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.

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.

The superduplex stainless steels (SDSS) are widely used in chemical, oil and gas industries, to pipelines and storage material facilities. In welding process or working in temperature elevated, secondary phases may appear in the form of precipitates, as the sigma phase (σ) which is an intermetallic compound. This compound is harmful to the properties of steel, deteriorating its mechanical properties, such as decreasing corrosion resistance and toughness. In this paper it is analyzed the formation, kinetics and microstructural evolution of sigma phase in SDSS UNS S32750 after isothermal aging at 700ºC, 750ºC and 800ºC. In this work sigma phase kinetics is studied by JMAK theory and by two microstructural path descriptors, SV, interfacial area per unit of volume between sigma phase and austenite, and <λ>, mean chord length of sigma, both in function of the VV, volumetric fraction of sigma, known in the literature as microstructural partial path (MP). The MP formulation is common in recrystallization studies, but so far has not been used in the sigma phase precipitation studies, being applied here for the first time. The results indicated that the sigma phase nucleates by site saturation with anisotropic linear impingement. This means that sigma phase nucleates on edges.