Corrosion Resistance Of Conversion Coatings Research Articles

Modeling and optimization of the corrosion resistance of Cr-free and Cr-based chemical conversion coatings on nickel foil by artificial neural network and response surface method

Chemical conversion treatment is one of the most powerful methods to prevent the corrosion of metals. However, determining the optimal addition amount of various active ingredients for the enhancement of corrosion resistance requires a large number of repeated experiments, which is very time-consuming and is also not helpful for figuring out the interactions between active ingredients. In the present study, Cr-free conversion coatings (CFCC) and Cr-based conversion coatings (CCC) were prepared on nickel foil and characterized by scanning electron microscopy and X-ray photoelectron spectroscopy to determine their morphology and composition of them. The corrosion resistance of conversion coatings was measured by salt spray test and further investigated using the response surface method (RSM) and artificial neural network (ANN) approach. The constructed models displayed high predictive accuracy, and the coefficient of correlation value of the RSM-CFCC, ANN-CFCC, RSM-CCC, and ANN-CCC models is 0.9441, 0.9513, 0.9535 and 0.9564, respectively. This comparative study recommends the ANN model best fits the experimental data for both CFCC and CCC. By using the ANN outputs, satisfactory results can be estimated with superior accuracy and efficiency while the RSM model can describe the interactions between factors.

Effects of nucleation pretreatment on corrosion resistance of conversion coating on magnesium alloy Mg-10Gd-3Y-0.4Zr

To improve corrosion resistance of phosphate chemical conversion coatings (PCCs) on magnesium alloy Mg-10Gd-3Y-0.4Zr, a new route was proposed. The phosphate conversion treatment was preceded by two nucleation pretreatment processes in the form of physical adhesion and chemical adsorption. The present study focused on the effects of two nucleation pretreatments of silane coupling agent KH-550 + nano-SiO2 sol process and Ti/Zr surface modification treatment on microstructure and corrosion resistance of PCCs on magnesium alloy Mg-10Gd-3Y-0.4Zr. Results demonstrate that nucleation pretreatment can prefabricate dispersed nuclei on substrate's surface, which significantly enhances the nucleation rate during the initial stage of coating formation, and decreases the crystal size of PCCs. Corrosion resistance of the PPCs was increased by a factor of 4. The Ti/Zr pretreatment demonstrates a superior application perspective due to the ease-of-fabrication and simplicity.

Study on corrosion resistance behavior and formation mechanism of Ce conversion coating on manganese

Ce conversion coatings have received extensive attention from researchers due to their excellent corrosion resistance and environmental friendliness. Here, Ce conversion coatings on manganese were prepared by treating in the cerium nitrate conversion solution. The influence of treating parameters was studied and optimized. The surface morphology, element composition and chemical state of Ce conversion coatings were studied by means of SEM, EDS, XRD and XPS. Electrochemical methods of polarization curve and electrochemical impedance were used to study the corrosion resistance of conversion coatings. The results showed that the Ce conversion coating contained mainly Mn, O and Ce, and its forms were manganese oxide and cerium oxide. Electrochemical tests showed that Ce conversion coating can not only effectively improve the corrosion resistance of manganese substrate, but also has good acid and alkali resistance. Discussion on the formation mechanism of cerium conversion coating shows that the conversion treatment is beneficial to inhibit the further oxidation of manganese to the higher valence state.

Effects of Nickel Additive on Micro-Arc Oxidation Coating of AZ63B Magnesium Alloy

Based on environmentally friendly and green manufacturing of magnesium alloys, micro-arc oxidation coatings were prepared in optimized metasilicate solutions on AZ63B magnesium alloys surfaces. Effect of nickel additive on working voltage, surface morphology, thickness, hardness, composition and corrosion resistance of micro-arc oxidation coating of magnesium alloys was studied. The surface morphology, composition, microstructure and corrosion resistance of conversion coatings were investigated by using scanning electron microscopy (SEM), energy dispersion spectrometry (EDS), X-ray diffraction (XRD) and electrochemical tests respectively. The results show that brown coating was prepared in electrolyte with the addition of nickel acetate, which can accelerate growth of micro-arc oxide film layer and make the smaller hole on the surface. The micro-arc oxide coating mainly consists of Mg2SiO4, MgSiO3, MgAl2O4, Ni2SiO4. Potentiodynamic measurements and electrochemical impedance spectroscopy (EIS) testify that the corrosion resistant of micro-arc oxidation film adding nickel acetate electrolyte increases significantly.

