Abstract Mineral coal is a fuel that can be used by thermoelectric plants to generate electricity, however, the solid residues of its combustion, so-called ash, cause erosion and corrosion in the internal components of boilers that operate at high temperatures, causing failures in the system. This study proposes a metallic coating based on Fe-Cr-Si, obtained using the electric arc thermal spray technique as an alternative for protecting the most affected parts of boiler tubes by the ash impact. Optical microscopy (OM), scanning electron microscopy (SEM), semi-quantitative chemical analysis by Energy Dispersive Spectroscopy (EDS), X-ray diffraction (XRD), roughness measurements, pull-off adhesion tests were used. Thermal diffusivity measurements and mathematical modelling were elaborated to evaluate the thermal exchange efficiency impact of the coating in thermoelectric plant boiler tubes. The results showed that the chemical components of the sprayed wire were well incorporated into a coating with a hardness of 730.54 ± 164.28 HV, approximately three times greater than the hardness of boiler pipes and only 11% less than the hardness of ash. XRD identified the presence of crystalline phases in the coating and adhesion tests showed that the failures were cohesive without substrate exposure. The presence of the coating reduced heat exchange of approximately 0.64% / tube length. These results showed that the proposed coating, which has a lower cost compared to others, be a promising solution to avoid unscheduled shutdowns in thermoelectric boilers.

Abstratc This work reports the production of low-tin Cu-Sn alloy coatings on carbon steel substrates using a bath containing CuCl2, SnCl2, and sodium citrate. In the first part of this study, the coatings were electrodeposited by direct and simple pulse current processes (DC and SPC, respectively). Different current density values were used, while the pulse frequency and duty cycle remained constant. Independent of the current mode used, low-tin Cu-Sn coatings, showing globular surface morphology and Cu6Sn5 as the main compositional phase, were produced. Both the current mode and the applied current density affected the anticorrosive properties of the coatings. The most protective DC and SPC coatings, showing Sn content < 3 wt.% and compact morphology, were prepared using j = 80 A m-2 and jc = 167 A m-2, respectively. High charge transfer resistance values were verified even after immersion for 24 h in 0.5 mol L-1 NaCl solution.

Resumo A Aspersão Térmica (ASP) é um processo para fabricação de revestimentos metálicos de baixo custo e alta produtividade. Esses revestimentos são usados em tubos de trocadores de calor para promover boa resistência a corrosão e ao desgaste. Este trabalho avaliou um revestimento metálico Fe-Cr-Nb-Ni-B depositado por ASP, nas condições sem selante e com selante à base de resina de silicone e alumínio, na proteção de tubos de aço quanto à corrosão e desgaste por atrito de partículas no ambiente de uma caldeira para geração de vapor d`água. Selantes ou esquemas de pintura, são soluções comumente usadas para melhorar a resistência à corrosão. As situações testadas expuseram o revestimento e substrato de amostras com e sem selante, em uma caldeira durante sete e doze meses. Utilizou-se técnicas de metalografia, microscopia eletrônica de varredura, análise química por espectroscopia de energia dispersiva e análise e processamento digital de imagens. Os resultados mostraram que embora o selante tenha sido removido do revestimento pelo atrito do fluxo de partículas presentes na água em operação na caldeira, o revestimento metálico manteve-se integro em relação à sua aderência ao substrato e camada de espessura depositada, sendo considerado uma solução promissora para a proteção de tubulações de caldeiras.

Abstract The Arc Spray Process (ASP) is a low-cost and high-productivity process for making metallic coatings. These coatings are used in heat exchanger pipes to promote good corrosion and wear resistance. This work evaluated a metallic coating of Fe-Cr-Nb-Ni-B alloy deposited by ASP to protect carbon steel pipes boiler to generate steam. Were considered the coating performance about corrosion resistance and the wear due to particles' friction. Two conditions were tested, coating with sealant based on silicone resin with aluminum and without sealant. Sealants or paint layers are commonly used to improve coatings corrosion resistance. Two samples for each condition tested remained in the boiler environment for seven and twelve months. Metallographic preparation techniques, Scanning Electron Microscopy, chemical analysis by Energy Dispersive Spectroscopy, and Digital Image Analysis and Processing were used. The results showed that sealant was removed from the coating by the friction of the flow of particles present in the boiler's water operation. Nevertheless, the metallic coating remained intact in its adhesion to the substrate and layer of deposited thickness. The proposed solution was considered a promising solution for protecting steel pipes of boilers.

