Effect of Molybdate on Corrosion Performance of Oxide Coating Produced on 7075 Al Alloy Using PEO

In this research, plasma electrolytic oxidation (PEO) coatings were prepared on 7075 Al alloy in a silicate-based solution with Na2MoO4 additive using a unipolar waveform at constant current density. The coatings displayed micro-pores, micro-cracks, pancake-like and crater-like features, and also solidified molten oxide particles on the surface. The coatings were majorly composed of Al2O3 (gamma, delta, and alpha), SiO2 (amorphous), and MoO3 phases, which confirms the incorporation of molybdenum in the case of additive-containing coatings. Molybdenum species were transported through cracks, channels, and micropores, as the ready access pathways into the coating and partly sealed the coating pores. The EIS technique was used to evaluate the long-term corrosion performance of the coatings up to 168 h of immersion in 3.5 wt.% NaCl solution. The results showed that the barrier action of the PEO coatings was highly enhanced by adding Na2MoO4 due to the higher resistance that alumina achieved to chlorine absorption and also its higher stability by the incorporation of MoO3. The coating formed in the presence of 5 g L-1 Na2MoO4 showed the highest thickness and the lowest porosity percent (15.15%), which provided the highest corrosion performance at long immersion times

Experimental characterization, machine learning analysis and computational modelling of the high effective inhibition of copper corrosion by 5-(4-pyridyl)-1,3,4-oxadiazole-2-thiol in saline environment.

An oxadiazole derivative with functional groups favouring its adsorption on copper surface, namely 5-(4-Pyridyl)-1,3,4-oxadiazole-2-thiol, has been explored as potential inhibitor of copper corrosion in 3.5 wt.% NaCl. Electrochemical evaluation by electrochemical impedance spectroscopy, potentiodynamic polarization and SVET reveals inhibition efficiencies exceeding 99%. Surface microscopy inspection and spectroscopic analysis by Raman, SEM-EDX and XPS highlight the formation of a compact barrier film responsible for long-lasting protection, that is mainly composed of the organic molecules. Machine Learning algorithms used in combination with Raman spectroscopy data were used successfully for the first time in corrosion studies to allow discrimination between corroded and inhibitor-protected metal surfaces. Quantum Chemistry calculations in aqueous solution and Molecular Dynamic studies predict a strong interaction between copper and the thiolate group and an extensive coverage of the metal surface, responsible for the excellent protection against corrosion

Effects of pulse current mode on plasma electrolytic oxidation of 7075 Al in KMnO4containing solution

Plasma electrolytic oxidation of 7075 Al alloy was performed in alkaline silicate electrolyte containing KMnO4as an additive using unipolar (U) and bipolar pulsed current modes (B1 and B2). The coating grown using the bipolar current with longer cathodic pulse time (B2) revealed a more dense foam-like surface morphology with fewer volcano areas providing higher thickness, which is mainly raised by reduction in discharge intensity. Also, a lower concentration of electrolyte elements (Si and Mn) was incorporated into the coating providing a brighter appearance. EIS results showed very high inner layer resistances for the coatings indicating this layer controls the overall corrosion performance of the coatings. In this way, the highest inner layer resistance along with very low value of constant phase element was achieved for the coating produced by B2, which is an indication for its lower porosity and higher thickness. Also, the more noble corrosion potential and lower passive current density observed for this coating in potentiodynamic polarization test confirms its stronger barrier effect against the chloride ingression where its corroded surface after long-term EIS test showed no significant damage. The lower penetration depth and subsequently the higher hardness values were achieved for the coatings produced by B2

Characterization and properties of PEO coatings on 7075 Al alloy grown in alkaline silicate electrolyte containing KMnO4a dditive

Plasma electrolytic oxidation (PEO) was used to modify the surface of 7075 Al alloy by applying a bipolar pulsed-current in a silicate based electrolyte containing 0 to 3 g L− 1 KMnO4. For the coating produced in the base electrolyte, crystalline γ-Al2O3 was the main phase, while the coatings formed at the presence of KMnO4 consisted of α-Mn2O3 containing Si and Al oxides. A pancake structure was observed on surface of the coating produced in the base electrolyte, while a foam feature along with some volcano-like areas was detected on the surface of the coatings produced in the presence of KMnO4. The KMnO4 changed the discharge type leading to diminish the pores at the metal/coating interface and also increased the coating thickness. The Nyquist plots of the coatings in 3.5 wt% NaCl solution at pH 4 showed two capacitive loops related to the outer porous and inner compact layers, where the resistance of inner layer was very high indicating that it could determine the overall corrosion performance. The inner layer resistance increased with increasing KMnO4 concentration. At prolonged immersion times, the barrier property of the coatings was lost to some extent, but the coating produced in 3 g L− 1 KMnO4 solution showed superior corrosion performance due to its lower permeability. The corrosion current density of the specimen coated in the presence of 3 g L− 1 KMnO4 was ~ 3 times lower than that observed for the specimen coated in the base electrolyte

