The Sealing Step in Aluminum Anodizing: A Focus on Sustainable Strategies for Enhancing Both Energy Efficiency and Corrosion Resistance

On the Use of Polymer-Based Composites for the Creation of Optical Sensors: A Review

Polymers are widely used in many areas, but often their individual properties are not sufficient for use in certain applications. One of the solutions is the creation of polymer-based composites and nanocomposites. In such materials, in order to improve their properties, nanoscale particles (at least in one dimension) are dispersed in the polymer matrix. These properties include increased mechanical strength and durability, the ability to create a developed inner surface, adjustable thermal and electrical conductivity, and many others. The materials created can have a wide range of applications, such as biomimetic materials and technologies, smart materials, renewable energy sources, packaging, etc. This article reviews the usage of composites as a matrix for the optical sensors and biosensors. It highlights several methods that have been used to enhance performance and properties by optimizing the filler. It shows the main methods of combining indicator dyes with the material of the sensor matrix. Furthermore, the role of co-fillers or a hybrid filler in a polymer composite system is discussed, revealing the great potential and prospect of such matrixes in the field of fine properties tuning for advanced applications.

Corrosion Resistance of Aluminum Alloy AA2024 with Hard Anodizing in Sulfuric Acid-Free Solution

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.

Influence of Hydrothermal Sealing on the High Cycle Fatigue Behavior of the Anodized 6082 Aluminum Alloy

For aluminum alloys, anodizing is a common electrochemical surface treatment to allow for protection against corrosion and wear. The produced conversion layers are first sealed in industrial processes to further enhance the corrosion protection by closing the coating surface pores. In their lifetime, anodized components often undergo cyclic loadings. However, despite the relevance of a sealing treatment, there is a lack of systematic studies regarding its influence on the fatigue behavior of anodized aluminum components. In this work, a 6082-aluminum alloy was anodized in sulphuric acid and the effect of the anodizing treatment with and without further hydrothermal sealing on the fatigue strength was investigated. The thickness and Martens hardness of the coatings were determined and the coating appearance in non-sealed and sealed conditions was analyzed by scanning electron microscopy prior to and after cyclically loading at R = −1. The fatigue strength was significantly decreased by the anodizing treatment, when compared to the bare substrate. However, hydrothermal sealing had a positive influence as the anodized and sealed condition attained a fatigue strength in the range of the bare aluminum. Distinct differences regarding the coating appearances, thickness, and hardness were not observed when comparing the non-sealed and the sealed conditions. After fatigue loading, numerous pronounced radial cracks were present in the anodic coating, but the number of cracks were significantly lower for the hydrothermally sealed coating. Fatigue failure occurred due to propagation of one crack from the coating towards the substrate, resulting in single-point crack initiation, which was similar to the fatigue fracture behavior of the bare aluminum substrate.

Chemical Structural Coherence Principle on Polymers for Better Adhesion

Composite materials are the most variative type of materials employed in almost every task imaginable. In the present study, a synthesis of a novel perfluoroalkyltriethoxysilane is reported to be used in creating composites with polyhexafluoropropylene—one of the most indifferent and adhesion-lacking polymers existing. The mechanism of adhesion of hexafluoropropylene is proved to be due to chemical structural coherence of perfluoroalkyltriethoxysilane to a link of polyhexafluoropropylene chain. The ability of perfluoroalkyltriethoxysilane to attach to surfaces was studied by FT-IR spectroscopy of modified glass microspheres. Although the perfluoroalkyltriethoxysilane surface modifier allowed partial adhesion of polyhexafluoropropylene, some detachment took place; therefore, the surface nanostructuring was used to increase its specific area by aluminum foil anodizing. An anodized aluminum surface was studied by scanning electron microscopy. The resulting composite consisting of anodized aluminum, perfluoroalkyl surface modifier, and polyhexafluoropropylene layer was proved to be stable, showed no signs of detachment, and is a promising material for usage in harsh environments.

Characterization of the Anodic Film and Corrosion Resistance of an A535 Aluminum Alloy after Intermetallics Removal by Different Etching Time

The objective of this study was to improve the corrosion resistance of an A535 alloy by removing intermetallics on the alloy surface by alkaline etching to improve the morphologies and properties of the anodic film that was sealed with different sealants. It was found that alkaline etching for 4 min was suitable for dissolving intermetallic particles and simultaneously providing sufficient roughness for the adhesion of an oxide film to the Al matrix. The effect of alkaline etching revealed that a decrease in the intermetallic fraction from 21% to 16% after etching for 2 and 4 min, respectively, corresponded to the increase in the surface roughness, thickness, and consistency of the anodic film. It was also demonstrated that the surface morphology of the anodic films after stearic acid sealing was more uniform and compact than that after nickel fluoride sealing. The electrochemical polarization curves and salt spray test proved that the alloy etched for 4 min and sealed with stearic acid had better corrosion resistance as compared with the aluminum alloy sealed with nickel fluoride.

