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 Mg2 SiO4, MgSiO3, Mg2 P2 O7, and MgAl2 O4 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)

Corrosion and wear resistance of coatings produced on AZ31 Mg alloy by plasma electrolytic oxidation in silicate-based K2TiF6 containing solution: Effect of waveform

In this research, plasma electrolytic oxidation coatings were prepared on AZ31 Mg alloy in a silicate-based solution containing K2TiF6 using bipolar and soft sparking waveforms with 10, 20, and 30% cathodic duty cycles. The coatings displayed a net-like surface morphology consisted of irregular micro-pores, micro-cracks, fused oxide particles, and a sintered structure. Due to the incorporation of TiO2 colloidal particles and the cathodic pulse repair effect, most of the micro-pores were sealed. Long-term corrosion performance of the coatings was investigated using electrochemical impedance spectroscopy during immersion in 3.5 wt.% NaCl solution up to 14 days. The coating grown by the soft sparking waveform with a 20% cathodic duty cycle having the lowest porosity (6.2%) and a sharp layer concentrated in F element at the substrate/coating interface shows the highest corrosion resistance. The friction coefficient of this coating has remained stable during the sliding even under 5 N normal load, showing relatively higher wear resistance than other coatings. The coating produced using the equivalent unipolar waveform, as the reference specimen, showed the highest friction coefficient and the lowest wear resistance despite its highest micro-hardness

Incorporation mechanism of colloidal TiO2 nanoparticles and their effect on properties of coatings grown on 7075 Al alloy from silicate-based solution using plasma electrolytic oxidation

Plasma electrolytic oxidation (PEO) was applied using a pulsed unipolar waveform to produce Al2O3-TiO2 composite coatings from sol electrolytic solutions containing colloidal TiO2 nanoparticles. The sol solutions were produced by dissolving 1, 3, and 5 g/L of potassium titanyl oxalate (PTO) in a silicate solution. Scanning electron microscopy, energy dispersive spectrometry, X-ray diffraction, and Raman spectroscopy were applied to characterizing the coatings. Corrosion behavior of the coatings was investigated using polarization and impedance techniques. The results indicated that TiO2 enters the coating through all types of micro-discharging and is doped into the alumina phase. The higher level of TiO2 incorporation results in the decrease of surface micro-pores, while the lower incorporation shows a reverse effect. It was revealed that the higher TiO2 content makes a more compact outer layer and increases the inner layer thickness of the coating. Electrochemical measurements revealed that the coating obtained from the solution containing 3 g/L PTO exhibits higher corrosion performance than that obtained in the absence of PTO. The coating produced in the absence of PTO consists of gamma-Al2O3, delta-Al2O3 and amorphous phases, while alpha-Al2O3 is promoted by the presence of PTO

Silicate and Hydroxide Concentration Influencing the Properties of Composite Al2 O3-TiO2 PEO Coatings on AA7075 Alloy

This work evaluates the effect of sodium meta-silicate pentahydrate (SMS) and potassium hydroxide concentrations on properties of Al2O3-TiO2 coatings produced through plasma electrolytic oxidation in a solution containing 3 g L−1 potassium titanyl oxalate, (PTO), using a unipolar waveform with constant current density. The surface and cross-section characteristics of PEO coatings including morphology, elemental distribution, and phase composition were evaluated using FESEM, EDS, and XRD techniques. Voltage-time response indicated the concentration of SMS and KOH had a significant effect on the duration of each stage of the PEO process. More cracks and pores were formed at the higher concentrated solutions that resulted in the incorporation of solution components especially Si into the coating inner parts. Ti is distributed throughout the coatings, but it had a dominant distribution in the Si-rich areas. The coating prepared in the electrolyte containing no silicate consisted of non-stoichiometric γ-Al2O3 and/or amorphous Al2O3 phase. Adding silicate into the coating electrolyte resulted in the appearance of α-Al2O3 besides the dominant phase of γ-Al2O3. The corrosion behaviour of the coatings was investigated using the EIS technique. It was found that the coating prepared in the presence of 3 g L−1 SMS and 2 g L−1 KOH, possessed the highest barrier resistance (~10 MΩ cm2), owing to a more compact outer layer, thicker inner layer along with appropriate dielectric property because this layer lacks the Si element. It was discovered that the incorporation of Ti4+ and especially Si4+ in the coating makes the dielectric loss in the coating

Effect of pulse current mode on microstructure, composition and corrosion performance of the coatings produced by plasma electrolytic oxidation on AZ31 Mg alloy

