L. Pezzato

Effect of process parameters of plasma electrolytic oxidation on microstructure and corrosion properties of magnesium alloys

Abstract In this work, a plasma electrolytic oxidation process was applied to AZ91 and AM50 magnesium alloys and commercially pure magnesium to produce a protective surface layer. The plasma electrolytic oxidation process was carried out in an alkaline phosphate solution with a DC power supply, using relatively high current densities and short treatment times. The influence of some important process parameters such as current density, treatment time and voltage was studied. The layers were characterised by scansion electron microscopy, X-ray diffraction and X-ray photoelectron spectrometry, in order to investigate the effect of the process parameters on the microstructure and chemical composition. The corrosion resistance properties of the obtained layers were investigated by potentiodynamic anodic polarization and electrochemical impedance spectroscopy tests. The current density, applied during the treatment, influenced the morphology and the thickness of the coatings, and, consequently, the corrosion resistance. The corrosion tests evidenced that the layers obtained with plasma electrolytic process provided a good corrosion protection to the magnesium and magnesium alloys.

Antibacterial effect of PEO coating with silver on AA7075

In this work, plasma electrolytic oxidation (PEO) coatings were produced on AA7075 using alkaline solution containing silicates compounds and silver micrometric particles in order to give to the coating an antimicrobial effect. In the optic of circular economy, silver chloride derived from the acid pre-treatment of electronic scraps was used as raw material and successively silver powders were synthesized from silver chloride solution using glucose syrup as reducing agent. The coatings were characterized by scanning electron microscope (SEM), X-ray diffraction analysis (XRD), X-ray photoelectron spectroscopy (XPS), potentiodynamic polarization test and antimicrobial tests. The results evidenced that the obtained coatings were homogenous and give to the samples higher corrosion resistance than untreated alloy. The silver particles, found both inside and outside of the pores that characterize the PEO layer, produced an efficacious antimicrobial effect both against E. coli and S. aureus.

Sealing of PEO Coated AZ91 Magnesium Alloy Using La-Based Solutions

In this work, solutions containing lanthanum salts were used for a post-treatment of sealing to increase the corrosion resistance of PEO coated AZ91 alloy. PEO coatings were produced on samples of AZ91 magnesium alloy using an alkaline solution containing sodium hydroxide, sodium phosphates, and sodium silicates. The sealing treatment was performed in a solution containing 12 g/L of La(NO3)3 at pH 4 at different temperatures and for different treatment times. Potentiodynamic polarization test, an EIS test, showed that the sealing treatment with solution containing lanthanum nitrate caused a remarkable increase in the corrosion resistance. The corrosion behavior was correlated with the surface morphology and elemental composition evaluated with scanning electron microscope (SEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). In particular, the sealing treatment at 50°C for 30 min resulted in being the most promising to increase the corrosion properties of PEO treated samples because of the formation of a homogeneous sealing layer, mainly composed of La(OH)3.

Effect of microstructure and residual stresses, generated from different annealing and deformation processes, on the corrosion and mechanical properties of gold welding alloy wires

Gold welding alloys, which are used in the production of both hollow and solid gold chains, affect the optical and mechanical properties of various gold products because the corrosion resistance of the individual links depends on these properties. It is important that welding alloys with high corrosion resistance do not degrade during or after the production process. The mechanical properties of gold welding alloy wires are strongly influenced by the alloy microstructure, which has a key role in both the machinability and the quality of the wires. In the presented work, various physical and mechanical properties of gold originating from different industrial deformation processes are evaluated. Specifically, various plastic deformation grades caused by different annealing and rolling steps are analyzed. The change of the temperature, time, and velocity parameters in the annealing and lamination processes leads to the formation of different levels of residual stresses in the material, which can generate a variation in the corrosion properties of the gold wires. The change in microstructure, due to the different annealing and rolling steps, is analyzed by optical microscope (OM) and SEM observations. The residual stresses are evaluated using XRD analysis, and the variations of the mechanical properties by micro-hardness tests. The corrosion resistance is evaluated by potentiodynamic polarization tests, in which an electrolyte solution is used to simulate human sweat. Small grain size and high homogeneity of the microstructure are preferred for the final products. In this study, the samples that have been produced are shown to have lower levels of residual stresses and feature higher corrosion resistance and more favorable mechanical properties.

Effects of atmospheric pressure plasma JET treatment on aluminium alloys

Abstract In this paper, the effect of surface treatments carried out with an atmospheric pressure plasma jet, generated by air, on the passive film of AA1071 and AA2024 was investigated. The samples of the aluminium alloys, before and after the surface plasma treatments, were analysed by secondary ions mass spectrometry depth profiling and by X-ray photoelectron spectroscopy. Potentiodynamic anodic polarisation tests and electrochemical impedance spectroscopy measurements were carried out to study the influence of the surface plasma treatments on the corrosion resistance properties of the Al alloys. The plasma treatments induced the oxidation of the surface and the formation of a thicker and denser passive layer, which exhibited higher corrosion resistance than the native film.

