Molybdate Conversion Coatings Research Articles

Ultra-thin inhibited molybdate conversion coatings for the protection of AMg3 aluminum alloy

Ultra-thin inhibited molybdate conversion coatings for the protection of AMg3 aluminum alloy

Corrosion inhibition of aluminum current collector with molybdate conversion coating in commercial LiPF6-esters electrolytes

A molybdate conversion coating (MCC) is developed to circumvent aluminum current collector (AlCC) corrosion in commercial LiPF6-esters electrolytes. The obtained MCC exhibits compact structure and good combination with AlCC matrix. Electrochemical tests show that MCC reduces the corrosion current density by an order of magnitude. Furthermore, in Li||NMC333 cells, the capacity retention of the cell with MCC-AlCC cathode is 75.6 % at 4.5 V high voltage, while that of control cell is only 7.2 %. The anticorrosion mechanism could be contributed to the stable artificial passivation layer which prevents HF erosion and the depth expansion of pitting corrosion.

Self-Healing Properties of Cerium-Modified Molybdate Conversion Coating on Steel

Environmentally friendly alternatives to chromium—phosphate/molybdate and cerium-modified phosphate/molybdate conversion coatings—were deposited on a carbon steel surface. Different surface analytic techniques were applied to obtain complementary information on the composition, element distribution morphology and inner structure of the coatings in order to establish the relationship between coating properties and corrosion performance. The higher protective and stronger self-healing abilities were found for phosphate/molybdate/cerium conversion coating deposited in a sulphate-containing solution. The protective barrier strength was found to be related with certain aspects of the coating morphology such like homogeneous distribution of fine crystallites and, hence, lower number of structural defects. The self-healing ability depended on both, the composition (higher amount of Ce(IV)) and micro-structural characteristics, such as defectiveness, of the conversion layer.

Effect of Molybdate Conversion Coating of Magnesium Alloy Reinforced by Micro-arc Oxidation

In order to enhance the corrosion resistance of molybdate conversion coating, the molybdate coating is subjected to micro-arc oxidation treatment in electrolyte with Na2SiO3 as a main salt. The electrochemical behavior and corrosion weight loss of different coating were studied. The surface morphology, element composition, roughness and phase composition of different coatings were analyzed by SEM, EDS, laser confocal microscopy and XRD, the growth process of different micro-arc oxidation coatings was explored. The results shows that the arcing voltage of the molybdate coating enhanced by micro-arc oxidation is significantly reduced, the corrosion potential is shifted to the right, and the corrosion weightlessloss of the coating is the lowest, compared with the molybdate coating and the micro-arc oxidation coating. Micro-arc oxidation on the surface of the molybdate coating is beneficial to the formation of MgAl2O4 and the production of new phase is MoSi2, which can better improve the corrosion resistance of the molybdate coating. The micro-arc oxide coating does not form a ceramic film with a pore structure at the moment of arcing. As the micro-arc oxidation time prolongs, the ceramic film is gradually formed. When the time reaches 60 s, the ceramic film is completely formed. In this paper, micro-arc oxidation treatment is performed on the basis of molybdate conversion coating to obtain micro-arc oxidation enhanced molybdate coating. The experiment is mainly divided into three parts: the first part, molybdate conversion treatment of the magnesium alloy substrate to obtain a molybdate conversion coating. The second part, the micro-arc oxidation treatment is performed on the surface of the molybdate coating, and discharge channels appear on the surface. The third part, micro-arc oxidation enhanced molybdate coating is obtained.

Corrosion Resistance and Formation Analysis of a Molybdate Conversion Coating Prepared by Alkaline Treatment on Aluminum Alloy 6063

A molybdate-based conversion coating (MBCC) was prepared on aluminum alloy 6063 (AA6063) by treatment in an alkaline conversion bath. The obtained MBCC is a gray and uniform conversion coating. Scanning electron microscope and electrochemical measurements were used to analyze the basic properties of the coating. X-ray photoelectron spectroscopy (XPS) was utilized to investigate the composition and formation process of the coating. The surface morphology and cross-sectional observation show that the coating grows rapidly under given conditions, its thickness can reach more than 30 μm in 10 min, and the MBCC prepared with the alkaline method is a non-cracked coating. The potentiodynamic polarization test indicates that the corrosion current of MBCC is more than one order of magnitude lower than the corrosion current of an untreated surface. The XPS measurement illustrates that the coating is made up of Al, Mo, O and F. According to the high-resolution XPS spectral analysis, the elements detected in the coating are in the form of Al2O3, AlF3, MoO3 and Al2(MoO4)3. Based on the XPS results, the formation process of the coating under alkaline conditions was discussed.

