Chemical Conversion Layer Research Articles

Assembly of Metal–Organic Chemical Conversion Layers as Ion Sieves along with Exposing Zn(002) Planes for Stable Zn Metal Anode

AbstractThe development of aqueous rechargeable Zn metal batteries, as one of the most promising large‐scale energy storage technology, is hindered largely by dendrite growth and surface passivation of Zn metal anode, which are deleterious to the battery life and Coulombic efficiency (CE). This report demonstrates that ethylenediamine tetramethylenephosphonic acid can in situ coordinate with Zn (EDTMP‐Zn) along with exposing (002) planes for highly reversible and stable Zn plating/stripping. The zincophilic EDTMP‐Zn layer may serve as ion sieves that homogenize Zn ion flux at the Zn anode surface and consequently induce uniform deposition of Zn. The hydrophobic groups in such functional layer is thought to circumvent the Zn anode surface from corrosion and hydrogen evolution reaction. EDTMP15‐Zn modified Zn anode (EDTMP15‐Zn@Zn) delivers a lifespan exceeding 1400 h at 5 mA cm−2, and 1 mAh cm−2 for Zn||Zn symmetric cell and improved CE of 99.7% over 1000 cycles in a Zn||Cu cell. The full cell coupled with NH4V4O10 cathode demonstrates improved rate performance and cycle stability.

Comparing three strategies for surface treatment of Mg coated by hexamethylene diamine tetra methylene phosphonic acid for corrosion protection

Manufacturing Mg and Mg‐based alloys with controllable degradation rate has always been a challenge. Surface modification is one of the best ways to acquire protection against corrosion. Previously, many approaches such as phosphating (Na3PO4), fluoridation (HF), and alkalization (NaOH) treatments have been used to regulate the corrosion rate. Here, we compare the corrosion rates of 3‐surface modified Mg samples coated with hexamethylene diamine tetra methylene phosphonic (HMDTMPA). The protection of those different layers was ranked in the order: phosphate > alkaline > fluoridation treatment. The chemistry of those chemical conversion layers was discussed in the light of scanning electron microscope (SEM), Fourier transform infrared spectroscopy (FTIR), and X‐ray photoelectron spectroscopy (XPS). The mechanism of tailoring a protective film of HMDTMPA on phosphated Mg was discussed according to the ability of P to bridge the substrate with the coating.

Comparative Investigation of the Corrosion Behavior and Biocompatibility of the Different Chemical Conversion Coatings on the Magnesium Alloy Surfaces

Due to their good biodegradability and biocompatibility, magnesium alloys are widely favored as the potential candidate for the biodegradable cardiovascular stent. However, the rapid degradation and the limited biocompatibility in vivo remain the main bottlenecks that inhibit their clinical applications. The construction of the chemical conversion coating on the magnesium alloy surface represents one of the effective strategies to control the degradation rate and enhance the biocompatibility. In the present study, the different chemical conversion layers were prepared on the magnesium alloy surface by chemical conversion treatment, including sodium hydroxide (NaOH), hydrofluoric acid (HF), phosphoric acid (H3PO4) and phytic acid (C6H18O24P6) treatment, and the corrosion behaviors and biocompatibility of the chemical conversion layers were comparatively investigated in detail. The results showed that the different chemical treatments can produce the different conversion layers on the magnesium alloy surfaces with a variety of physicochemical characteristics, corrosion resistance and biocompatibility, and all treatments can enhance the corrosion resistance to varying degrees. The hydrophilicity and corrosion resistance of the sodium hydroxide-treated magnesium alloy were the best among all the materials. Although the hydrofluoric acid-treated magnesium alloy had produced a hydrophobic coating, the corrosion resistance still needed to be improved. Magnesium alloys treated by sodium hydroxide showed a selective promotion of albumin adsorption, while the other samples simultaneously promoted albumin and fibrinogen adsorption. For the blood compatibility, the hemolysis rates of all of the treated materials were reduced to below 5%. The samples treated by phytic acid had the smallest hemolysis rate, and the NaOH-treated magnesium alloy had the least amount of platelet adhesion and activation. An appropriate microenvironment for cell growth could be achieved by the chemical conversion treatment, according to the results of the endothelial cell adhesion and proliferation, and the NaOH-treated surface showed the best endothelial cell growth behaviors among all of the samples. In summary, the corrosion resistance and biocompatibility of the magnesium alloy were significantly improved by the sodium hydroxide treatment, and thus this treatment can be used as a pretreatment for the surface modification of the magnesium alloy in order to further enhance the biocompatibility when used as the cardiovascular implants.

