Chromate Conversion Research Articles

Development of Non Chromate Conversion Coatings on AZ31 Alloy: Effect of Nickel Overcoat

Permanganate-based conversion coating, using nitric acid as an oxidative additive has been performed and analysis shows the reduction products of permanganate mainly MnO2, and oxidation products of magnesium (MgO and Mg(OH)2). Stannates-based coating has also been obtained in high basic medium using acetate- pyrophosphate additives. Isolated grains formed having a quasi-spherical form which have been identified as mixed magnesium and tin oxides. Particles very rich in tin are observed and discussed in terms of standard reduction potentials E0red.

Anticorrosive behaviour of Cr(VI)-free surface pretreatments applied on magnesium alloys

Among the anticorrosive protection technologies for magnesium alloys, the development of paint systems is a pressing need especially for the automotive and aeronautical industries. Conventional technologies are based on the use of Cr(VI) based compounds, both in pretreatments and primers, but for health and environmental reasons these technologies are being abandoned. An added problem in the painting of magnesium alloys is the lack of adhesion and the high reactivity of magnesium substrates, which may lead to underfilm corrosion and detachment of the paint system. Therefore, the selection of a suitable pretreatment is crucial.Our research seeks to evaluate the anticorrosive behaviour of Cr(VI)-free surface pretreatments applied on different magnesium alloys, AZ31, AZ91 and AM60 and to analyse whether they could be protective enough to assure the successful protection of the substrate without the application of a topcoat. The considered alternatives are four chemical conversion coatings including two coatings obtained by plasma electrolytic oxidation (PEO). A chromate conversion coating has been also applied as reference. Different corrosion tests have been carried out in both natural (marine and urban atmospheres) and accelerated environments: salt fog, condensing humidity and cyclic test (salt fog/dry/humidity). Electrochemical characterization by electrochemical impedance spectroscopy (EIS) has been also performed. The results show a notably better anticorrosive behaviour of PEO coatings compared to chemical conversion coatings for all the considered magnesium alloys in both, natural and accelerated environments. However, none of the studied coatings seem to successfully protect magnesium alloys in aggressive atmospheres (marine) without the application of a topcoat. On the other hand, although PEO coatings have shown an acceptable good behaviour in very low aggressive environments (urban), it would be also recommended the application of a topcoat.

Cerium-based conversion coatings on aluminium alloys: a process review

Rare earths are among the most promising options for replacing chromate conversion coatings on aluminium. In nearly three decades of research, several hundred papers have been published in the area, the bulk of which has never been reviewed. This paper reviews the literature on rare earth coating processes, with particular emphasis on those based on cerium. It is concluded that several process areas are poorly understood, some of which are critical to further progress in the field. These include the development of industrially suitable pretreatments, technologies for coating non-aerospace alloys and seals to enhance corrosion performance and paint adhesion.

The influence of Ce-based coatings as pretreatments on corrosion stability of top powder polyester coating on AA6060

Cerium-based conversion coatings (CeCCs) are one of the most prospective alternatives to the widely used chromate conversion coatings (CCCs) due to their anticorrosion efficiency, environmentally friendly nature and low cost. In this work, the CeCCs on AA6060 were prepared by immersion into aqueous cerium salt solutions at room temperature, and subsequently post-treated in heated phosphate solution. The effect of counter ion (nitrate and chloride) on the coating properties was studied testing CeCCs as sole or conversion layers for the top polyester coating. Since the 60μm thick polyester coating was applied, an artificial defect of 0.8mm hole was introduced to faster assess the differences between pretreatments. The system with CCC pretreatment was used as reference. Corrosion properties were investigated in 0.5M NaCl solution by electrochemical impedance spectroscopy while the adhesion strength was measured by NMPR (N-methyl-2-pyrrolidone) and pull-off tests. As shown, the post-treated chloride-based CeCC offered better protection than crack-free thin nitrate-based CeCC, when used as sole coatings. On the other hand, it was brought to evidence that in combination with top powder polyester coating, the CeCC deposited from nitrate solution exhibited better protection compared to protective system pretreated with chloride-based one. Excellent polyester coating adhesion was found independently on aluminium surface pretreatment.