The corrosion and tribocorrosion resistance of PEO composite coatings containing α-Al2O3 particles on 7075 Al alloy

Plasma electrolytic oxidation (PEO) of 7075 Al alloy was carried out in silicate base electrolyte containing 200 nm diameter α-Al2O3 particles for producing composite coatings. The process was performed under a soft-sparking regime using a pulsed bipolar signal with several concentrations of α-Al2O3 particles. It was found that the incorporation of α-Al2O3 particles into the coating did not significantly alter the thickness and roughness of the coating. However, the α-Al2O3 particles were detected on surface of the composite coatings. Corrosion tests showed significant improvement in corrosion performance of the composite coatings due to the efficient pore blocking provided by α-Al2O3 particles, which enhances the barrier performance of both inner and outer layers of the coatings. However, the long-term EIS measurements showed that the performance of composite coatings becomes close to that of particle-free coating after 56 days of immersion in chloride containing solution. Tribocorrosion tests showed that adding 3 g·l−1 of α-Al2O3 particles to the electrolyte bath decreased the lost wear volume of the resulted coating from 30 to 10 mm3 (×10−3). Higher α-Al2O3 particles concentration (i.e. 7 g·l−1) showed detrimental effect on both corrosion and tribocorrosion performance of the coating

Effects of pulse current mode on plasma electrolytic oxidation of 7075 Al in Na2WO4 containing solution: From unipolar to soft-sparking regime

Plasma electrolytic oxidation coatings were produced on 7075 Al alloy in a silicate based solution containing sodium tungstate using unipolar, usual bipolar and soft-sparking bipolar pulsed current regimes. X-ray diffraction proved that the coatings contain gamma alumina and metallic tungsten. EDS results showed that the tungsten content decreases with increasing the negative half cycle. Scanning electron microscopy showed that the pancake and volcano-like were dominant morphologies for the coatings produced by the unipolar and bipolar current regimes, respectively. Under unipolar current regime, a band of large pores is observed at the metal/coating interface, while they became discrete by applying the bipolar current regime and their population decreased significantly with increasing the negative half cycle. The coating produced by soft-sparking regime showed the highest corrosion resistance in chloride containing solution, resulting also very effective toward localized corrosion, due to a synergistic effect of W incorporation and employed waveform. Incorporation of tungsten resulted in the formation of hard coating (up to 1900 Hv) with dark appearance, therefore promising for thermal control applications

Effect of electrochemical parameters on wear and tribocorrosion capabilities of the PEO coatings generated through pulsed waveforms on AZ91 magnesium alloy

In this study, PEO coatings were produced on AZ91 Mg alloy using pulsed waveforms at different electrical parameters such as type (unipolar and bipolar), cathodic duty cycle (20 and 40 %), and voltage amplitude (350 and 400 V) to assess wear and tribocorrosion resistance. The coating bath contained NaAl2O4 and NaF. The coating phase composition, microstructure, and chemical composition of the coatings were evaluated using GXRD, SEM, and EDS. Dry wear behavior was studied using the reciprocating ball-on-flat device, while tribocorrosion behavior was evaluated by immersing the coated specimens in 3.5 wt% NaCl solution and reading the OCP before, during, and after sliding. The worn area in both cases was also examined. The results showed that the bipolar waveform with the higher cathodic duty cycle (40 %) provided the best tribocorrosion and wear performance by forming MgF2 (as a corrosion-resistant phase) and MgAl2O4 (as a hard phase). At all three waveforms, the higher coating voltage (i.e. 400 V) enhanced the wear and tribocorrosion resistance by forming thicker coatings with higher amounts of MgF2 and MgAl2O4 phases