Non‐chromate conversion process for zinc coating with durable hydrophobicity and enhanced corrosion resistance

AbstractA Zn‐based coating with durable hydrophobicity and good corrosion resistance was formed on a mild steel substrate, which involves electroplating Zn from a non‐aqueous electrolyte, followed by passivation in an oleic acid (OA) solution. The electrodeposited Zn coatings were porous, which facilitated the formation of a chemical conversion layer of Zn oleate (ZO) during OA passivation. The Zn coating after passivation had a two‐layer structure, which included an outer layer of ZO with a thickness of ∼26 μm and an inner layer of Zn with a thickness of ∼6 μm. The outer layer ZO is a type of metal soap with a smooth surface and durable hydrophobicity, such that water droplets can easily slip off its surface. Corrosion testing and electrochemical measurements in 3.5 wt.% NaCl aqueous solution indicate that the Zn coating after OA passivation exhibits outstanding anti‐corrosion properties compared with those exhibited by pure Zn coating. The corrosion products and mechanism of the two‐layer coating were explored. This study shows that smooth metal oleate coatings can provide hydrophobicity and corrosion resistance simultaneously to mild steel substrates.

Superhydrophobic fluoride conversion coating on bioresorbable magnesium alloy – fabrication, characterization, degradation and cytocompatibility with BMSCs

A micro-nano structure CaF2 chemical conversion layer was prepared on fluoride-treated AZ31 alloy, then the composite fluoride conversion film (CaF2/MgF2) was modified by stearic acid (SA) and fabricated a superhydrophobic surface. The fluoride-treated magnesium, fluoride conversion film and superhydrophobic coating were characterized by SEM, EDS, XRD and FTIR. The properties of coatings’ adhesion and corrosion resistance were evaluated via tape test and electrochemical measurement. The cytocompatibility of the MgF2, CaF2 and super-hydrophobic CaF2/SA surface was investigated with bone marrow-derived mesenchymal stem cells (BMSCs) by direct culture for 24 h. The results showed that the superhydrophobic fluoride conversion coating composed of inner MgF2 layer and the outer CaF2/SA composite layer had an average water contact angle of 152°. SA infiltrated into the micro-nano structure CaF2 layer and formed a strong adhesion with CaF2 layer. Furthermore, the super-hydrophobic coating showed higher barrier properties and corrosion resistance compared with the fluoride conversion film and fluoride-treated AZ31 alloy. The BMSC adhesion test results demonstrated MgF2 CaF2 and CaF2/SA coatings were all nontoxic to BMSC. At the condition of in direct contact with cells, MgF2 showed higher cell density and enhanced the BMSCs proliferation, while CaF2 and CaF2/SA coating showed no statistically difference in cell density compared with glass reference but the CaF2 and CaF2/SA coating were not conducive to BMSCs adhesion.

Novel preparation system of an organic–inorganic hybrid chemical conversion layer with high chlorine tolerance: A characterization and mechanism study

The quality of the pretreatment layer on steel was obviously affected by the chloridion concentration in the layer bath. In this study, the effects of chloridion on the micro- anti-corrosion performance, structure and chemical composition of polyacrylic acid (PAA) based layer were investigated. PAA-Zn layer and PAA-Zn-NaCl layers were fabricated on steel surface through immersion the steel in the layer bath without and with different concentrations of chloridion for 20 min. The electrochemical impedance spectroscopy and polarization curve results indicated that the anti-corrosion performance of the PAA–Zn layer could be improved due to the existence of chloridion in the layer bath and reached the best anti-corrosion performance when the concentration of chloridion was 2 wt%. Scanning electron microscopy results reflected that the thickness of the PAA-Zn-2%NaCl layer was higher than that of PAA-Zn layer. X-ray photoelectron spectroscopy results demonstrated that the concentrations of iron and zinc for the PAA-Zn-2%NaCl layer were higher than that of the PAA-Zn layer. The all experimental results indicated that chloridion could accelerate the steel corrosion during the initial immersion and thus promote PAA ionization and produce numerous iron ions. The formed PAA anions could chelate iron and zinc ions to form layers on steel, leading to the high chlorine tolerance characteristic of the PAA layer bath.

