Papers by Author: Isolda Costa

Abstract: Pre-sintering for removal of lubricants is one of the most important steps in processing of powder metallurgy materials to achieve corrosion resistance. This is often the most neglected step in the sintering process. Incomplete removal of these organic compounds may result in sensitization of sintered parts. The aim of this study is to investigate the effect of lubricant removal process on the corrosion resistance of sintered AISI 304 L stainless steel filters. Pre-sintering was carried out at 450° C and sintering at 1150° C. As lubricant, 1% wax was used. The corrosion resistance of sintered samples was evaluated by polarization and tests, optical and scanning electron microscopy. The results showed that lubricant removal was not complete and during sintering sensitization of the stainless steel occurred decreasing the localized corrosion resistance of the stainless steel.

Abstract: 2xxx Al alloys are particularly sensitive to localized corrosion in chloride environments and in order to maintain their integrity, minimize maintenance needs and repairs, and to maximize component life, protective treatments are required.Anodizing is an electrochemical process based on the growth of the Al oxide layer by applying anodic potentials. One of the alternatives is tartaric/sulphuric acid (TSA) anodizing, which is environmentally compliant and provides corrosion resistance properties, compatible with the requirements of the aerospace industry with appropriate paint adhesion.In this study, AA2524-T3 specimens were anodized in a tartaric-sulfuric acid bath (TSA) and subsequently protected by application of a hybrid sol–gel coating. The sol–gel coating was prepared using a solution with high water content (58 %v/v) and obtained by the hydrolysis and condensation of tetraethoxysilane (TEOS) and 3-glycidoxypropyltrimethoxysilane (GPTMS). The corrosion resistance evaluation of both unsealed and coated samples was carried out in a sodium chloride solution by EIS as a function of immersion time. The results were also fitted using electrical equivalent circuits.

Abstract: The effect of hydrothermal treatment time on sealing and corrosion resistance of the AA7475-T761 anodized aluminium alloy has been investigated in this study. The hydrothermal treatments tested are environmental compatible without chromium ions involved. Anodizing was carried out by a tartaric-sulphuric anodizing (TSA) process and this was followed by hydrothermal treatments for partial sealing, in various solutions. The effect of propyleneglycol (PRG) and/or cerium ions in the hydrothermal treatment solution was evaluated. Four treatment times were tested, specifically, 2.5, 5.0, 7.5 and 10 min. The corrosion resistance of the anodized and treated samples was evaluated by Electrochemical Impedance Spectroscopy (EIS) and the anodic layers formed were characterized by Scanning Electron Microscopy (SEM). The EIS results showed that the hydrothermal treatments in solutions with cerium ions resulted in similar impedances for periods of treatment from 5 to 10 min whereas in the solutions with PRG the impedance increased with time of treatment from 2.5 to 10 min showing a slower kinetics of anodic layer sealing. However, the fastest kinetics of sealing were associated to the treatments that combined two steps, one in PRG and other in cerium containing solutions with similar impedances obtained from 2.5 to 10 min of treatment. Surface evaluation by SEM showed that the porosities in the anodic layer were not sealed for the periods of hydrothermal treatments corresponding to 2.5 min. The presence of cerium in hydrothermal treatment had a beneficial effect on the stability of the anodic layer formed and provided a healing effect on the corroding sites.

Abstract: New treatments for replacement of chromate require lower toxicity and corrosion protection. This study aims to investigate the influence of the combination of a Ce conversion coating (CCCe) with glycol molecules on the corrosion resistance of the AA2024-T3 clad (AA1230). The corrosion resistance of surface treated and untreated samples was evaluated by electrochemical techniques (electrochemical impedance spectroscopy, polarization tests and open circuit potential). These tests were complemented by salt spray tests to accelerate the corrosive effects of weathering. The surfaces were analyzed after corrosion tests by scanning electron microscopy with X-ray energy dispersive detector (SEM - EDX). The results of the CCCe samples in combination with glycol were compared with that of the surface with chromate layer and the results showed that the CCCe treatment is a candidate for replacement of chromating with the advantage that it does not generate toxic residues. The self-healing capacity of the new treatment tested was indicated by the increased formation of corrosion products deposition on top of Fe rich intermetallis in the AA1230 clad with time of exposure to the electrolyte.