Plasma electrolytic oxidation (PEO) coatings were grown on AZ31 Mg alloy in a silicate-based electrolyte containing KF using unipolar and bipolar (usual and soft-sparking) waveforms. The coatings were dual-layered consisting of MgO, MgF2 and Mg2SiO4 phases. Surface morphology of the coatings was a net-like (scaffold) containing a micro-pores network, micro-cracks and granules of oxide compounds. Deep pores were observed in the coating produced by unipolar and usual bipolar waveforms. The soft-sparking eliminated the deep pores and produced the lowest porosity in the coatings. It was found that the corrosion performance of the coatings evaluated using EIS in 3.5 wt. % NaCl solution is mostly determined by the inner layer resistance, because of its higher compactness. After 4 days of immersion, the inner layer resistances were almost the same for all coatings. However, the coatings produced by unipolar and usual bipolar waveforms showed sharp decays in inner layer resistances after 1 week and even the barrier eect of outer layer was lost for the unipolar-produced coating after 3 weeks. The low-frequency inductive loops appeared after a 3-week immersion for all coatings indicated that the substrate was under local corrosion attack. However, both coatings produced by soft-sparking waveforms provided the highest corrosion performance

Synergistic effect of W incorporation and pulsed current mode on wear and tribocorrosion resistance of coatings grown by plasma electrolytic oxidation on 7075 Al alloy

Ceramic coatings were grown by plasma electrolytic oxidation on 7075 Al alloy using unipolar and bipolar pulsed current waveforms with 20 and 40% cathodic duty cycles, from a silicate-based bath without and with the addition of Na2WO4. Pancake-like morphology was dominant on the coatings grown by unipolar waveform, while the bipolar waveforms promoted volcano-like morphology, increased the roughness of the coating surface and the formation of more compact layers. The coatings produced using the bipolar waveforms provided higher resistances toward both tribocorrosion and dry sliding conditions, while further improvement was achieved by the presence of tungsten. The coatings produced in tungstate containing bath using the bipolar waveform with 40% cathodic duty cycle provided the best performance in both sliding conditions, showing ∼90% reduction in volume loss comparing to the coating produced in additive-free electrolyte using unipolar waveform

The multi-effects of K2TiF6 additive on the properties of PEO coatings on AZ31 Mg alloy

Plasma electrolytic oxidation of AZ31 Mg alloy was performed in an alkaline silicate-based solution containing various concentrations of K2TiF6 using unipolar waveform at a constant voltage. The surface morphology of all coatings was rough and contained a micro-pore network, micro-cracks, and granules of oxide compounds in different diameters and micro-pore levels. The coating is composed of MgO, MgF2, Mg2SiO4, SiO2 (amorphous), and TiO2 (crystalline and amorphous) phases. The average thickness of the coatings was increased by adding the K2TiF6, where EDS results showed that a skinny fluoride-rich passive layer forms. With increasing time and reaching the final potential, the TiO2 colloidal particles absorbed physically by the oxide erupted into the discharge channels allowing the formation of a thicker and more compact layer coating along with the pore sealing. Incorporation of silicate ions develops amorphous SiO2 in the coating that also partially seals the pores in the coating. Long-term corrosion performance of the coatings was investigated using EIS during 7 days of immersion in 3.5 wt% NaCl solution. It was found that the barrier performance of the PEO coatings was improved by adding K2TiF6 up to 5 g l−1. The coating produced in the presence of 5 g l−1 K2TiF6 displayed the lowest porosity percent (13.04%) with an appropriate thickness, which provided the highest barrier performance at both short- and long-immersion times

The importance of type of Ti-based additives on the PEO process and properties of Al2O3-TiO2 coating

Al2O3-TiO2 coatings were obtained from a silicate-based electrolyte using pulsed bipolar current by PEO process. Nano-particle titania (NP-TiO2) and potassium titanyl oxalate (PTO) were used as Ti-based additive sources in the PEO electrolytic solution, separately. The coatings were characterized using scanning electron microscope, energy dispersive spectroscopy, and an X-ray diffractometer. The mechanical properties of the coatings were investigated using nanoindentation and ball-on-disk tests. SEM results showed that the PTO developed a more compact inner layer besides the increase of coating thickness. However, NP-TiO2 created an inner layer with less thickness but with higher compactness, without any effect on the outer layer morphology. XRD and Raman spectroscopy analyses showed that the NP-TiO2 had inert incorporation into the alumina. However, TiO2 pro-duced by PTO had reactive incorporation into alumina and made a polymeric titanium oxide structure on the coating with doped rutile and anatase phases. Nanoindentation and tribology analyses approved that the Ti incorporation through PTO provides appropriate mechanical properties owing to the more compact and thicker inner layer. The mechanism of PTO performance in the PEO process was discussed regarding its effect on coating characteristics