EIS and LEIS investigation of aging low carbon steel with Zn–Ni coating

Electrochemical Impedance Spectroscopic technique (EIS) and Local Electrochemical Impedance Spectroscopic technique (LEIS) have been used to investigate the corrosion behavior of zinc–nickel+chromate conversion coating on carbon steel. The measurements were conducted in chloride media, 0.6M and 10mM NaCl solutions with coating with and without artificial defect. An electrical equivalent circuit was proposed to analyze the EIS spectra in 0.6M NaCl solution. LEIS was also performed in the vicinity of a scratch exposing the carbon steel to the corrosive solution. It was shown that the local impedance measurement was very useful to characterize in situ the electrochemical reactivity around a defect.

Molybdate-Based Conversion Coatings for Aluminum Alloys Part I: Coating Formation

In order to prevent the corrosion of aluminum alloys, a chromate conversion coating (CCC) is applied to the surface. However, chromate is a carcinogen. Thus, the development of chromate-free and environmentally-friendly replacement coatings is currently being pursued. In this study, a molybdate conversion coating (MoCC) was developed on aluminum alloy AA2024-T6. The coating ennobled the corrosion potential of AA2024-T6 and protected the underlying aluminum alloy via anodic inhibition. Scanning electron microscopy revealed the surface morphology to consist of a mud cracked pattern that is similar to what would be seen on a sample coated with a chromium conversion coating. A dense underlying protective layer was observed when examined using optical and scanning electron microscopes.

Molybdate-Based Conversion Coatings for Aluminum Alloys Part II: Coating Chemistry

In order to prevent the corrosion of aluminum alloys, a chromate conversion coating (CCC) is applied to the surface. However, chromate is a carcinogen and substitutes are being studied for its replacement. In part I of this study, the authors described the development of an environmentally friendly and corrosion protective molybdate conversion coating (MoCC) for aluminum alloy AA2024-T6. In this part, MoCC is characterized and a structure is proposed. Multiple techniques were used to characterize the coating chemistry. Raman and Fourier Transform-Infrared spectroscopies showed the formation of a molybdate based conversion coating on the aluminum alloy surface. X-ray photoelectron spectroscopy (XPS) showed the coating to be composed of multiple molybdenum based species: MoO2, Mo2O5, MoO4 2- and MoO3. Further, MoCC was determined to consist of two layers; the surface layer was composed predominantly of oxidized Mo(VI) whereas the inner layer mostly consisted of reduced Mo(IV) and Mo(V) species.

Aluminothermic reduction of Cr2O3 contained in the ash of thermally treated leather waste

In this study the viability of utilising ashes with high chromium oxide content, obtained by thermal treatment of footwear leather waste, in the production of low-carbon ferrochromium alloy (Fe-Cr-LC) by aluminothermic reduction was investigated. The following key-factors were selected for process modelling: the quantity of aluminium (Al) employed in the reaction, the iron amount added, the iron compound (Fe and/or Fe2O3) used, and the chromic acid addition. The process was investigated using a 2(4) full factorial design where the percentage of Cr2O3 reduced was used as the response. Variance analysis was employed to determine the significant effects and to validate the obtained model. The model was useful for finding the optimal operating conditions, including the maximisation of chromium conversion and the gross margin. Both resulted in similar process conditions, with 76.8±12.3% of chromium being reduced to the metallic phase, and 1.65±0.52 USD (kg ash)-1 as the gross margin. The qualities of some alloys obtained were investigated by scanning electron microscopy coupled with energy dispersive X-ray spectroscopy analysis (SEM/EDS). The results showed that the main problem for these alloys in a standard specification was the P and S content, suggesting that a pre-treatment is required.

Decanoate conversion layer with improved corrosion protection for magnesium alloy

A novel decanoate conversion layer (DCL) with improved hydrophobicity for the corrosion protection of Mg alloy is fabricated and tested. XRD, EDX and FTIR results suggest that DCL comprises (CH3(CH2)8COO)2Mg and Mg(OH)2. The contact angle of DCL increases with increased cycling number and reaches ca. 120° at the 30th cycle. The corrosion protection of DCL is much better than that of a decanoate inhibition layer, and close to that of a chromate conversion layer. With increased thickness, improved compactness and hydrophobicity, the decanoate conversion is proven to be a green and effective way to protect Mg alloys from corrosion.

