Electrochemical assessment of the self-healing properties of Ce-doped silane solutions for the pre-treatment of galvanised steel substrates

The present work aims at assessing the electrochemical behaviour of galvanised steel (GS) substrates pre-treated with bis-[triethoxysilylpropyl] tetrasulfide silane (BTESPT) doped with cerium nitrate. Furthermore, the work aims at evaluating the self-healing properties of the dopant and discussing the possible mechanisms involved in this process. The study was performed by electrochemical impedance spectroscopy (EIS) and by the scanning vibrating electrode technique (SVET), during immersion in NaCl solutions. X-ray photoelectron spectroscopy (XPS) was also used to complement the electrochemical results. The results show that the protective behaviour of the pre-treatments based on Ce-doped silane solutions is dependent on the concentration of the dopant. The results also show that the dopant improves the anti-corrosion performance of the silane coatings formed on galvanised steel substrates.     

Composition and behaviour of cerium films on galvanised steel

The composition and corrosion performance of galvanised steel treated by immersion in cerium nitrate solution was investigated by electrochemical techniques and surface analysis. The surface film consists of a mixture of Ce(III) and Ce(IV) compounds, being very rich in Ce(III) in the first instants of the deposition process and becoming gradually enriched in the more oxidised form, Ce(IV). The presence of this film on the surface hinders the corrosion reaction by reducing the rate of both the cathodic and the anodic reactions. The film becomes thicker but more uneven when the time of film growth increases, with the development of defects in the film, which in contact with electrolyte behaves anodic with respect to the covered areas of the surface. These thicker films have revealed lower resistance to corrosion initiation.     

Analytical and microscopic characterisation of modified bis-[triethoxysilylpropyl] tetrasulphide silane films on magnesium AZ31 substrates

In this work solutions of bis-[triethoxysilylpropyl] tetrasulphide silane modified with cerium nitrate are used as pre-treatments for the AZ31 Mg alloy.     

Cerium salt activated nanoparticles as fillers for silane films: Evaluation of the corrosion inhibition performance on galvanised steel substrates

The present work investigates the electrochemical behaviour of galvanised steel substrates pre-treated with bis-[triethoxysilylpropyl] tetrasulfide silane (BTESPT) solutions modified with SiO₂ or CeO₂ nanoparticles activated with cerium ions. The electrochemical behaviour of the pre-treated substrates was evaluated via electrochemical impedance spectroscopy in order to assess the role of the nanoparticles in the silane film resistance and capacitance. The ability of the Ce-activated nanoparticles to mitigate corrosion activity at the microscale level in artificial induced defects was studied via scanning vibrating electrode technique (SVET). Complementary studies were performed using potentiodynamic polarisation. The results show that the presence of nanoparticles reinforces the barrier properties of the silane films and that a synergy seems to be created between the activated nanoparticles and the cerium ions, reducing the corrosion activity. The addition of CeO₂ nanoparticles was more effective than the addition of SiO₂ nanoparticles.     

Analytical characterization of silane films modified with cerium activated nanoparticles and its relation with the corrosion protection of galvanised steel substrates

Galvanised steel substrates were pre-treated in bis-1,2-[triethoxysilyilpropyl]tetrasulphide silane solutions containing SiO₂ or CeO₂ nanoparticles activated with cerium ions. The surface composition was investigated by infrared spectroscopy. The film thickness was determined by scanning electron microscopy. The results showed that the barrier properties of silane films modified with nanoparticles depend upon the concentration of nanoparticles. The results also showed that the silane film thickness increases when the nanoparticles are activated with cerium ions. The anti-corrosion behaviour of the cerium activated nanoparticles was also investigated at the microscale level, in artificial induced defects, using the scanning vibrating electrode technique (SVET). The substrates treated with the silane coating modified with CeO₂ nanoparticles revealed improved corrosion behaviour comparatively to the coatings modified with SiO₂ nanoparticles. X-ray photoelectron spectroscopy and Auger electron spectroscopy experiments carried out on the defects after immersion in NaCl solutions revealed the presence of a surface film containing zinc corrosion products and cerium/ceria compounds.     