Biodegradable and Bioactive Orthopedic Magnesium Implants with Multilayered Protective Coating.

Biocompatible and biodegradable magnesium (Mg) and its alloys possess excellent potential as orthopedic implant biomaterials, but they corrode too quickly inside the human body to maintain such functions as supporting and fixation. In this study, we have successfully prepared a composite biomedical material with Mg as the substrate and a multilayered coating on the metal, which contains a chemical conversion layer of fluoride or phosphate, an adhesion layer, and a layer of biodegradable polylactic acid (PLA). The recorded weight loss and the release of ionic magnesium (Mg2+) in the simulated body fluid (SBF) demonstrate that the corrosion rate of the protected Mg has been reduced to 1/10 that of the unprotected bare Mg. The corrosion rate of the protected Mg can be adjusted with either the chemical conversion type or the thickness of the PLA layer. The bare and the protected Mg has been manufactured as an implant for bone defect repair of Sprague-Dawley rats. The X-ray and micro-CT images show an improved anticorrosion of the protected Mg in vivo, and the histological analysis indicates that the Mg2+ ions released at an appropriate rate are beneficial for bone growth. The preliminary tests prove that the biodegradable Mg has been effectively protected by the innovative multilayered coating, and the implant has met the required biological functions such as biocompatibility and promotion of bone growth.

Influence of GPTMS Precursor Concentration as Pretreatment of Galvanized Steel

This work aims to evaluate the influence of the alkoxy precursor concentration on the morphological and electrochemical characteristics of the chemical conversion layer formed on a galvanized steel surface. Different films were obtained by the dip coating method varying precursor concentrations in 2, 5 and 10%. According to the SEM and EIS results the corrosion resistance property increases for higher GPTMS precursor concentrations.

Biofunctionization of biodegradable magnesium alloy to improve the in vitro corrosion resistance and biocompatibility

Due to its superior mechanical and biodegradable properties, magnesium alloy shows great promise in medical devices such as cardiovascular stents and bone grafts. However, the rapid corrosion and the limited biocompatibility hinder its clinical applications. Because the corrosion and biocompatibility are closely related with the surface properties of biomaterials, a novel surface modification strategy was proposed in the present study. The magnesium alloy was firstly treated by NaOH to form a passive chemical conversion layer to improve the acute corrosion resistance, and then 16-Phosphonohexadecanoicacid (16-Pho) was introduced on the modified surface by the self-assembly approach. Poly (ethylene glycol) (PEG), fibronectin (Fn) and fibronectin/heparin (FH) were successively immobilized on the magnesium alloy (AZ31B) to improve the biocompatibility. Attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) and X-ray photoelectron spectra (XPS) results showed that the above molecules were successfully immobilized on the surface. The hydrophilicity and corrosion resistance were significantly improved by alkali heat treatment. Decreased trend of water contact angle and better corrosion resistance were clearly observed after the following immobilization of the molecules. The biocompatibility was also improved by the surface modifications. Alkali heat treatments gave rise to low hemolysis rate, less platelet adhesion and prolonged clotting time, and the following surface modification further improved the blood compatibility due to the introduction of the different molecules. In addition, good cytocompatibility to endothelial cells was obtained after the surface modifications, especially, Fn and FH effectively enhanced cell attachment and growth. Therefore, this study may give clues to obtain desirable corrosion resistance and biocompatibility simultaneously on the magnesium alloys, which would expand their potential applications in biomaterials and biomedical devices, especially for cardiovascular biomaterials.