Preparation & Microscopic Characterization of Non-Chrome Chemical Conversion Film in Aluminum Surface

A chromium-free environmental protection aluminum surface treatment technology was developed by theoretical analysis and a large number of experiments. Add zirconium ions and cerium ions to the treatment solution, besides adding fluoride, aluminum and hydrogen peroxide, etc. According to the orthogonal test obtained a non-chromate film-formation process of environmental friendly aluminum. The characterization methods including SEM, XPS and XRD were applied to study and analyze the morphology, composition, phase, and corrosion resistance of phosphate film, then discussed the film-forming reaction mechanism. Results showed that chemical conversion film formed on the aluminum surface was uniform, compact and stronger anti-corrosion could replace the traditional, more toxic chromate conversion film.

PREPARATION AND CHARACTERIZATION OF ANTI-CORROSION SELF-HEALING AND HYDROPHOBICCOATING ON ALUMINUM ALLOY

Aluminum alloy has been widely used as the basic material in many industries because of its low density and high strength to weight ratio. Poor resistance to corrosion, however, limits the variety of practical applications of aluminum alloy. Traditionally, this problem was faced with the use of chromate conversion coatings which result in effective corrosion protection because of the self-healing property. However, the presence of toxic hexavalent chromium compounds makes those coatings very hazardous to the environment. Consequently, it is necessary to develop new environmentally compliant, chromate (VI)-free alternatives. Recently, sol-gel technology provides a new promising approach to prepare protective coatings on aluminum alloy. It has many advantages, such as the simple operation and environmental protection, etc. In this work, an Ormosil coating was developed on aluminum alloy through the sol-gel method using triethoxyoctylsilane (TEOCS) and tetraethylorthosilicate (TEOS) as the precursors. The sol-gel coatings were deposited by dip-coating method on aluminum alloy sub- strate. The sol-gel coatings doped with different concentrations of cerium salt (Ce(NO3)3·6H2O) were investigated. Surface morphology, wettability, anti-corrosion and self-healing of the sol-gel coatings non-doped and doped cerium salt were characterized by using atomic force microscopy, contact angle measurements, electrochemical impedance spectroscopy and scanning electrochemical microscopy. The hydrophobic of the coatings was evaluated by means of contact angle measurements. The corrosion resistance of the coatings was investigated by means of electrochemical impedance spectroscopy mea- surements, and the anti-corrosion, self-healing properties were discussed based on equivalent circuit fitting. The corrosion behavior of the damaged sol-gel coatings was studied by scanning electrochemi- cal microscopy. The results indicated that it had a marked effect on the surface morphology, corrosion resistance and hydrophobicity when cerium salt was added to the Ormosil sol-gel coating. The con- tact angle of the Ormosil sol-gel coating is about 92.0 ◦ . Cerium salt doped coatings have a better hydrophobicity due to marked improvement of the surface morphology. This positive effect was more

Corrosion Protection of Hot Dip Galvanized Steel in Mortar

Corrosion of steel in concrete is one of the major causes of structure degradation, requiring expensive maintenance. The using of hot dip galvanized steel (HDGS) has been recognized as one effective measure to increase the service life of reinforced concrete structures in marine environmental. However, HDGS corrodes in contact with high alkaline environment of fresh concrete. Although this initial corrosion process allows the formation of a protecting layer barrier, the corrosion that occurs initially is harmful and chromate conversion layers are usually used to prevent it. Due to toxicity of Cr(VI), these kinds of pre-treatments have been forbidden and hybrid coatings have been proposed as alternatives [1-3]. To evaluate the performance of these coatings, beyond the laboratory characterization, in situ tests in real conditions should be performed. An electrochemical system to measure the macrocell current density (i gal ) was designed to evaluate the degradation of HDGS coated samples with different organic-inorganic hybrid films, embedded in mortar during 70 days, using an automatic data acquisition system. This system revealed to be feasible and highly sensitive to coatings degradation. Also, allow distinguishing different hybrid coatings with different thicknesses.

