Papers by Author: Olandir Vercino Correa

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: The aim of the present work was to study the influence of beta phase precipitates content and distribution on the corrosion behavior of the AZ91D magnesium alloy, using samples subjected to solution annealing and aging. The morphology of the beta phase was observed using optical microscopy and scanning electron microscopy. The volume fraction of each phase was also determined. Potentiodynamic polarization tests and immersion tests were carried out, indicating an increase of the corrosion resistance with the volume fraction of the beta phase and its distribution on the grain boundaries of the alpha-phase. Solution annealed samples were more susceptible to corrosion.

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: In most engineering fields there is increasing demand for materials with enhanced physical properties. Components of complex systems exposed to severe environments often require coatings for adequate thermal insulation and to prevent wear, erosion and/or oxidation. Coatings prepared with nanostructured material have, in general, exhibited higher hardness and strength compared to conventional coatings. This paper presents the microstructure, mechanical properties, thermal stability and erosion-oxidation resistance of HVOF (High Velocity Oxygen Fuel) sprayed coatings of Cr3C2-X(Ni20Cr) prepared using nanocrystalline and as-received (AR) powder as feedstock. Details about nanocrystalline powder synthesis and characterization are also presented.

Abstract: Cr3C2-NiCr coatings have been used for corrosion and wear resistant applications. In the last decade, thermal sprayed coatings using nanostructured feed stock of other materials has exhibited higher hardness, strength and corrosion resistance. Hence, it is anticipated that nanostructured Cr3C2-NiCr coatings will also exhibit these properties and therefore posses improved performance characteristics. Preparation of nanostructured feed stock powders is the first step in the synthesis of nanostructured coatings and mechanical milling is an effective process to obtain the powders. Preliminary studies in which commercial Cr3C2-NiCr powders were milled in hexane and gaseous nitrogen revealed that particle and crystallite size were significantly smaller in powders milled in nitrogen. This paper presents the effect of hexane content and milling parameters on Cr3C2-Ni20Cr powder characteristics. Use of just sufficient hexane as the milling media reduced significantly the particle and crystallite sizes.

Abstract: Particle reinforced metal matrix composite (MMC) coatings have been developed due to property combinations such as increased hardness, high creep/fatigue resistance as well as superior wear and oxidation resistance. MMC coatings can be produced by various techniques, which include powder metallurgy (plasma spray and HVOF processes), liquid metal processes and electrodeposition. This paper presents the development of the electrodeposition process to obtain nickel-alumina composites. The process conditions were optimized and based on these gradient nickel-alumina composites were prepared. Microstructural characterization and hardness determinations were carried out. The effect of process parameters on the amount and distribution of alumina particles as well as morphology of the deposits with and without alumina gradients are presented and discussed.

Abstract: The erosion-oxidation (E-O) behavior of high velocity oxy fuel (HVOF) sprayed Ni20Cr alloy as well as WC and Cr3C2 cermet coatings on a steel substrate were studied. The E-O tests were carried out in a rig with specimen assemblies that were rotated through a fluidized bed of erodent particles in the temperature range 500-850°C and with erodent impact velocities of 2.5-19.5 ms-1. Alumina powder (~200Am) was used as the erodent. The E-O resistance of the coatings was determined as wastage, as a function of temperature. The three coatings did not exhibit any significant change in E-O at temperatures up to 500-600°C. At higher temperatures, wastage increased with temperature, reached a maximum at 700°C and then decreased with further increase in temperature. Different E-O regimes were identified. The specimen surfaces were examined in a scanning electron microscope and their roughness determined. E-O maps have been drawn that define conditions under which the coatings undergo low, moderate and severe wastage.

Abstract: This study investigates the in vitro corrosion and cytotoxicity response of AISI 316L stainless steel produced by powder injection molding (PIM) technology in a solution that simulates physiological fluids (MEM) by electrochemical techniques and neutral red uptake cytotoxicity assay. The results were compared with those of AISI 316L produced by conventional metallurgy. Both steels showed high corrosion resistance and no toxic effect in the cytotoxicity test. The corrosion products were analyzed by instrumental neutron activation analysis (INAA). The surfaces of the alloys were evaluated before and after corrosion test by scanning electron microscopy and a passive behaviour was indicated supporting the results from other techniques.