Electrochemical analysis of molybdate conversion coating on hot‐dip galvanized steel in various growth stages

The electrochemical behavior of molybdate conversion coatings in various growth stages was investigated by electrochemical impedance spectroscopy and equivalent circuits. The chemical composition and microstructure were characterized by SEM/energy dispersive spectroscopy, atomic force microscopy, and AES. Neutral salt spray tests complemented the information. The results indicated that the growth process of the molybdate conversion coating was accompanied by the growth of micro cracks, consisting of three stages: in the early and middle stages, the protective property of the coatings increased with increasing treatment time and then decreased when the cracks developed in the last stage. On the basis of the observation of the microstructure and the analysis of the electrochemical impedance spectra of the coatings in various stages, a simplified growth process model of the molybdate conversion coating on hot‐dip galvanized steel was created. Copyright © 2017 John Wiley & Sons, Ltd.

Treatment of molybdate solutions by electrodialysis: The effect of pH and current density on ions transport behavior

Abstract Molybdate conversion coatings are seen as a viable alternative to produce chromate-free coatings. By the coating process, molybdate containing wastewater are generated and electrodialysis could be applied for salts recovery and process water reuse. In fact, the electrodialysis efficiency for these purposes is associated to the ions transport properties through the ion-exchange membranes. In this work the transport behavior of molybdate ions through a HDX200 heterogeneous anion-exchange membrane was studied using the electrochemical characterization technique of chronopotentiometry. The study was carried out with 0.005 M Na 2 MoO 4 solutions at different pH values: 4.8, 6.5 and 12.0. Differences in the chronopotentiometric curves shape as well as in the limiting current density and ohmic electrical resistance values were observed and explained with basis on the molybdenum speciation diagram. The obtained results revealed that, according to the solution pH and current density conditions, polynuclear anions can be formed leading to a membrane-poisoning phenomenon, exposing limitations and suitable conditions of using the electrodialysis in the treatment of molybdate wastewater.

A high anticorrosive chromium-free conversion coating prepared with an alkaline conversion bath on electroless Ni–P coating

A molybdate conversion coating (MCC) thin film was deposited on electroless Ni–P (ENP) coating by treating in an alkaline chemical conversion bath. A chromate conversion coating (CCC) was also prepared for comparison. Surface morphology of the conversion coatings exhibited no obvious difference with the as-plated ENP coating. Acid exposure test indicated that the oxidation resistance of the MCC was improved about 45 times higher than that of untreated ENP coating, respectively. The corrosion current density (icorr) values for both passivated surfaces were dramatically reduced by conversion treatment. However, MCC specimen showed the lowest icorr value of 0.19μAcm−2 compared to 6.55μAcm−2 for ENP and 0.38μAcm−2 for CCC. EIS studies revealed that the MCC thin film possessed the highest charge transfer resistance (Rct) value, about 5 times higher than the CCC. XPS analysis indicated that the MCC thin film was made up of Ni, P, O, and Mo; the values of binding energy (BE) implied that both Ni and P participated in the formation of conversion coating thin film and the Mo was in the form of MO3.

Corrosion behavior of molybdate conversion coatings on AZ31 magnesium alloy in NaCl solution

Purpose – This paper aims to study the corrosion behavior of molybdate conversion coatings on AZ31 magnesium alloy in NaCl solution. Design/methodology/approach – The open circuit potential curves, anodic polarization curves, electrochemical impedance spectroscopy and immersion tests were used to study the corrosion behavior of molybdate conversion coatings on AZ31 magnesium alloy in NaCl solution. Findings – The molybdate conversion coatings were homogeneous and uniform. The corrosion resistance of AZ31 magnesium alloy had been improved evidently through the molybdate conversion treatment. Originality/value – The molybdate conversion coatings possess favorable feasibility in practice for the corrosion protection of AZ31 magnesium alloys in NaCl solution.

Study of formation of molybdate conversion coatings on AZ31 magnesium alloy

Molybdate conversion coatings are considered as an alternative to chromium conversion coatings for improving the corrosion resistance of AZ31 magnesium alloy. In this study, the formation process of molybdate conversion coatings on AZ31 magnesium alloy was investigated using an open circuit potential curve, scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). The influence of processing parameters of molybdate conversion coatings was studied using electrochemical impedance spectroscopy (EIS) in 3·5 wt-%NaCl solution as a function of pH value, temperature and time. In addition, the mass gain of the coating was also monitored during formation. The results show that the conversion coatings formed at 65°C for 12 min and pH 3 have excellent corrosion resistance properties. Molybdate conversion coatings formed on AZ31 magnesium alloy exhibit a network morphology with many microcracks on the surface of the coatings. The coating is composed of O, Mo, F, Na, P, Mg and Al. The weight gain of the coating during formation follows a parabola-like curve.

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.

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.