Silver-Doped Anodic Alumina with Antimicrobial Properties—Synthesis and Characterization

The incessant need for the elimination of pathogenic viruses and multi-drug resistant bacteria has been a critical issue during recent decades, and requires the creation of new antimicrobial materials. Our study describes the production of silver-modified anodic alumina substrates by two methods, and estimation of their bactericidal activity. Aluminum oxide coatings were obtained via an anodization process of low-purity aluminum in an acidic bath for different time periods. The realization of silver infiltration into the pores of the alumina layers was carried out employing two different routes—electrochemical deposition, and in situ thermal reduction. The obtained films were characterized using scanning electron microscopy (SEM). Changes in the surface morphology and thickness of the initial oxide structures after hot water sealing procedure were observed. The presence of silver inside the pores of the alumina layers was also assessed. It was found that silver electrodeposition resulted in greater surface saturation. Large silver accumulations were observed on the thinner anodic films which experienced electroplating for longer time periods. Finally, the antibacterial activity of the modified alumina structures against Gram-negative (Escherichia coli) and Gram-positive (Bacillus cereus) bacteria was evaluated. The results demonstrate that silver deposits acquired by the electrochemical technique improve the bactericidal efficiency of the anodic aluminum oxide (AAO) layers. On the contrary, alumina structures with chemically embedded Ag particles did not show significant antibacterial properties. Overall, the present studies demonstrate that biological activity of silver-doped AAO films depends on the techniques used for their modification.

Preparation and Preliminary Evaluation of Silver-Modified Anodic Alumina for Biomedical Applications

The present study reports a specific method for preparation of silver-modified anodic alumina substrates intended for biomaterial applications. Al2O3 coatings were obtained by anodization of technically pure aluminum alloy in sulfuric acid electrolyte. Silver deposition into the pores of the anodic structures was carried out employing in situ thermal reduction for different time periods. The obtained coatings were characterized using scanning electron microscopy (SEM), potentiodynamic scanning after 168 h in 3.5% NaCl solution and bioassays with human fibroblast and NIH/3T3 cell lines. The modified alumina substrates demonstrated better biocompatibility compared to the control anodic Al2O3 pads indicated by increased percent cell survival following in vitro culture with human and mouse fibroblasts. The Ag-deposition time did not affect considerably the biocompatibility of the investigated anodic layers. SEM analyses indicated that mouse NIH/3T3 cells and human fibroblasts adhere to the silver-coated alumina substrates retaining normal morphology and ability to form cell monolayer. Therefore, the present studies demonstrate that silver coating of anodic alumina substrates improves their biocompatibility and their eventual biomedical application.

Rapid antibacterial activity of anodized aluminum-based materials impregnated with quaternary ammonium compounds for high-touch surfaces to limit transmission of pathogenic bacteria

Infections caused by multidrug-resistant bacteria are a major public health problem. Their transmission is strongly linked to cross contamination via inert surfaces, which can serve as reservoirs for pathogenic microorganisms. To address this problem, antibacterial materials applied to high-touch surfaces have been developed. However, reaching a rapid and lasting effectiveness under real life conditions of use remains challenging. In the present paper, hard-anodized aluminum (AA) materials impregnated with antibacterial agents (quaternary ammonium compounds (QACs) and/or nitrate silver (AgNO₃)) were prepared and characterized. The thickness of the anodized layer was about 50 μm with pore diameter of 70 nm. AA with QACs and/or AgNO₃ had a water contact angle varying between 45 and 70°. The antibacterial activity of the materials was determined under different experimental settings to better mimic their use, and included liquid, humid, and dry conditions. AA-QAC surfaces demonstrated excellent efficiency, killing >99.9% of bacteria in 5 min on a wide range of Gram-positive (Staphylococcus aureus, Clostridioides difficile, vancomycin-resistant Enterococcus faecium) and Gram-negative (streptomycin-resistant Salmonella typhimurium and encapsulated Klebsiella pneumoniae) pathogens. AA-QACs showed a faster antibacterial activity (from 0.25 to 5 min) compared with antibacterial copper used as a reference (from 15 min to more than 1 h). We show that to maintain their high performance, AA-QACs should be used in low humidity environments and should be cleaned with solutions composed of QACs. Altogether, AA-QAC materials constitute promising candidates to prevent the transmission of pathogenic bacteria on high-touch surfaces.