Comparison of the behaviours of chromate and sol–gel coatings on aluminium

Differences between chromate conversion coatings and sol–gel coatings formed on aluminium and aluminium–copper alloys are highlighted with respect to the formation of cracks, the effect of the substrate composition on coating thickness and the influence of curing in air on coating re‐growth. For sol–gel coatings, it is shown that (i) cracking can be eliminated by inclusion of a sufficient amount of organic moieties, (ii) the coating thickness is unaffected by copper and (iii) coating re‐growth occurs readily after curing. These attributes are shown to contrast with the significantly different behaviours of chromate conversion coatings. Copyright © 2012 John Wiley & Sons, Ltd.

Effective corrosion inhibition performance of Ce3+ doped sol–gel nanocomposite coating on aluminum alloy

Sol–gel nanocomposite coatings have been investigated for Alclad AA2024 as a potential replacement for the hazardous and banned hexavalent chromate conversion coatings. The sol–gel coatings were prepared using a sol obtained by the hydrolysis and condensation of methyltriethoxysilane and colloidal silica. They were doped with cerium nitrate inhibitor to provide active corrosion protection and were deposited by spray-coating method. The sol–gel process conditions were optimized to obtain smooth and homogenous crack free films. The chemical composition and microstructure of the coatings were studied using EDX and FESEM respectively. The effect of cerium nitrate concentration on the corrosion protection efficiency of the sol–gel coatings was evaluated using potentiodynamic polarization, electrochemical impedance spectroscopy and salt spray tests. The results indicated that cerium nitrate doped coatings were more resistant to corrosion than undoped coating. The coatings doped with 6000ppm cerium nitrate rendered improved protection after longer immersions due to the inhibitive action of Ce3+.

Sol–gel processing of hybrid nanocomposite protective coatings using experimental design

Environmentally friendly hybrid nanocomposite sol–gel coatings as substitutes for chromate conversion coatings have attracted a great deal of attention recently. The coatings derived from hydrolysis and condensation of polymerizable silane precursors tetraethylorthosilicate (TEOS) and 3-glycidoxypropyltrimethoxysilane (GPTMS) were deposited onAA5083 substrates by a dip coating technique. Statistical design of experimental (DoE) methodology based on Taguchi orthogonal design has been used to study and optimize compositional and process parameters using multifactor analysis of variance (ANOVA) analysis method and the adhesion strength of coatings to the substrate as per pull off test has been used as a response. Adhesive strength of sol–gel coatings to the substrate was evaluated using pull off and tape tests. Bending, impact resistance and pencil scratch tests were employed to characterize mechanical properties of different coatings. The surface morphology and chemical composition of the hybrid films were studied by atomic force microscopy (AFM) and Attenuated Total Reflectance-Fourier Transform Infrared Spectroscopy (ATR/FTIR), respectively. Optical microscopy (OM) and field emission scanning electron microscopy (FE-SEM) were used to investigate structure of the coatings. The results show an increase of the hydrolysis water content at a constant organic/inorganic molar ratio and other variable parameters increases adhesion of coatings to substrate, and optimum coatings exhibit excellent mechanical properties as well as adhesive to the substrate.

Using Silane Films to Produce an Alternative for Chromate Conversion Coatings

Recently, many efforts have been made to find environmentally friendly substitutes for carcinogen Cr6+ in the pre-treatments of metal substrates. Silane films are one of the important candidates for this purpose. The use of silane films on metallic substrates provides corrosion protection as a result of excellent adhesion and good barrier properties. Their corrosion protection can be improved by adding suitable dopants. There are various methods for applying these films on the metal substrates (sol-gel dip coatings and electrodeposited coating). In this review, several main researches in the field of silane films have been indicated and their results have been analyzed.

