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This work evaluates the dry sliding behavior of anodic aluminum oxides (AAO) formed during one traditional hard anodizing treatment (HA) and two golden hard anodizing treatments (named G and GP, respectively) on a EN AW-6060 aluminum alloy. Three different thicknesses of AAO layers were selected: 25, 50, and 100 μm. Prior to wear tests, microstructure and mechanical properties were determined by scanning electron microscopy (VPSEM/EDS), X-ray diffractometry, diffuse reflectance infrared Fourier transform (DRIFT-FTIR) spectroscopy, roughness, microhardness, and scratch tests. Wear tests were carried out by a pin-on-disc tribometer using a steel disc as the counterpart material. The friction coefficient was provided by the equipment. Anodized pins were weighed before and after tests to assess the wear rate. Worn surfaces were analyzed by VPSEM/EDS and DRITF-FTIR. Based on the results, the GP-treated surfaces with a thickness of 50 μm exhibit the lowest friction coefficients and wear rates. In any case, a tribofilm is observed on the wear tracks. During sliding, its detachment leads to delamination of the underlying anodic aluminum oxides and to abrasion of the aluminum substrate. Finally, the best tribological performance of G- and GP-treated surfaces may be related to the existence of a thin Ag-rich film at the coating/aluminum substrate interfaces.

Based on the analysis of known methods of surface hardening of aluminum alloys (chromium plating, plasma electrolytic oxidation, hard anodizing), the prospects for pulsed hard anodizing are shown both for improving the functional characteristics of alloys and for large-scale implementation of this method. The purpose of this work is to show the possibility of pulsed hard anodizing to improve the serviceability of low-strength aluminum alloy 1011 under conditions of abrasive and sliding wear. The influence of the pulsed anodizing temperature on the phase-structural state of the synthesized layers, their abrasive wear resistance, and tribological characteristics in various lubricants were established, and the mechanism of wear of these layers was proposed. It is shown that with an increase in the temperature of pulsed anodizing, the wear resistance of the synthesized layers increases, and their abrasive wear resistance decreases. The negative effect of lubricating media on the wear resistance of the synthesized layers compared to tests under dry conditions was shown, and an explanation for this phenomenon is proposed. A significant (up to 40 times) increase in wear resistance in dry friction of anodized low-strength aluminum alloy 1011 compared to high-strength aluminum alloy 1050 was shown.

Aluminum alloys cannot be used in aggressive corrosion environments application. In this paper, three different surface coating technologies were used to coat the 6082-T6 aluminum alloy to increase the corrosion resistance, namely Plasma Electrolytic Oxidation (PEO), Plasma Spray Ceramic (PSC) and Hard Anodizing (HA). The cross-sectional microstructure analysis revealed that HA coating was less uniform compared to other coatings. PEO coating was well adhered to the substrate despite the thinnest layer among all three coatings, while the PSC coating has an additional loose layer between the coat and the substrate. X-ray diffraction (XRD) analysis revealed crystalline alumina phases in PEO and PSC coatings while no phase was detected in HA other than an aluminum element. A series of electrochemistry experiments were used to evaluate the corrosion performances of these three types of coatings. Generally, all three-coated aluminum showed better corrosion performances. PEO coating has no charge transfer under all Inductive Coupled Plasma (ICP) tests, while small amounts of Al3+ were released for both HA and PSC coatings at 80 °C. The PEO coating showed the lowest corrosion current density followed by HA and then PSC coatings. The impedance resistance decreased as the immersion time increased, which indicated that this is due to the degradation and deterioration of the protective coatings. The results indicate that the PEO coating can offer the most effective protection to the aluminum substrate as it has the highest enhancement factor under electrochemistry tests compared to the other two coatings.

Aerospace applications and energy saving strategies in general raised the interest and study in the field of lightweight materials, especially on aluminum alloys. Aluminum alloy itself does not have suitable wear resistance. Therefore, improvements of surface properties are required in practical applications, especially surface hardness when aluminum is in contact with other parts. In this work, first Al7075-T6 was coated using hard anodizing technique in different parameters condition and the surfaces hardness of hard anodizing-coated specimens were measured using microhardness machine. Second, fretting fatigue life of AL7075-T6 was investigated for both uncoated and hard anodized specimens at the highest surface hardness obtained. Third, a fuzzy logic model was established to investigate the effect of hard anodizing parameters, voltage, temperature, solution concentration, and time on the anodized AL7075-T6. Four fuzzy membership functions are allocated to be connected with each input of the model. The results achieved via fuzzy logic model were verified and compared with the experimental result. The result demonstrated settlement between the fuzzy model and experimental results with 95.032 % accuracy. The hardness of hard anodizing-coated specimens was increased up to 360 HV, while the hardness of uncoated specimens was 170 HV. The result shows that hard anodizing improved the fretting fatigue life of AL7075-T6 alloy 44 % in low-cycle fatigue.

In the automobile sector, pistons are anodized on the crown, on ring grooves, and also on skirts to improve its wear resistance and corrosion resistance properties. In this work, we have carried out crown anodizing along with skirt anodizing simultaneously to study the wear resistance of anodized samples. The surfaces are anodized with three different processes such as soft anodizing, hard anodizing, and microarc oxidation (MAO) or well known as plasma arc oxidation (PAO). All the three processes differ in their respective procedures. The hardness value and microstructure of all the samples were tested to find wear resistance values and the effect of coatings on the samples. In wear testing, piston samples are rubbed on cast iron to denote its wear resistance based on weight loss per unit time. Reciprocating wear testing is also carried out on every sample for testing their wear resistance value.

