least part of the Fe is oxidized. When applying more cathodic potentials, the fraction of Fe and O increases, grain size is reduced and an amorphous/nanocrystalline phase forms. The Fe content is pH dependent in this potential region. At even more cathodic potentials, overpotential deposition of Fe in the bcc structure sets in, the O content is reduced. The results are discussed with respect to thermodynamics in the Fe-Pt-(O) system. The concepts of underpotential deposition of Fe during Fe-Pt alloy deposition and of hydroxide formation due to a pH rise at the cathode are evaluated. These simple models can only partially describe the deposition process in agreement with the compositional and structural information obtained and a coupled mechanism is proposed.     

Electrophoretic deposition of nanocomposites formed from polythiophene and metal oxides

An electrophoretic deposition (EPD) procedure was adopted for the cathodic preparation of thin films of conducting polymer/metal oxide nanocomposites with a core-shell structure. The deposition process was investigated at different potentials and in various solvents. The mechanism and kinetics of the electrophoretic deposition were studied via quartz crystal microbalance (QCM) and zeta-potential measurements.The properties of the composite layers were studied by electrochemical methods (cyclic voltammetry, impedance spectroscopy) and photocurrent measurements. The reversible redox potential of polythiophene films was about 0.75 V_SCE. The p-type semiconducting behaviour of the reduced polythiophene was studied by photocurrent measurements. In the case of using TiO₂ (n-type semiconductor) as a core material, an n/p heterojunction was observed. In the photocurrent spectra the maximum of the cathodic peak of polythiophene was found around λ = 500 nm (2.5 eV), depending on the applied potential. It is in agreement with the results of UV-vis optical spectra of deposited layers and of pressed pellets. The flatband potential of polythiophene in the heterojunction with TiO₂, obtained from photocurrent measurements, was 0.53 V_SCE.     

Corrosion resistance of ternary NiP based alloys in sulfuric acid solutions

NiP based alloy films were prepared by autocatalytic deposition and their structure, chemistry and corrosion behaviors in sulfuric acid solutions were studied as a function of their composition. The as-prepared Ni-based alloys are nanocrystalline, and their grain size decreases with increasing P content. Addition of a third element (W or Mo) influences the observed grain size. At low anodic overpotential NiP based alloys present a lower exchange current and lower reactivity than Ni, both improving with increasing P content. Contrary to Ni however, the NiP based alloys do not passivate at higher anodic overpotentials. Addition of W to NiP alloys can improve their corrosion resistance, while addition of Mo has little or no beneficial effects on corrosion properties.     

Effects of some thioureas on the dissolution of aluminium alloys in nitric acid

A study has been made on the inhibitive efficiency of some thioureas (urea, thiourea, phenylthiourea and naphthylthiourea), in the concentration range 0.0025 to 2%, in relation to dissolution of some aluminium alloys (1060, 1100, 3003 and 5052) in 20% nitric acid solution at temperatures of 25, 35 and 45° C. It was found that the structures of the thioureas affected the inhibitive efficiency but the mechanism of inhibition remained the same. The compounds exhibited maximum protection in the concentration range 0.025 to 0.03%. A gradual lowering in the efficiency of the compounds above the concentration of 0.03% has been observed. At the concentration of 1.5%, urea appears to be a corrosion accelerator and causes localized attack on the alloy surface; the other compounds still showed some degree of inhibition. The additives were most efficient towards 1060 alloy, followed by 1100, 3003 and 5052 alloys. The protecting power of all the compounds other than urea was found to improve with increase in temperature. All the compounds appeared to inhibit corrosion by adsorbing at cathodic sites. A distinctive hump (i.e. a current maximum) is observed in the steady-state potentiostatic anodic polarization curves of the alloys. This hump is found to be affected by the inhibitors. At higher concentrations, the compounds act as cathodic depolarizers, which is confirmed by measuring steady-state corrosion potentials and cathodic polarization diagrams.     

Integrated AFM and SECM for in situ studies of localized corrosion of Al alloys

Rolled 3xxx series Al alloys, e.g., EN AW-3003, are generally used as fin or tube material in heat exchangers for automobiles. With reducing fin thickness, maintaining fin material integrity is of increasing importance. This study aimed at exploring the differences in intrinsic corrosion properties between EN AW-3003 and a newly developed Al-Mn-Si-Zr fin alloy using state-of-the-art local probing techniques. Volta potential mapping of both alloys by scanning Kelvin probe force microscopy (SKPFM) indicates a cathodic behaviour of constituent intermetallic particles (>0.5 μm) relative to the alloy matrix. Compared to EN AW-3003, the Al-Mn-Si-Zr alloy has a smaller number of particles with large Volta potential difference relative to the matrix. In situ atomic force microscopy (AFM) measurements in slightly corrosive solutions showed extensive localized dissolution and deposition of corrosion products on EN AW-3003, and only a small number of corroding sites and “tunnel-like” pits on Al-Mn-Si-Zr. Probing the ongoing localized corrosion process by integrated AFM and scanning electrochemical microscopy (SECM) revealed more extensive local electrochemical activity on EN AW-3003 than on Al-Mn-Si-Zr. In all, the lower corrosion activity and smaller tunnel-like pits resulted in lower material loss of the Al-Mn-Si-Zr alloy, a beneficial property when striving towards thinner fin material.     

Influence of deposition conditions on the protective behavior of tetraethyl orthosilicate sol–gel films on AA5754 aluminum alloy

The aim of this work is to compare the effect of two deposition methods, dip-coating and electrophoretic deposition, on the characteristics of tetraethyl orthosilicate (TEOS) sol–gel films on AA5754 aluminum alloy, especially in what concerns to their resistance against corrosion. The influence of pH bath on the hydrolysis of the system was analyzed. Moreover, the effect of some experimental parameters, such as deposition time and deposition voltage, was also evaluated. The results showed that pre-hydrolysis rate of the orthosilicate bath depends on the solution pH, and that at pH 2 complete hydrolysis of the solution was obtained after only 20 min. Moreover, it was observed that electrodeposited TEOS films provided better corrosion resistance than films obtained by dip-coating. The improved corrosion resistance was ascribed to a higher uniformity and density of the polysiloxane films and to an in situ modification of the aluminum–polysiloxane interface by the cathodic voltage.     

Preparation and characterization of hydroxyapatite coating by magnetron sputtering on Mg–Zn–Ag alloys for orthopaedic trauma implants

The aim of the present paper was to evaluate the effect of hydroxyapatite coatings on the two types of Mg–Zn–Ag alloys as a possible solution to control magnesium alloy degradation. The coatings were prepared by the radio frequency magnetron sputtering method at a deposition temperature of 300 °C. To perform this evaluation, the coated alloys were immersed in a simulated body fluid solution at body temperature (37 ± 0.5 °C) to determine the corrosion resistance through electrochemical and immersion tests. Moreover, the investigation also consisted of the evaluation of microchemical, mechanical, and morphological properties. The deposition temperature of 300 °C was enough to obtain a crystalline hydroxyapatite structure with a Ca/P ratio close to the stochiometric one. The adhesion of coatings was not influenced by the nature of Mg–Zn–Ag alloys, so similar values for both coated alloys were found. The results showed that the coating was homogonous deposited on the Mg–Zn–Ag alloys and the corrosion resistance of uncoated magnesium alloys was improved.