Influence of silica nanoparticles added to an organosilane film on carbon steel electrochemical and tribological behaviour

The electrochemical behaviour and tribological properties of carbon steel coated with bis-[trimethoxysilylpropyl]amine (BTSPA) filled with SiO₂ were evaluated. The silane film filled with SiO₂ was prepared by adding different SiO₂ concentrations. The electrochemical behaviour of the coated steel was mainly evaluated by means of open-circuit potential (E_OC), electrochemical impedance spectroscopy (EIS) and polarization curves, in 0.1 mol L⁻¹ NaCl solution. Structural and morphological characterizations were made by optical, electron and atomic force microscopy (AFM). E_OC and EIS data showed that sample filled with 300 ppm SiO₂ presented the highest E_OC and total impedance value. AFM measurements showed a homogeneous particle distribution of SiO₂ particles. Nanohardness measurements showed SiO₂ promoted an increase of the hardness mean value (1.70 ± 0.11 GPa to non-filled BTSPA and 2.21 ± 0.05 GPa for sample filled with 300 ppm SiO₂). Silane films when filled with SiO₂ particles improved the corrosion resistance of the steel substrate. The optimum SiO₂ particles concentration in silane solution is 300 ppm SiO₂. Incorporation of an extra amount of silica into BTSPA film led to degradation of the corrosion protection of the film to the substrate.     

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.     

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.     

Growth mechanisms of cerium layers on galvanised steel

In recent times, a good deal of research efforts have been devoted to develop new environmentally friendly corrosion inhibitors as an alternative to the classic systems based on chromates. In this way, rare earth salts have demonstrated a high efficiency in different metal/corrosive systems. Lanthanide salts are classified as cathodic inhibitors, although, it is not clarified the species which contribute to the formation, in the cathodic areas, of the oxide/hydroxide of the lanthanide ion. In this work, the inhibitor behaviour of CeCl₃ for galvanised steel in aerated NaCl solutions has been investigated. SEM and EDS studies showed that Ce is initially stochastically dispersed onto the metallic surface, forming a continuous film on its surface after prolonged immersion time. Electrochemical calculations seem to indicate that, under experimental conditions only Ce³⁺ is expected on the metallic surface and this is consistent with experimental findings.     

Electrochemical study of modified bis-[triethoxysilylpropyl] tetrasulfide silane films applied on the AZ31 Mg alloy

This work investigates the protective behaviour of bis-[triethoxysilylpropyl] tetrasulfide silane pre-treatments on the AZ31 Mg alloy. The silane solution was modified by the addition of cerium nitrate or lanthanum nitrate in order to introduce corrosion inhibition properties in the silane film.The corrosion behaviour of the pre-treated AZ31 magnesium alloy was studied during immersion in 0.005 M NaCl solution, using electrochemical impedance spectroscopy and the scanning vibrating electrode technique (SVET). The electrochemical experiments showed that the presence of cerium ions or lanthanum ions improve the protective behaviour of the silane film. The SVET experiments evidenced that the presence cerium in the silane film led to an important reduction of the corrosion activity.The results demonstrate that either cerium ions or lanthanum ions can be used as additives to the silane solutions to improve the performance of the pre-treatments for the AZ31 magnesium alloy.     

Hydrodynamic effect on the behaviour of a corrosion inhibitor film: Characterization by electrochemical impedance spectroscopy

Influence of hydrodynamic conditions on the behaviour of an inhibitor film used in the high speed mechanical field is studied. Experimentations are realized with a Rotating Disk Electrode (RDE). Electrochemical Impedance Spectroscopy (EIS) is carried out at the corrosion potential and also at anodic and cathodic over voltages under various rotation rates and after different immersion times. EIS technique is used to determine the beneficial effect of hydrodynamic conditions on the film formation. This technique characterizes also inhibitive layer modifications for high electrode rotation speeds and after long immersion times. Different analyses, XPS and AFM, were carried out and have confirmed film structure evolution with flow conditions.     