This work is devoted to anodic oxidation at low temperatures in the sulfuric acid electrolyte of the foundry aluminum alloy AK9ch, with a high silicon content. Optimal anodizing conditions were chosen for obtaining coatings with a hardness of more than 300 HV and a thickness of about 40 microns. With an increase in the thickness of the coatings by increasing the current density or anodizing time, their hardness begins to decrease. The resulting hard coatings are planned to be used on large parts obtained by casting aluminum alloy AK9ch.

Hard anodizing is used to improve the anodic films’ mechanical qualities and aluminum alloys’ corrosion resistance. Applications for anodic oxide coatings on aluminum alloys include the space environment. In this work, the aluminum alloys 2024-T3 (Al-Cu), 6061-T6 (Al-Mg-Si), and 7075-T6 (Al-Zn) were prepared by hard anodizing electrochemical treatment using citric and sulfur acid baths at different concentrations. The aim of the work is to observe the effect of citric acid on the microstructure of the substrate, the mechanical properties, the corrosion resistance, and the morphology of the hard anodic layers. Hard anodizing was performed on three different aluminum alloys using three citric–sulfuric acid mixtures for 60 min and using current densities of 3.0 and 4.5 A/dm2. Vickers microhardness (HV) measurements and scanning electron microscopy (SEM) were utilized to determine the mechanical characteristics and microstructure of the hard anodizing material, and electrochemical techniques to understand the corrosion kinetics. The result indicates that the aluminum alloy 6061-T6 (Al-Mg-Si) has the maximum hard-coat thickness and hardness. The oxidation of Zn and Mg during the anodizing process found in the 7075-T6 (Al-Zn) alloy promotes oxide formation. Because of the high copper concentration, the oxide layer that forms on the 2024-T6 (Al-Cu) Al alloy has the lowest thickness, hardness, and corrosion resistance. Citric and sulfuric acid solutions can be used to provide hard anodizing in a variety of aluminum alloys that have corrosion resistance and mechanical qualities on par with or better than traditional sulfuric acid anodizing.

An investigation has been carried out in order to study the fretting fatigue behavior of a 2014-T6 aluminum alloy, which has been coated with a commercial hard anodizing of approximately 20-25 μm in thickness. The hardness (HV) was significantly improved up to about 380 after hard anodizing coating while the hardness value of original 2014-T6 was 175. Fretting reduced drastically the fatigue life of samples in both conditions, substrate and coated conditions. The application of such a coating to the substrate may increase the fretting fatigue life in comparison with the uncoated samples in low-stress region for rotating bending fatigue loading while at higher stresses the effect of anodizing is reversed. This may be result from early initiation of cracking of hard anodizing film due to high-stress concentration resulting from bulk stresses. On the other hand, the increase in fretting fatigue life in low-stress region may be probably attributed to low coefficient of friction that prevents metal-to-metal contact, which may result in higher fretting fatigue life because of retardation of crack initiation resulting from lower stress concentration compared to the substrate.

In the aeronautical industry, Al-Cu alloys are used as a structural material in the manufacturing of commercial aircraft due to their high mechanical properties and low density. One of the main issues with these Al-Cu alloy systems is their low corrosion resistance in aggressive substances; as a result, Al-Cu alloys are electrochemically treated by anodizing processes to increase their corrosion resistance. Hard anodizing realized on AA2024 was performed in citric and sulfuric acid solutions for 60 min with constant stirring using current densities 3 and 4.5 A/dm2. After anodizing, a 60 min sealing procedure in water at 95 °C was performed. Scanning electron microscopy (SEM) and Vickers microhardness (HV) measurements were used to characterize the microstructure and mechanical properties of the hard anodizing material. Electrochemical corrosion was carried out using cyclic potentiodynamic polarization curves (CPP) and electrochemical impedance spectroscopy (EIS) in a 3.5 wt. % NaCl solution. The results indicate that the corrosion resistance of Al-Cu alloys in citric acid solutions with a current density 4.5 A/dm2 was the best, with corrosion current densities of 2 × 10−8 and 2 × 10−9 A/cm2. Citric acid-anodized samples had a higher corrosion resistance than un-anodized materials, making citric acid a viable alternative for fabricating hard-anodized Al-Cu alloys.

The current work delves into the interactive effects of hard anodizing variables, including electrolyte concentration, temperature, current density, and time, on the microstructural, mechanical, and tribological characteristics of 6061 aluminum alloy. The results demonstrate that the final film characteristics are controlled by a kinetic balance between electrochemical oxide formation and chemical dissolution. The influence of electrolyte concentration and temperature was found to be non-monotonic, while a substantial synergistic effect between current density and time was observed. A maximum hardness of 679HV and a thickness of 59 μm was achieved using a sulfuric acid concentration of 190 g/L, an electrolyte temperature of -2 °C, a current density of 4.4 A/dm 2 , and a duration of 60 min. These were considered the optimal anodizing conditions. Tribological examination confirmed the enhanced film’s tribological behavior, exhibiting noticeably lower mass loss and a more stable coefficient of friction than the bare substrate. This enhancement is attributed to a shift in the wear mechanism from severe adhesive wear on the substrate to milder abrasion and, at high loads (50 N), brittle fracture on the hard anodic film.