RESUMO Este trabalho teve como objetivo quantificar e avaliar através de técnicas de análise e processamento digital de imagens (PDI) defeitos formados em revestimentos metálicos à base de ligas ferro-cromo e ferro-cobalto, obtidas por aspersão térmica à arco elétrico (AT). Imagens obtidas por microscopia óptica (MO) foram processadas e analisadas utilizando-se o software FIJI (Image J) para quantificação de poros, redes de óxidos e trincas. Também foram feitas medidas de microdureza Vickers (HV) e testes de corrosão segundo a norma ASTM-G48. Foram realizados dois testes de corrosão em meio de cloreto férrico: um expondo-se apenas o revestimento, onde a resistência a corrosão foi avaliada qualitativamente através de imagens de MO e a possível presença de pites de corrosão e outro expondo-se o conjunto revestimento e substrato, neste caso, a resistência a corrosão foi avaliada quantitativamente através de medidas na espessura da camada de revestimento antes e depois da exposição. Foi realizada uma avaliação microestrutural por MO e microscopia eletrônica de varredura (MEV). Os resultados obtidos por MO, MEV e microdureza indicaram uniformidade nas camadas depositadas e porcentagem de defeitos variando entre 3,7% e 4,3% para os revestimentos estudados. Todos mostraram possuir excelente resistência à corrosão em meio de cloreto férrico, que é considerado agressivo aos aços inoxidáveis, exceto para a liga de revestimento à base de ferro-cobalto quando encontrou-se combinada ao cobre e com o substrato também exposto ao meio corrosivo

ABSTRACT The aim of this work was to quantify and evaluate, through techniques of analysis and digital image processing (PDI), defects formed in metallic coatings based on ferro-nickel, iron-chromium and cobalt-chromium alloys obtained by thermal arc-electric spraying. Images obtained by optical microscopy (OM) were processed and analyzed using the FIJI software (Image J) for quantification of pores, oxide networks and cracks. Measurements of Vickers microhardness (HV) and corrosion tests according to the ASTM-G48. Two corrosion tests were carried out in ferric chloride medium: one exposing only the coating, where the corrosion resistance was evaluated qualitatively through OM images and the possible presence of corrosion pits and the other exposing the coating and substrate set, in this case, the corrosion resistance was quantitatively evaluated through measurements on the thickness of the coating layer before and after exposure. An assessment of the microstructural morphology by optical microscope (OM) and scanning electron microscope (SEM) was made. The results obtained by OM and SEM indicated that the coatings showed uniformity throughout the deposited layer and low number of defects (oxides, cracks and porosities). The percentage of defects found varied between 3.7% and 4.3% for the coatings studied. All showed excellent resistance to corrosion in ferric chloride, which is considered aggressive to stainless steels, except for the cobalt-based coating alloy when it was combined with copper and with the substrate also exposed to the corrosive environment.

Abstract: In the present work we have studied the effects of nitrogen addition on the equilibrium of the volume fraction ratio austenite/ferrite and on the structural properties of weldments produced by gas tungsten arc welding (GTAW). The welded joints in tubes UNS S32707 of hyper-duplex steel were produced using argon and nitrogen gas welding, with nitrogen added in ratios of 1.5, 2.5, 3.5, 4.5, and 5.5%. Microstructural characterization of this material under the different processing condition was conducted by means of optical microscopy, scanning electron microscopy, local chemical analysis by X-ray energy dispersive spectroscopy, electron backscatter diffraction Vickers microhardness test, and digital image processing. All welding conditions resulted in welded joints with equiaxial grains microstructure containing austenite both at the boundaries and in the ferrite matrix. Microhardness measurements did not show significant variation in relation to the base metal. The use of welding gases with higher percentages of nitrogen resulted in increases in the austenite volume fraction in the order of 39 to 57% when considering the nitrogen content ranging from 1.5 to 5.5%.

Geopolymers are exclusively mineral nature and are considered an alternative to materials based on Portland clinker, whose production accounts for about 5% of anthropogenic CO2 emissions in the world. The geopolymer cement concrete (GCC) may be prepared from natural oxide-aluminosilicates such as metakaolin (MK), or synthetic, such as fly ash (FA) together with active silica contained in the rice husk ash (RHA). The fracture properties of the Portland cement concrete (PCC) with 25 MPa and 50 MPa, and of three different geopolymeric concretes with the same strength Classes were determined for comparative analysis. The aim of this study is to provide support to begin the use of geopolymers in the reinforced concrete precasting Industry. Three-point bending tests of notched beams with a/d (notch depth/beam depth) of 0.5 from RILEM TC80-FMT Recommendations were used to determine the critical values of K, G, R and J-integral for crack propagation under mode I. The results showed that the geopolymeric concretes exhibit similar mechanical behavior and fracture properties higher that those determined in PCC for the same strength class.