Effect of Molybdate Bath Service Life on Corrosion Resistance of Conversion Coating Deposited on Hot Dip Galvanized Steel

A conversion coating was obtained in the molybdate based bath on hot dip galvanized steel. The influence of the bath service life and bath component variation on the corrosion resistance of the conversion coating were investigated. Corrosion resistance of the coating was evaluated by electrochemical technique in 3.5%NaCl. It is found that the anodic branch of polarization curves presented in passivation character. Electrochemical impedance spectroscopy (EIS) results confirm that the radius of the capacity impedance for molybdate film decreased with treating area, which reveals that the bath life and bath component variation affected anti-corrosion performance of the coating remarkablely. The bath constituent was analysed by inductively coupled plasma atomic emission spectrometry (ICP), and the results show that the bath will lose the passivation ability after treating 1.16m2 sample area for new prepared treating solution of 500ml. The surface morphologies of the coatings were observed using scanning electron microscopy (SEM), and the element composition of the coatings was detected using energy dispersive X-ray (EDX). The investigation results indicate that uniform and compact film is formed on the surface of the galvanized steel, and the film is composed of zinc,oxygen, phosphor and molybdenum.

Effect of cerium additive on aluminum-based chemical conversion coating on AZ91D magnesium alloy

Based on environmentally friendly and recycling of magnesium alloys, chemical conversion coatings were prepared in aluminum nitrate solutions with cerium nitrate additive on AZ91D magnesium alloys surfaces. Effect of additive's concentration on the surface morphology, composition and corrosion resistance of aluminum-based conversion coating on magnesium alloys was studied. The surface morphology, composition, microstructure and corrosion resistance of conversion coatings were investigated using scanning electron microscopy (SEM), X-ray energy dispersion spectrometry (EDS), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and electrochemical tests respectively. The results show that the cerium content of the conversion coatings surface increased with increasing additive concentration. The conversion coatings’ morphologies first gradually became dense and the micro-cracks on the coating surface became narrow with the increase of the additive concentration. Then the coatings’ morphologies became bad and the micro-cracks widened after the additive concentration reached 0.005mol/L. When the additive concentration was 0.005mol/L, the conversion coating consists of Al(OH)3, Al2O3, Mg(OH)2, MgO, CeO2 and Ce2O3; the conversion coating surface morphology was the densest and the micro-cracks were the narrowest, and the corrosion resistance was also the best.

Influencing Factors of Phytic Acid Conversion Coating on AZ91 Magnesium Alloy

AZ91 magnesium alloy is selected as the matrix metal. SEM, EDS, Tafel and EIS were adopted and spot test was carried out to investigate the influence of pH value, phytic acid concentration, temperature,conversion time on corrosion resistance of conversion coatings. Result shows that process of phytic acid forming is the controlled metal corrosion process and the uneven surface pattern reflects the different chemical properties of two-phase current on magnesium alloy surface, which is implied by the conversion surface. It is proved by the test that pH value is committed the most to the corrosion resistance of the conversion coating, and the next is the phytic acid concentration. It is found that the corrosion resistance of the conversion coating is the best when pH value of the conversion solution is 4.5; The conversion coating has little influence on the cathode reaction dynamics, however, more importantly, it changes the dynamics of the alloy electrode anode solution reaction, therefore, the alloy’s anode current density decreases while the corrosion resistance of magnesium alloy is enhanced.

Effect of alternating voltage treatment on the microstructure and corrosion resistance of stannate conversion coating on AZ91D alloy

A new stannate conversion coating (CC) on AZ91D alloy was synthesized by the application of alternative voltage (AV) treatment technique. By using AV technique, the formation process of CC can be controlled. SEM results indicated that a continuous and compact dual-layer CC was formed on alloy surface after AV treatment. TEM results revealed that the inner layer consists of magnesium–aluminum–stannum nano-crystals and amorphous, meanwhile, the outer layer is primarily a mixture of magnesium–stannum hydroxides and oxides with amorphous structure. The electrochemical experimental results revealed that AV treatment significantly improved the corrosion resistance of CC, which attributed to the distinguishing microstructure of AV-CC.

Computer modeling of the nonchromate treatment of aluminum alloys by neural networks

We analyze the thermodynamic characteristics and specific features of the course of reactions of oxometalate reduction on the aluminum surface and the properties of the final products (the specific electric resistance of oxides). The absence of a correlation between the ranged series of these parameters led to the necessity of using artificial intellect for modeling the processes of formation of conversion coatings. We obtained and visualized in 3D space the dependences of the corrosion resistance of D16 alloy on the oxometalate concentration, pH, the time of sealing, and test duration. Artificial neural networks based on multilayer perceptrons were used for predicting the corrosion resistance of conversion coatings.