Preparation and Corrosion Resistance Property of Molybdate Conversion Coatings Containing SiO2 Nanoparticles

In this paper, molybdate conversion coatings containing SiO2 nanoparticles were successfully prepared on AZ31 magnesium alloy. The effect of SiO2 nanoparticles on the weight of molybdate conversion coatings was investigated. The surface morphology of molybdate conversion coatings containing SiO2 nanoparticles was examined by scanning electron microscopy (SEM). The polarization curves, electrochemical impedance spectroscopy (EIS) and weight loss measurements were employed to evaluate the corrosion resistance of molybdate conversion coatings containing SiO2 nanoparticles in 3.5 wt% NaCl solution. The results demonstrate that the weight of molybdate conversion coatings reaches a maximum value when the concentration of SiO2 nanoparticles is 4 g L−1. The SiO2 nanoparticles are uniformly distributed in the molybdate conversion coatings. The results of weight loss measurements and electrochemical methods show that the addition of SiO2 nanoparticles in the conversion treatment has significantly improved the corrosion resistance property of molybdate conversion coatings.

Self-Healing Nature of Molybdate Conversion Coatings for Aluminum Alloys

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Effect of Additives on Structure and Corrosion Resistance of Molybdate Conversion Coatings Deposited on AZ31B Mg Alloy

Molybdate conversion coatings were prepared on AZ31B Mg alloy in a molybdate based solution with additives of sodium fluoride (NaF) or lanthanum nitrate (La(NO3)3). The effects of F- and La3+ additives on the morphology and on the corrosion resistance of the molybdate coatings were investigated by scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and potentiodynamic polarization analysis. The results revealed that these additives could accelerate the deposition of molybdenum on the surface of AZ31B magnesium alloy. It can be seen that the composite conversion coatings consist of many spherical particles. It is also found that the corrosion resistance is greatly improved after molybdate treatment. The films formed in F- additive solution showed lower corrosion current density than in La3+ additive solution.

Novel approaches in designing high performance nano and nano-composite coatings for industrial applications

The first part of this paper reports the author recent research to develop eco-friendly coatings based on salts like silica, ceria, vanadia, or molybdate conversion coatings for aluminium alloys and AI-composite as alternatives to toxic chromate-based systems. The effect of surface modification prior to applying the coatings was studied in details. Different coatings techniques were used such as free immersion, electrochemical activation and sol-gel method. The second part reports the challenges in designing high performance nanocomposite coatings over steel via electroless deposition technique. Newly developed Ni-P nanoparticle alumina and Ni-P-W were studied. Results confirmed the vital role of the surface modification prior to applying the coatings on the corrosion resistance and adhesion performance of chemical conversion coatings. A new coating systems based on etching the substrate surface with dilute alkaline solution followed by oxide thickening in boiled water prior to applying the coatings was found promising to improve the adhesion as well as the corrosion resistance.

Preparation of Molybdate Conversion Coatings on the Anodizing Surface of Aluminum Alloy

Molybdate conversion coatings were prepared on the anodized surfaces of aluminum alloy substrates, and the corrosion resistance in 3.5% NaCl solution was also investigated. The effects of the main parameters on the quality of the coatings were discussed. The morphology and chemical composition of the conversion coatings was examined by SEM and EDS. SEM observation showed that the coatings were composed of small sheet grains with tiny cracks between them. EDS analysis showed that the main element of the coatings was molybdenum. The results of polarization curve showed that the coatings effectively inhabited the pitting corrosion of aluminum alloy substrates. In the end the possible conversion mechanism was discussed.

SVET investigation into use of simple molybdate passivation treatments on electrodeposited zinc coatings

Chromate replacement in passivation processes has formed the basis of much research, molybdates having already been studied as one entirely chromium free alternative. Molybdate conversion coatings from three different solutions have been examined using the scanning vibrating electrode technique (SVET) to determine whether molybdates are capable of 'self-repair', one of the beneficial features of chromates. Additionally, morphological studies and neutral salt spray corrosion tests have been used to characterise the samples. Chromate passivated samples have been used as the standard against which the molybdates have been assessed throughout the study. SVET line scans conducted over a period of ∼74 min suggest that molybdates are capable of some form of self-repair when damaged. However, the anodic potential perturbation during repassivation is larger than that observed with chromates.

Structural and Electrochemical Properties of Al-Added Molybdate Conversion Coatings on Zinc

Molybdate conversion coatings were deposited on zinc substrate in the presence of Al to improve the corrosion resistance and their structural and electrochemical properties were characterized with various analytical techniques. Infrared reflectance spectroscopy analysis indicated that Al existed as Al(OH) 3 in the molybdate conversion coatings. It was found that the addition of Al to molybdate conversion coatings hindered the oxygen reduction and hydrogen evolution reactions effectively compared with pure molybdate conversion coatings, resulting in the enhanced anticorrosive property.