Improving Corrosion Resistance and Biocompatibility of Magnesium Alloy by Sodium Hydroxide and Hydrofluoric Acid Treatments

Owing to excellent mechanical property and biodegradation, magnesium-based alloys have been widely investigated for temporary implants such as cardiovascular stent and bone graft; however, the fast biodegradation in physiological environment and the limited surface biocompatibility hinder their clinical applications. In the present study, magnesium alloy was treated by sodium hydroxide (NaOH) and hydrogen fluoride (HF) solutions, respectively, to produce the chemical conversion layers with the aim of improving the corrosion resistance and biocompatibility. The results of attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) and X-ray photoelectron spectroscopy (XPS) indicated that the chemical conversion layers of magnesium hydroxide or magnesium fluoride were obtained successfully. Sodium hydroxide treatment can significantly enhance the surface hydrophilicity while hydrogen fluoride treatment improved the surface hydrophobicity. Both the chemical conversion layers can obviously improve the corrosion resistance of the pristine magnesium alloy. Due to the hydrophobicity of magnesium fluoride, HF-treated magnesium alloy showed the relative better corrosion resistance than that of NaOH-treated substrate. According to the results of hemolysis assay and platelet adhesion, the chemical surface modified samples exhibited improved blood compatibility as compared to the pristine magnesium alloy. Furthermore, the chemical surface modified samples improved cytocompatibility to endothelial cells, the cells had better cell adhesion and proliferative profiles on the modified surfaces. Due to the excellent hydrophilicity, the NaOH-treated substrate displayed better blood compatibility and cytocompatibility to endothelial cells than that of HF-treated sample. It was considered that the method of the present study can be used for the surface modification of the magnesium alloy to enhance the corrosion resistance and biocompatibility.

Phytic acid coating on Mg-based materials for biodegradable temporary endoprosthetic applications

Mg-based materials are especially attractive as biodegradable implants, but they degrade so fast in physiological media that some corrosion protection is needed. This paper evaluates the long-term biodegradation kinetics of powder metallurgy Mg and AZ31 alloy coated with a phytic acid chemical conversion layer and tested in a saline phosphate buffer solution (PBS) at 37 °C. The morphology and chemical nature of the conversion coating and the corrosion products were analysed by SEM/EDX, XRD and FTIR. Corrosion resistance of the as-received and coated specimens was determined by hydrogen release, mass loss and potentiodynamic polarisation curves. The results indicate that the inherent heterogeneity and porosity of the powder metallurgy magnesium specimens, both as-received and coated with phytic acid, promoted rapid hydrogen release and led to their premature fragmentation and complete dissolution before 250 h of immersion. In contrast, the phytic acid coating on AZ31 improves corrosion resistance in long-term tests up to 336 h without affecting biocompatibility, biodegradability and resorbability for use as a temporary endoprosthetic implant.

Electrochemical Evaluation of Hot-Dip Galvanized Steel (HDGS) Coated with Organic-Inorganic Hybrid Sol-Gel in Cementitious Materials

Chromate based chemical conversion layers (CCC) are commonly used as surface treatments as they exhibit enhanced corrosion resistance and improved coating adhesion. However, the restrictions imposed on the use of hexavalent chromium, due to health and environmental concerns, has led to R&D efforts in the last few decades to search for new coating materials that fulfill the regulations and economic constraints imposed. Among these new coating materials, sol-gel methods are shown to be environmentally and economically advantageous in producing coatings that can provide effective corrosion protection. This work describes the studies performed for applying a sol-gel method to the production of organic-inorganic hybrid (OIH) coatings over HDGS reinforcement. The evaluation of their efficiency as a corrosion protection layer, to prevent the initial excessive reaction between mortar and the zinc, during the first days after concreting was studied.

크롬 프리 친환경 유/무기 하이브리드 코팅액에 의한 냉연강판의 내식특성

In the past, a very popular method for reducing the corrosion on zinc involved the use of chemical conversion layer coatings based on . However, there is an important problem with using chromium salts as a result of restrictive environmental protection legislation. This study investigated the optimum condition for galvanized steel using an organic/inorganic solution with a Ti composition. In the case of a fixed heat treatment time, the corrosion resistance values of LR-0727(1) and LR-0727(2) were improved as the heat treatment temperature increased, and the optimum minimum temperature decreased with the heat treatment time. At the optimum heat treatment condition of two coating solutions, the heat treatment time of the LR-0727(1) solution was shorter than LR-0727(2) for the same heat treatment temperature. LR-0727(1) coated specimens did not show desquamation, and all of the specimens showed a good adhesive property. In contrast, in the case of the LR-0727(2) coated specimens, desquamation arose. Therefore, the adhesive property of LR-0727(1) was superior to that of LR-0727(2). The pencil hardness had a 3H average for all of the coating solutions and heat treatment conditions. In the case of a corrosion resistance test with boiling water, the coated specimens of LR-0727(1) were discolored, but LR-0727(2) was not. Finally, LR-0727(1) was more moisture proof than LR-0727(2).