Corrosion Behavior of AA2055 Aluminum-Lithium Alloys Anodized in the Presence of Sulfuric Acid Solution

The aim of this work was to evaluate the corrosion behavior of the AA2055 Aluminum-lithium alloy anodized in a sulfuric acid (H2SO4) bath, varying the current density of 0.19 and 1 A·cm−2 and why the sealing solution was water (H2O) and sodium dichromate (Na2Cr2O7). Anodized samples were exposed to a 10 vol.% H2SO4 solution and the electrochemical technique used was electrochemical impedance spectroscopy. Scanning electron microscopy and X-ray photoelectron spectroscopy were employed to characterization of the anodizing layer, determinate morphology and thickness of coatings. The Na2Cr2O7 sealing solution tends to increase the charge transfer resistance and produces a more homogeneous and compact passive oxide layer, and imparts a corrosion inhibition protection to the AA2055. SEM observations indicated that the morphology and thickness of the anodic films formed on AA2055 aluminum-lithium alloy anodized have the best results for both current densities.

Dry Sliding Behavior of an Aluminum Alloy after Innovative Hard Anodizing Treatments

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.

A Review on Anodizing of Aerospace Aluminum Alloys for Corrosion Protection

Aluminum alloys used for aerospace applications provide good strength to weight ratio at a reasonable cost but exhibit only limited corrosion resistance. Therefore, a durable and effective corrosion protection system is required to fulfil structural integrity. Typically, an aerospace corrosion protection system consists of a multi-layered scheme employing an anodic oxide with good barrier properties and a porous surface, a corrosion inhibited organic primer, and an organic topcoat. The present review covers published research on the anodic oxide protection layer principles and requirements for aerospace application, the effect of the anodizing process parameters, as well as the importance of process steps taking place before and after anodizing. Moreover, the challenges of chromic acid anodizing (CAA) substitution are discussed and tartaric-sulfuric acid anodizing (TSA) is especially highlighted among the environmentally friendly alternatives.

Corrosion Resistance of Hard Coat Anodized AA 6061 in Citric–Sulfuric Solutions

Aluminum is a material widely used in aeronautical and transport industries due to its excellent mechanical and corrosion resistance properties. Unfortunately, aluminum alloys are susceptible to corrosion, which limits their use in some corrosive environments. The aim of this work is to characterize hard coat film fabricated by anodizing in a citric–sulfuric acid system using electrochemical techniques. The anodization process was carried out using an aluminum alloy AA 6061 anodization bath: a mix of citric and sulfuric acid solutions were used. For the anodizing process, two current densities were used, 1 and 7.2 A·cm−2. Anodized specimens obtained under different conditions were exposed to a 3.5 wt.% NaCl solution, and their electrochemical behavior was studied by electrochemical impedance spectroscopy (EIS) and cyclic potentiodynamic polarization (CPP) according to ASTM G106-15 and ASTM G5-13, respectively. Scanning electron microscopy (SEM) was employed to determinate the morphology and thickness of coatings. The results showed improved corrosion resistance in 6061 aluminum anodized in citric–sulfuric acid electrolyte compared to those anodized in sulfuric acid solution.

Infrared Absorption Study of Zn–S Hybrid and ZnS Ultrathin Films Deposited on Porous AAO Ceramic Support

Infrared (IR) spectroscopy is a powerful technique to characterize the chemical structure and dynamics of various types of samples. However, the signal-to-noise-ratio drops rapidly when the sample thickness gets much smaller than penetration depth, which is proportional to wavelength. This poses serious problems in analysis of thin films. In this work, an approach is demonstrated to overcome these problems. It is shown that a standard IR spectroscopy can be successfully employed to study the structure and composition of films as thin as 20 nm, when the layers were grown on porous substrates with a well-developed surface area. In contrast to IR spectra of the films deposited on flat Si substrates, the IR spectra of the same films but deposited on porous ceramic support show distinct bands that enabled reliable chemical analysis. The analysis of Zn-S ultrathin films synthesized by atomic layer deposition (ALD) from diethylzinc (DEZ) and 1,5-pentanedithiol (PDT) as precursors of Zn and S, respectively, served as proof of concept. However, the approach presented in this study can be applied to analysis of any ultrathin film deposited on target substrate and simultaneously on porous support, where the latter sample would be a reference sample dedicated for IR analysis of this film.