Investigation by electrochemical impedance spectroscopy of filiform corrosion of electrocoated steel substrates

This work aims at studying by electrochemical impedance spectroscopy (EIS) the susceptibility to filiform corrosion of low carbon steel covered by cataphoretic coating. The determination of the exposed metallic area variations of scratched samples during ageing test is an estimation of the disbonding of the coating and/or the filiform corrosion. This area can be evaluated by electrochemical impedance spectroscopy (EIS). A simplified electrical equivalent model used to estimate the exposed metallic area is valid if the corrosion products are correctly dissolved before characterization. Furthermore the steel is a very complex substrate and thus many parameters must be optimized in order to remove the corrosion products before EIS measurements.     

Influence of pores on the quality of a silicon polyester coated galvanised steel system

Silicon polyester coated steel plate is frequently used because of its good corrosion resistance under various conditions. However, if the application of the coating and/or the curing process is carried out too fast, evaporating solvent is enclosed in the coating, forming pores. The aim of this study is to investigate the influence of different types of pores on the corrosion resistance of the above-mentioned coating system by means of electrochemical impedance spectroscopy (EIS). Three different phases in the degradation process could be observed. An almost intact coating area is investigated as reference coating. By simultaneous interpretation of the measured corrosion potential, the evolution of the coating capacitance and its resistance, it can be concluded that this system provides a very good corrosion resistance in a saline environment. A coating containing only small macro-pores (2 μm<Ø<100 μm) always seems to be in the first degradation phase. Their presence hardly influences the corrosion resistance of the coating. Finally, large macro-pores (Ø>100 μm) can even reach the underlying zinc surface. They will strongly decrease the corrosion protection behaviour of the coating.     

Electrochemical impedance spectroscopy study of the effect of curing time on the early barrier properties of silane systems applied on steel substrates

Silane-based pretreatments have emerged as a valuable ecologically acceptable alternative to the chromating process. The application process of these pretreatments involves a series of fundamental variables whose optimisation is imperative in order to form a silane layer with good behaviour properties at the interface. One of the most important variables is the silane layer curing process, during which an interfacial network that delays the entry of aggressive species to the metallic surface is formed. The present work studies the effect of curing time on the barrier properties of two types of silane, 3-aminopropyltriethoxysilane and bis-3-triethoxysilylpropylamine, applied on steel substrates, with or without the subsequent application of an alkyd-base paint. Electrochemical impedance spectroscopy (EIS) shows how the silane layer curing process influences its barrier properties and thus its corrosion protection properties.     

Comparative EIS and XPS studies of the protective character of thin lacquer films containing CR or P salts formed on galvanised steel, galvanneal and galfan substrates

X-ray photoelectron spectroscopy (XPS) is used to analyse variations during exposure to humidity and UV radiation (UVCON test) in the chemical composition of the outer surface of organic coatings (lacquers) containing phosphating or chromating reagents applied on galvanised steel, galvanneal and galfan substrates. By means of electrochemical impedance spectroscopy (EIS) measurements the protective character of the coatings analysed by XPS is studied and an attempt is made to establish possible relationships between the chemical composition of the surface of the lacquered substrates after exposure to the UVCON test and their electrochemical characterisation in immersion in a 3% NaCl solution. In general, the formation of defects or the loss of adhesion of the lacquer film leads to a significant reduction in charge transfer resistance values and, at the same time, an increase in interfacial capacitance values. The special behaviour of the galfan/lacquer “with chromating reagents” system is associated with the presence of a thin insulating film of chromium and aluminium oxides at the base of the pores in the lacquer. Interfacial capacitance values tend to evolve in close correspondence with the content of some elements on the surface of the materials.     

Effect of PgTPhPBr on the electrochemical and corrosion behaviour of 304 stainless steel in H2SO4 solution

Weight-loss, potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) measurements were used to study the inhibition of 304 stainless steel corrosion in 1 M H₂SO₄ at 50 °C by propargyltriphenylphosphonium bromide (PgTPhPBr). The inhibiting effects of propyltriphenylphosphonium bromide (PrTPhPBr) and propargyl alcohol (PA) were also studied for the sake of comparison. For the investigated compounds, Tafel extrapolation in the cathodic region gave a corrosion inhibition efficiency of 98% at 1 × 10^–3 M. Adsorption of both PgTPhPBr and PA was found to follow Frumkin's isotherm while adsorption of PrTPhPBr obeys that of Temkin. In the anodic domain, PgTPhPBr acted as a good passivator. The impedance spectra recorded at the corrosion potential (E _cor) revealed that the charge transfer process in the inhibited and uninhibited states controls corrosion of 304 stainless steel.     