유/무기 하이브리드 코팅액에 의한 냉간압연강판의 내식특성

In the past, a very popular way to reduce the corrosion rate of zinc was the use of chemical conversion layers based on . However, the use of chromium salts is now restricted because of environmental protection legislation. Previous research investigated the optimum corrosion resistance of galvanized steel treated with an organic/inorganic solution containing Si. The result showed that the optimum corrosion resistance occurred by heat treatment of in 5 min. In this study, one organic and three hybrid organic/inorganic coating solutions were applied to cold-rolled (CR) carbon steel. The coatings were then evaluated for corrosion resistance under a salt spray test. The coating solutions examined in this study consisted of urethane-only, urethane-Si, urethane-Si-Ti, and urethane-Si-Ti-epoxy. The results of the 7 h salt spray test showed that the urethane-Si-Ti and urethane-Si-Ti-epoxy coating solutions had superior corrosion resistance on CR steel.

Crystallization of a chemical conversion layer that forms on AZ91D magnesium alloy in carbonic acid

This investigation studied the crystallization of a chemical conversion layer that formed on AZ91D Mg alloy in carbonic acid. The layer was an amorphous precursor to a crystalline Mg–Al layered double hydrotalcite, which improved the corrosion resistance of the alloy. The precursor must be treated with a strongly alkaline solution, causing the leaching out of amphoteric Al 3+. The leaching step evidently caused crystallization from an amorphous precursor layer to a crystalline coating. Nuclear magnetic resonance analysis indicated that Al 3+, which was originally randomly coordinated with surrounding OH − in the precursor, coordinated octahedrally with OH − groups in the crystalline conversion coating.

Si 변성 유/무기 하이브리드 코팅액에 의한 아연도금강판의 내식특성

Galvanized steel has gone through a chemical process to keep it from corroding. The steel gets coated in layers of zinc because rust will not attack this protective metal. For countless outdoor, marine, or industrial applications, galvanized steel is an essential fabrication component. The reduction of the corrosion rate of zinc is an important topic. In the past, a very popular way to reduce the corrosion rate of zinc was to use chemical conversion layers based on . However, a significant problem that has arisen is that the use of chromium salts is now restricted because of environmental protection legislation. Therefore, it is very important to develop new zinc surface treatments that are environmentally friendly to improve the corrosion resistance of zinc and adhesion with a final organic protective layer. In this study, a Urethane solution (only Urethane 20 wt.%; S-700) and an organic/inorganic solution with Si (Si polysilicate 10 wt.% + Urethane 10 wt.%; LRO-317) are used. Based on the salt spray test of 72 h, S-700 and LRO-317 had a superior effect for the corrosion resistance on EGI and HDGI, respectively.

Corrosion properties of AZ31 magnesium alloy and protective effects of chemical conversion layers and anodized coatings

The corrosion properties of AZ31 magnesium alloys were studied by potentiodynamic polarization curves and electrochemical impedance spectroscopy(EIS) techniques, meanwhile, the protective properties of two environmentally protective types of chemical conversion layers and anodized coatings of AZ31 magnesium alloys were also discussed. The component of chemical conversion bath is NaH 2O 4 12H 2O 20 g/L, H 3PO 4 7.4 mL/L, NaNO 2 3 g/L, Zn(NO 3) 2·6H 2O 5 g/L and NaF 1g L, and components of the anodization bath is Na 2SiO 3 60 g/L, C 6H 5Na 3O 72H 2O 50 g/L, KOH 100 g/L and Na 2B 4O 7·2H 2O 20 g L. The results show that the corrosion resistance of AZ3 1 magnesium increases with the increase of pH value of the corrosive medium. For the chemical conversion layer acquired at 80 °C, 10 min is the best processing time and the charge transfer resistance of the chemical conversion layer is enhanced nearly by 10 times. The optimum processing time for the anodization of AZ31 is 60 min, the charge transfer resistance value of the anodized sample at the early immersion stage is nearly 26 times of that of the blank sample and the corrosion type of the anodized samples is pitting.