The Effect of Electrolytic Solution Composition on the Structure, Corrosion, and Wear Resistance of PEO Coatings on AZ31 Magnesium Alloy

Plasma electrolytic oxidation coatings were prepared in aluminate, phosphate, and silicate-based electrolytic solutions using a soft-sparking regime in a multi-frequency stepped process to compare the structure, corrosion, and wear characteristics of the obtained coatings on AZ31 magnesium alloy. The XRD results indicated that all coatings consist of MgO and MgF2, while specific products such as Mg2SiO4, MgSiO3, Mg2P2O7, and MgAl2O4 were also present in specimens based on the selected solution. Surface morphology of the obtained coatings was strongly affected by the electrolyte composition. Aluminate-containing coating showed volcano-like, nodular particles and craters distributed over the surface. Phosphate-containing coating presented a sintering-crater structure, with non-uniform distributions of micro-pores and micro-cracks. Silicate-containing coating exhibited a scaffold surface involving a network of numerous micro-pores and oxide granules. The aluminate-treated sample offered the highest corrosion resistance and the minimum wear rate (5 × 10−5 mm3 N−1 m−1), owing to its compact structure containing solely 1.75% relative porosity, which is the lowest value in comparison with other samples. The silicate-treated sample was degraded faster in long-term corrosion and wear tests due to its porous structure, and with more delay in the phosphate-containing coating due to its larger thickness (30 µm)

Effect of Anodic Deposition Parameters on Electrochemical Behavior and Microstructure of Mn-Ni Oxide As a Pseudocapacitive Electrode

Hybrid supercapacitors, or briefly pseudocapacitors, are an emerging class of energy storage devices, with the capability of providing both high power and energy density. In contrast to conventional electric double layer capacitors, which accumulate charge mostly through an electrostatic mechanism, pseudocapacitors utilize the pseudocapacitance arising from reversible Faradic reactions occurring at the electrode surface. Although various metal oxides have been investigated as active material for pseudocapacitors, greater attention has been given to manganese oxides. In recent years, also Mn-based binary oxides have been studied for pseudocapacitance, in particular, mixed oxides comprising Ni or Co as the other component. In the present work, Mn-Ni oxide thin films were deposited potentiodynamically at scan rate of 100 mVs-1, pH=7 and room temperature, on a stainless steel substrate. Undoubtedly, a survey of the literature shows that a systematic study on the effect of operating parameters on the properties of the mixed oxide thin films is still lacking. Accordingly, this work was undertaken with the aim of exploring the processing space for these oxides studying in particular the effects of the electrolyte Ni to Mn molar ratio, the deposition peak potential and the deposited mass on chemical composition, microstructure and capacitance behavior of Mn-Ni oxide thin films. The binary oxide deposits were characterized by energy dispersive X-ray spectroscopy (EDX) and scanning electron microscopy (SEM). The electrochemical behavior of the Mn-Ni oxide electrodes was studied by cyclic voltammetry (CV) at 5, 20, 50 and 100 mVs-1 and electrochemical impedance spectroscopy (EIS) in 1 M Na2SO4electrolyte. Expectedly, with increasing the Ni to Mn molar ratio in solution (from 0.1 to 10), the molar fraction of Ni in the oxide film was found to increase, resulting, on the one hand, in smoothing of the surface morphology and, on the other hand, in a change of the specific capacitance through a maximum of 145 Fg-1 at approximately 12% at Ni fraction and CV scan rate of 20 mVs-1. With further increase of the Ni fraction in the oxide to about 19% at, the specific capacitance started to decrease gradually. Changing the peak potential in the potentiodynamic deposition from +1.2 to +1.6 V vs. Ag/AgCl, the CV curves revealed a box-like shape for all values of the peak potential, while the capacitance showed a decreasing trend. EIS results revealed that charge transfer resistance increased with increasing peak potential and the lowest Rct was found for sample prepared at peak potential of 1.2 V vs. Ag/AgCl. The mass load per unit area of the film was controlled by the number of cycles. Finally, specific capacitance of 196 Fg-1 was obtained at scan rate of 5 mVs-1 for the Mn-Ni oxide film deposited potentiodynamically (5 cycles, 100 mVs-1, pH=7 and RT) in solution with Ni to Mn molar ratio of 2. At the high scan rate of 100 mVs-1, the specific capacitance was 135 Fg-1 showing good retention of the capacitance at high rate