In situ electrochemical studies of forming-induced defects of organic coatings on galvanised steel

A new electrochemical setup is presented for in situ measurements during uniaxial forming of thin film coated metal substrates. This approach allows the formability analysis of a zinc pigmented organic coating on a galvanised steel substrate. The aim is to monitor the formation of defects during the forming process. The setup comprises an electrochemical microcapillary cell in a three-electrode arrangement and a miniaturised linear stretching device. The development of forming-induced defects is monitored in situ by applying electrochemical impedance spectroscopy (EIS) and also microscopically analysed by means of field emission scanning electron microscopy (FE-SEM). The studies were supported by GOM^® grid measurements and finite element simulations of model sample forming degrees. The established technique enables the evaluation of the correlation between forming degree and degradation of the barrier properties of organic coatings.Finally a phosphating process on the unformed and formed specimen is electrochemically and microscopically analysed to correlate the respective defect size with its local reactivity. The results show that stretching-induced defects occur at the interface between spherical Zn particles and the epoxy binder matrix. The defect size increases with increasing strain values. The phosphating process leads to the nucleation of phosphate crystals especially in the forming-induced defects and thereby reduces the free zinc in the defect area. The kinetic of the phosphating is accelerated with increasing size of the defect.     

Corrosion protection of new composite polymer coating for carbon steel in sulfuric acid medium by electrochemical methods

In this paper, electropolymerization technique has been used for the obtained of new composite: polypyrrole – dioctyl sulfosuccinate sodium/poly N-ethylaniline (PPY-AOT/PNEA) coatings over carbon steel of type OLC 45 electrode for anticorrosion protection. The PPY-AOT/PNEA coatings were successfully synthesized onto OLC 45 electrode by galvanostatic deposition from aqueous solutions 0.1 M NEA, 0.1 M PY, 0.01 M AOT and 0.3 M H₂C₂O₄ solution at different current densities (5, 3 and 1 mA/cm²) in different molar ratio. The anionic surfactant (AOT) as a dopant ion used during electropolymerization can have a significant result on the anticorrosion protection of the composite film by hindering the penetration of aggressive ions. The polymeric composite coatings have been analyzed by cyclic voltammetry (CV), Fourier transform infrared (FT-IR) spectroscopy and scanning electron microscopy (SEM) methods. The corrosion resistance of PPY-AOT/PNEA coated carbon steel has been examined by potentiodynamic polarization techniques and electrochemical impedance spectroscopy (EIS) methods in 0.5 M H₂SO₄ solutions. The data of the corrosion samples demonstrated that PPY-AOT/PNEA coatings assure a great anticorrosion protection of OLC 45 electrode in corrosive media. The corrosion rate of PPY-AOT/PNEA coated OLC 45 has been indicated to be ~9 times lower than of uncoated electrode. The corrosion protection effectiveness of the composite coating is more than 89%. The best efficiency is accomplished of PPY-AOT/PNEA obtained by electrodeposition at 5 mA/cm² current densities applied in molar ratio 5:1.     

Electrochemical stability and surface analysis of a new alkyd paint with low content of volatile organic compounds

This paper presents the results regarding the electrochemical stability and surface analysis of a new alkyd paint with low content of volatile organic compounds in comparison with a conventional alkyd paint. The alkyd films were realized on carbon steel substrate. Dry films thickness in 30–35 μm range was obtained. The experimental tests were carried out in 3% NaCl solution at the 25 °C temperature. The electrochemical techniques (electrochemical impedance spectroscopy (EIS), potentiodynamic and Tafel polarization) and surface analysis techniques (AFM, SEM, EDX) were used to evaluate protective properties of the tested paint films. The correlation of all experimental results established that the new alkyd paint with low content of volatile organic compounds presents better protective performances and lower permeability than conventional alkyd paint. AFM, SEM and EDX indicated a compact, homogenous, non-porous and adherent coating.