EIS study of organic coating on zinc surface pretreated with environmentally friendly products

The life time of many steel structure can be remarkably improved by protecting the steel with zinc layers. However, also the zinc coating can be involved by corrosion phenomena with the consequence that some steel surface is unprotected. The reduction of the corrosion rate of zinc is therefore an important topic. These results can be obtained by introducing zinc alloys with lower corrosion rate (ZnNi, ZnFe, etc.) or by protecting the zinc surface with organic or inorganic layers able to reduce the corrosion rate. In the past a very popular way to reduce the corrosion rate of zinc was the use of chemical conversion layers based on Cr 6+, able to increase the passivation tendency of the zinc (chromating). This procedure is quite effective also for improving the adhesion of organic coatings deposited on the zinc surface, but there is the important problem that the use of chromium salts is now restricted because of environmental protection legislation. It is therefore very important to develop new zinc surface treatments environmentally friendly to improve the corrosion resistance of zinc and the adhesion with the final organic protective layer. In this paper a characterisation of environmentally friendly conversion treatments based on Cr 3+ for zinc surface will be reported in comparison with traditional based Cr 6+ pretreatments on different zinc layers protected by organic coatings. The samples were studied using EIS measurements, and the data analysis was mainly based on the discussion of the mathematical combination (ratio, product, etc.) of different parameters of the equivalent electrical circuit model. This approach was found more useful, in order to compare the performance of different materials, in comparison to the simple discussion of the numerical values of the parameters, being these values generally influenced by random defects present in the samples, affecting the measured impedance. The results showed that the performance (adhesion and corrosion protection) of good formulated Cr 3+ based pretreatments are not far from the results, which it is possible to obtain with industrial Cr 6+ pretreatments and therefore Cr 3+ conversion layers can be considered an interesting alternative to the traditional ones.

Degradation mechanism of electrodeposited coatings in alkaline solution

Electrodeposited organic coatings are used as primers or as a single-layer coating for corrosion protection of domestic appliances. In this application, it is important to avoid any possible coating degradation that could be caused by detergent solution which may accidentally come in contact with the coating. These kinds of coatings are often tested by immersion of the coated metal sheet with a cross scribe in a detergent solution (pH 11, temperature 74 °C) for about 10 days, measuring the extent of delamination away from the scribe at the end of the test. In these tests, a high dispersion of data is noted: specimens that have been prepared to be equivalent show a range of delamination from almost zero to very high levels, unacceptable for the application. The aim of this work is to define the reasons for the anomalous and premature failure of some specimens. The experimental work was carried out using mainly electrochemical techniques and surface analysis. A mechanism of delaminative degradation has been proposed, based on the chemical conversion layer (zinc phosphate) of the steel substrate, the porosity of the organic coating (dependent on the deposition conditions) and the particular environment (high temperature and pH). The final results reveal the cause of the problem and allow a definition of a testing approach, based on electrochemical impedance measurements, capable of checking the coating quality during production. This test is conducted in well controlled conditions and with a simplified data analysis procedure.

Chromate-free conversion coating

“Chromium free” aluminium conversion treatments usually produce a very thin chemical conversion layer; moreover they do not possess the well known inhibition properties of chromates. Hence, the corrosion protection of the substrate is, in general, less effective than that offered by the thicker chromate layers. However, the aluminium/paint adhesion is noticeably improved by the use of fluotitanate pretreatment baths. Our previous works also showed that the behaviour of fluotitanate pretreatments can be similar to that of chromatized aluminium surfaces when the aluminium is painted with a protective thermoplastic polyester resin. The aim of this work is to study new bath formulations and treatment procedures in order to further improve the effectiveness of this kind of environmentally friendly aluminium pretreatment. Some production parameters such as surface degreasing and pickling, pH and salt concentration of the bath and time of immersion were varied in order to obtain more efficient conversion layers. Aluminium alloy sheets were pretreated in fluotitanate baths and for comparison, in traditional chromate baths or they were only degreased and pickled; these samples were then studied with and without the application of organic coatings by the use of electrochemical impedance spectroscopy. The painted samples were coated with epoxy or vinyl resin types. The coatings were kept sufficiently thin (10–20 μm) in order to speed up undercoating corrosion thus highlighting the pretreatment action. By the use of proper testing solutions and of suitable fitting procedures of the impedance data, it was possible to identify the contribution of the pretreatment to the corrosion resistance of the painted aluminium alloys.