Abstract: 2XXX and 7XXX high strength aluminum alloys are the most used materials for structural parts of aircrafts due to their high strength/weight ratio. Their joining procedure is an engineering challenge since they present low weldability. Friction Stir Welding (FSW) is a joining technology developed in the early 90 ́s. It is a solid-state welding process, without the use of fillers or gas shield, that eliminates conventional welding defects and has been considered of great interest for application in the aircraft industry. FSW of aluminum alloys results in four regions of different microstructures, specifically: the base material (BM), the heat affected zone (HAZ), the thermo-mechanically affected zone (TMAZ), and the nugget zone (NZ). The complex microstructure of the weld region leads to higher susceptibility to localized corrosion as compared to the BM even when similar alloys are joined. The welding of dissimilar alloys in its turn results in even more complex microstructures as materials with intrinsically different composition, microstructures and electrochemical properties are put in close contact. Despite the great interest in FSW, up to now, only few corrosion studies have been carried out for characterization of the corrosion resistance of dissimilar Al alloys welded by FSW. The aim of this study is to investigate the corrosion behavior of aluminum alloy 2024-T3 (AA2024-T3) welded to aluminum alloy 7475-T761 (AA7475-T761) by FSW. The evaluation was performed in 0.01 mol.L^-1 by means of open circuit potential measurements, polarization techniques and surface observation after corrosion tests.

Abstract: Stainless steel (SS) powders are used in the preparation of sintered SS products. One of the applications of sintered SS products is as filters in the petrochemical and food processing industries. In these industries, the SS filters are subject to severe conditions associated with the removal of solid particles from the fluid. Hence, SS filters should have adequate mechanical strength and high corrosion resistance. Welding can be used to manufacture SS filters. In this study, sintered AISI 316L specimens were welded using the TIG (Tungsten Inert Gas) process. The weld joints were examined by optical microscopy and by scanning electron microscopy. Electrochemical polarization measurements were carried out to evaluate the influence of welding on the corrosion resistance of sintered filters.

Abstract: - Ferromagnetic stainless steels (SS) produced by powder metallurgy (PM) techniques have been investigated as potential candidates for dental prosthesis applications in replacement of magnetic attachments made of noble and expensive alloys. Two SS were investigated: SS 17-4 PH produced by powder injection (PIM) and SS PM2000 obtained by mechanical alloying. In vitro cytotoxicity analysis of the two SS showed no cytotoxic effects. The magnetic retention force of both tested SS was also evaluated and they were comparable to noble commercially available material that is in use at the moment. The corrosion resistance of both SS was evaluated by electrochemical techniques in sodium phosphate buffer solution (PBS) at 37°C. The AISI 316L SS was also tested under the same conditions for comparison reasons. SS samples tested showed passive behaviour in the electrolyte, but they also presented susceptibility to pitting. The best pitting resistance was associated to the PM2000 whereas the 17-4PH PIM showed the highest pitting susceptibility among the tested steels. The results pointed out that the PM2000 SS might be considered a potential candidate for substitution of high cost magnetic alloys used in dental prosthesis.

Abstract: Bipolar plates play main functions in PEM fuel cells, accounting for the most part of the weight and cost of these devices. Powder metallurgy may be an interesting manufacturing process of these components owing to the production of large scale, complex near-net shape parts. However, corrosion processes are a major concern due to the increase of the passive film thickness on the metal surface, lowering the power output of the fuel cell. In this work, the corrosion resistance of PIM AISI 316L stainless steel specimens was evaluated in 1M H2SO4 + 2 ppm HF solution at room temperature during 30 days of immersion. The electrochemical measurements comprised potentiodynamic polarization and electrochemical impedance spectroscopy. The surface morphology of the specimens was observed before and after the corrosion tests through SEM images. The material presented low corrosion current density suggesting that it is suitable to operate in the PEM fuel cell environment.

Abstract: The hydrogenation decrepitation process was employed to produce sintered magnets using a mixture of two alloys. The effect of niobium and boron content on the magnetic properties of Pr14FebalCo16BxNby-type sintered magnets was studied. Niobium and boron have a significant effect on the magnetic behavior of these permanent magnets. The optimum amount of boron was 6 at% and niobium should be kept below to 0.50 at%. The squareness factor (0.90) has been improved considerably and good overall magnetic properties (Br=1320 mT, iHc=700 kAm-1 and (BH)max=315 kJm-3) have been achieved for the sintered magnets prepared from the Pr14Fe63.85Co16B6Nb0.15 alloy.

Abstract: In this work, porous titanium samples processed by powder metallurgy and coated with biomimetic coatings, obtained during different periods of immersion in a simulated body fluid (SBF), were tested for corrosion resistance in a phosphate buffer solution (PBS). Uncoated samples were also tested for comparison. The corrosion resistance of both types of titanium samples was evaluated by electrochemical impedance spectroscopy and potentiodynamic polarisation curves. The electrochemical results indicated the formation of a surface film on the porous Ti samples with immersion in the SBF solution and this biomimetic film increased their corrosion resistance. This film helps osteointegration besides increasing corrosion resistance.