Synergistic effect of cerium conversion coating and phytic acid conversion coating on AZ31B magnesium alloy

A novel environmentally-friendly composite coating with a cerium conversion coating deposited on the phytic acid conversion sample was formed on an AZ31B magnesium alloy. The optimum processing parameters of the composite coating on the AZ31B magnesium alloy were investigated by hydrogen evolution method. The surface morphology, chemical nature and corrosion resistance of the composite coating were examined by SEM, EDS, FTIR, XRD, electrochemical measurements and hydrogen evolution method. The results show that the optimum processing parameters of the composite coating on the AZ31B magnesium alloy are cerium nitrate solution concentration of 0.06mol/L, treating time of 30min and treating temperature of 50°C. The composite coating has better uniformity than the phytic acid conversion coating and it is composed of O, C, P, Mg, Al and Ce. The P–OH, HPO42−, PO43−, CeO2 and a little Ce(OH)2 exist in the surface of the composite coating and the composite sample has best corrosion resistance than the untreated sample, chromate conversion sample and phytic acid conversion sample. The synergistic mechanism was also discussed.

Effect of chromate conversion coatings on the adhesion and corrosion resistance of painted 5083 aluminum alloy

The surfaces of 5083 aluminum alloy were coated with chromate conversion coatings. Then epoxy paints were applied to the chromate and nonchromate treatment surfaces. The adhesion, hardness, and corrosion resistance of the coated samples were evaluated by using adhesion tape tests, pencil hardness tests, salt spray, and electrochemical technique. The results indicated that the adhesion of coating systems (including chromate treatment) was better than the adhesion of nonchromate conversion coatings, whereas the hardness of the coating systems was identical. The salt-spray results showed that the corrosion propagation from the scratched area in the painted samples with chromate treatment was lower than for the nonchromate coated surfaces. In addition, the electrochemical corrosion measurements indicated that the coating system including chromate treatment presented lower corrosion current densities, higher polarization resistance, and less negative corrosion potential than the corresponding values obtained for the nonchromate conversion coated surfaces.

Experimental Assessment of Chromium Mobilization from Polluted Soil by Washing

Chromium contamination in soils mainly results from the discharge of chromium-containing wastes from several industrial activities. Chromium remediation in soils may involve extraction from the soil matrix by washing with an aqueous solution containing a chelating agent. Since any extraction treatment strongly affect soil chemical and physical properties, and the metals distribution into the different soil fractions, one of the major risks of applying this remediation approach may be the increase in the oxidation potential of the soil and, as a consequence, the enhancing of trivalent chromium conversion into the soluble and more toxic hexavalent form. This oxidation potential, in fact, will strongly depends upon several soil characteristics, mainly organics amount and manganese oxide content. The objectives of this paper were therefore to investigate the effectiveness of selected unspecific chelating agents (Ethylenediaminetetraacetic acid, EDTA, Ethylenediamine-N,N\'-disuccinic acid, EDDS, and Nitrilotriacetic acid, NTA) towards total chromium removal and to investigate the effect of the extraction treatment on the potential chromium oxidation capacity of the soil. Experimental batch tests of chromium extraction from soil slurries were performed at different pH and at a 1:10 liquid vs. solid ratio: chromium concentration was determined on the extracted solutions, and mass balances were performed. Results show that using EDTA and EDDS, the calculated maximum chromium extraction efficiencies were 25 % and 27.5 % respectively, depending upon the pH of the extracting solution, while a lower removal rate was observed by using NTA. According to the results of oxidation tests performed after the extractions, as a consequence of soil characteristics modifications induced by the extraction process, the soil showed a different potential of Cr(III) oxidation, depending on the operating conditions of the washing treatment.

Improved corrosion protection of aluminium alloy AA 2024 by sol–gel hybrid coatings after surface pretreatment by silanisation

Sol–gel derived organic–inorganic hybrid coatings have been investigated as a potential replacement for the environmentally hazardous chromate conversion coatings for corrosion protection of aerospace aluminium alloy AA 2024. Sol–gel hybrid coatings were prepared using a sol mixture (1∶1 volume ratio) of 3-glycidoxypropyl trimethoxysilane and tetraethoxysilane and doped with cerium nitrate and deposited on AA 2024 by dip coating and spray coating techniques. The effect of surface pretreatment on the corrosion protection efficiency of the sol–gel hybrid coatings was studied. The corrosion resistance of the coatings in 3·5% sodium chloride solution was evaluated by potentiodynamic polarisation measurements, electrochemical impedance spectroscopy and salt spray tests. The results showed that a sol–gel coating system with a simple silanisation as surface pretreatment exhibited improved corrosion protection for AA 2024.