Microstructure and Mechanical Properties of a 18Kt 5N Gold Alloy After Different Heat Treatments

In precious metals industry, heat treatments play a key role in the production process, since the type and the sequence of these treatments produce the right combination of hardness and workability. The production sequence usually involves rolling, plastic deformation, heat treating, and surface finishing. In this work, different heat treatments were performed on samples of 18Kt 5N gold–copper–silver alloy, and the evolution of the microstructure and the microindentation hardness was analyzed. The heat treatments were performed after the first and the second rolling process (a softening heat treatment) and before the final surface brushing and polishing (a hardening heat treatment). Two different softening heat treatments were tested: a continuous annealing treatment performed in a belt furnace and a solution heat treatment in static furnace. Two different hardening treatments were studied: an aging heat treatment and a treatment of solubilization followed by an aging treatment. The microstructural observation and mechanical analysis showed that the solution treatment in static furnace produced smaller and more homogeneous grain size than a continuous annealing treatment, whereas the hardness was not reduced. This could be very important because the reduction in the grain size results in a marked decrease in the orange peel defects on the final products. It was also found that between the two hardening treatments, solubilization and aging produced an increase of about 90 HV in the hardness with a significant reduction in the number of superficial defects during the final surface finishing steps. The optimization of the heat treatments, studied in this work, produces a significant decrease in the total number of defects at the end of the production cycle.

Plasma electrolytic oxidation coatings with fungicidal properties

ABSTRACT Plasma electrolytic oxidation (PEO) is a relatively new surface treatment that permits to increase the corrosion and wear resistance of metals. One of the peculiarities of the process is the possibility to incorporate into the produced coating the particles suspended in the electrolyte. In this work, micrometric silver particles were added in the PEO coating to innovatively give a fungicidal effect to the samples due to the presence of silver, whose fungicidal effect is well known. PEO coatings were produced on aluminium alloy 7075 using an electrolyte containing sodium silicate and silver particles. The thickness, morphology, composition and corrosion resistance of the obtained coatings were studied. The fungicidal properties were evaluated with contact tests of 2 and 4 h using Candida albicans ATCC 10231. Silver particles were homogeneously distributed in the coating and the silver-containing samples were characterised by improved corrosion resistance and high fungicidal activity.

Effect of the composition and production process parameters on the microstructure, residual stresses, and mechanical and corrosion properties of gold alloys used in industrial jewelry processes

Gold alloys, used in the production of both hollow and solid gold chains, influence the optical and mechanical properties of various gold products. Also, the microstructure of the alloys strongly influences the mechanical properties, which have a key role in both the machinability and quality of the plates. In the present work, different compositions of gold alloys and various industrial deformation processes (annealing and rolling steps) were analyzed and optimized. The change in the production parameters and in the compositions of the alloys may lead to the formation of different levels of residual stresses within the material, which can generate a variation in the behavior of gold sheets. The microstructures after the different production processes were analyzed by OM and SEM observations, whereas the variation of mechanical properties by microhardness tests. The residual stresses were evaluated using XRD analysis and the corrosion resistance by potentiodynamic polarization tests. The results showed that with the optimized process, a higher homogeneity of the microstructure, with an increase of the quality of semi-finished products and without ruptures under roller trains, was obtained. Moreover, the grain refiner was changed and a totally non-magnetic gold alloy was developed. Finally, to improve the weldability of the final chains, a different gold welding alloy, with a lower melting point, was developed and optimized.

Effect of Secondary Phases Precipitation on Corrosion Resistance of Duplex Stainless Steels

Duplex Stainless steels (DSS) are biphasic austeno-ferritic steels in which the best combination of mechanical and corrosion resistance properties is achieved for almost equal volume fraction of the phases. These steels are classified according to their pitting corrosion resistance, assessed by the PREN index (Pitting Resistance Equivalent Number) which, although qualitatively, is widely employed as comparison. The present work is aimed to study the pitting resistance of four DSS grades (SAF 2101, 2304, 2205 and 2507) in the as-received condition and after isothermal aging in the critical range 750°C-900°C, to highlight the effect of secondary phases precipitation on the corrosion behavior. The materials were potentiodynamically tested in artificial seawater (pH7) at room temperature and the corresponding Critical Pitting Temperatures (CPT) were determined according to ASTM G150. Secondary phase precipitation mainly affected the lean duplex grades whereas the high-alloyed DSS were more stable even if large precipitation occurred.

Grey Anodizing of a Grade 5 Titanium Alloy: Study of Process Parameters

Anodizing is one of the most promising surface treatments for lightweight metals as titanium because it can increase wear and corrosion-resistance, as well as provide aesthetic appearance and electrical insulation.Three different types of anodizing can be performed on titanium alloys: type I for elevated temperature forming, type II as anti-galling application and type III for coloured surfaces. The type II anodization, called also grey anodizing, is often requested in aerospace or biomedical applications. It is characterized by the use of alkaline electrolyte and it is standardized according to the SAE AMS 2488D norm. However, in literature it is difficult to find information about the process parameters of grey anodizing.In this work, different parameters of the grey anodizing process on a grade 5 titanium alloy were investigated and optimized, in order to obtain an anodized layer with the desired properties, in terms of corrosion resistance, thickness of the coating and wear properties. In particular, the effect of current and voltage applied, treatment time, temperature and electrolyte composition on the characteristics of the anodized layer was studied.The thickness, the composition, the morphology and the adhesion of the protective layers were characterized by SEM/EDS analysis. The chemical and phase composition were analyzed by XRF and XRD techniques. The corrosion resistance of the samples was investigated by potentiodynamic polarization and electrochemical impedance spectroscopy tests. The best results were obtained using as electrolyte a solution containing sodium hydroxide, titanium dioxide, sodium silicate and activated charcoal, with the formation of an anodized layer mainly constituted by titanium oxides and silicates. Intermediate treatment times and ambient temperature were the best conditions to produce the anodized layer. The sample with the best performances showed a homogeneous protective layer about 5 μm thick and was characterized by a lower corrosion current density, higher corrosion potential and polarization resistance, compared with the other anodized samples.