A Survey on the Oxidation Behavior of a Nickel-Based Alloy Used in Natural Gas Engine Exhaust Valve Seats

This study reports the oxidation behavior of a Ni-based alloy used in the manufacture of valve seats for automotive engine exhaust systems. Isothermal thermogravimetric analyses were carried out at temperatures of 660, 740, 860, and 900 °C under an oxygen atmosphere for up to 1 h. At 660 and 740 °C, only one stage was observed during the whole time studied. At this stage, the oxide layer was formed mainly by NiO + Cr2O3, following a linear oxidation law with a rate constant (Kl) on the order of magnitude of 10−6 kg/m2s and an apparent activation energy (Ea) of ~47 kJ/mol. At 860 and 900 °C, an identical first stage was observed with a transition to a different stage. In the second stage, the oxidation layer was composed of Cr2O3, and a parabolic oxidation law was followed with a rate constant (Kp) on the order of 10−8 kg2/m4s and Ea of ~128 kJ/mol. Moreover, the Ni-based alloy formed a dense and compact oxide layer after oxidation, with no apparent cavities, pores, or microcracks. Characterization techniques such as Scanning Electron Microscopy (SEM), Energy-Dispersive X-ray Spectroscopy (EDS), Fourier Transform Infrared Spectroscopy (FTIR), and Raman Spectroscopy were carried out to characterize the formed oxide layer

Effect of secondary phase precipitation on the corrosion resistance of UNS N26455 superalloy

The UNS N26455 is a superalloy of the Ni-Cr-Mo system used on offshore platforms as components of injection pumps. This alloy has low-carbon content and high levels of Cr and Mo. This composition gives the material high resistance to various forms of corrosion in different types of environments. However, in some aggressive conditions containing high concentrations of chloride, high temperatures and presence of corrosive gases, the alloy UNS N26455 may show localized corrosion. As an aggravating factor, prolonged exposure of this alloy at high temperatures may favor the preferential precipitation of intermetallic phase P. This precipitate is rich in Cr and Mo and changes the composition of the alloy in the matrix/precipitate interface, and in turn promotes localized corrosion of the material. In this work, the effect of P phase precipitation in localized corrosion resistance was studied using electrochemical techniques: anodic potentiodynamic polarization, cyclic polarization, potentiostatic technique, crevice repassivation potential (THE), electrochemical impedance spectroscopy (EIS) and Mott-Schottky (MS). The experiments were carried out in environments containing high concentrations of chloride ions (100.000 e 180.000 ppm), with and without CO2 saturation, and at temperatures of 25 °C, 40 °C and 75 °C. The potentiodynamic results indicated that the alloy UNS N26455 in as-cast condition is extremely resistant to pitting corrosion. On the other hand, increasing the concentration of precipitates in the microstructure of the alloy, active dissolution occurs in potentials above 250 mVAg/AgCl. After potentiostatic testing and analysis by SEM, it was found that the dissolution occurs at the interface matrix/precipitate. The repassivation potential is not affected by the concentration of P phase, however, the weight loss obtained in the crevice corrosion test increased with the addition of the second phase in the microstructure of the alloy. The EIS results showed that the polarization resistance decreases with increasing P precipitates, indicating changes in the characteristics of the passive viii film. MS experiments showed that the passive film formed on alloy UNS N26455 shows n-type semiconducting behavior and the donor density increases with increasing concentration of the second phase.Financiadora de Estudos e ProjetosA liga UNS N26455 é uma liga do sistema Ni-Cr-Mo que é utilizada em componentes de bombas de injeção de plataformas offshore . Esta liga possui baixo teor de C e teores elevados de Cr e Mo. Esta composição confere ao material elevada resistência a várias formas de corrosão em diferentes tipos de ambientes. No entanto, em algumas condições agressivas contendo altas concentrações de cloreto, elevadas temperaturas e presença de gases corrosivos, a liga UNS N26455 pode sofrer corrosão localizada. Como agravante, a exposição prolongada deste material em altas temperaturas pode favorecer a precipitação preferencial da fase intermetálica P. Este precipitado é rico em Cr e Mo e modifica a composição da liga na região de interface matriz/precipitado, e por sua vez favorece a corrosão localizada do material. Neste trabalho, o efeito da precipitação da fase P na resistência à corrosão localizada foi estudado utilizando as técnicas eletroquímicas: polarização anódica potenciodinâmica, polarização potenciodinâmica cíclica, técnica potenciostática, potencial de repassivação de frestas (THE), espectroscopia de impedância eletroquímica (EIS) e Mott-Schottky (MS). Os ensaios foram realizados em soluções contendo altas concentrações de íons cloreto (100.000 e 180.000 ppm), na condição saturada em CO2 e sem saturação, e nas temperaturas de 25 °C, 40 °C e 75 °C. Os resultados potenciodinâmicos indicaram que a liga UNS N26455 na condição bruta de fusão é extremamente resistente à corrosão localizada. Por outro lado, com o aumento da concentração de precipitados na microestrutura da liga, ocorre a dissolução ativa da fase P em potenciais acima de 250 mVAg/AgCl. Após ensaios potenciostáticos e análise por MEV, verificou-se que a dissolução da fase P ocorre na interface matriz/precipitado. O potencial de repassivação não é afetado pela concentração de fase P, no entanto, a perda de massa obtida nos ensaios de corrosão por fresta aumentou com o aumento de segunda fase na microestrutura da liga. Os resultados de EIS mostraram que a resistência à polarização diminui com o aumento de precipitados P, indicando modificações nas características do filme passivo. Os ensaios de MS mostraram que o filme passivo formado na liga UNS N26455 nas condições estudadas apresenta comportamento semicondutor tipo-n e a densidade de doadores aumenta com o aumento da concentração de segunda fase

GLASSPANACEA: UM SOFTWARE EFICIENTE PARA A FORMULAÇÃO DE MATERIAIS CERÂMICOS

GlassPanacea is an efficient software tool that combines several attractive technical features with ease of use. Its configuration leads to the intuitive handling and learning with accurate results, providing the users with flexibility in the selection of suitable chemicals for the formulation of glassy, partially glassy or crystalline ceramic materials, as well as speed and accuracy in the calculation of the relative proportions of each chemical in a batch. The software runs directly from an executable file with multiplatform support. Hence, it can be used on different operating systems, such as Windows, Linux and Mac OS, without installation. One of its highlights is the user-friendly interface that enables immediate application, even for operators with little computer experience. This makes GlassPanacea a very valuable tool for students, researchers and engineers who work on the development of ceramic materials using different synthesis techniques, such as melting, solid-state reaction, sintering and sol-gel processing. The archive containing the software, information for use and logo can be downloaded, free of charge, from http://www.certev.ufscar.br/research-1/glasspanacea-glass-and-ceramic-formulation-software

GLASS·PANACEA: AN EFFICIENT SOFTWARE FOR THE FORMULATION OF CERAMIC MATERIALS

<p></p><p>GlassPanacea is an efficient software tool that combines several attractive technical features with ease of use. Its configuration leads to the intuitive handling and learning with accurate results, providing the users with flexibility in the selection of suitable chemicals for the formulation of glassy, partially glassy or crystalline ceramic materials, as well as speed and accuracy in the calculation of the relative proportions of each chemical in a batch. The software runs directly from an executable file with multiplatform support. Hence, it can be used on different operating systems, such as Windows, Linux and Mac OS, without installation. One of its highlights is the user-friendly interface that enables immediate application, even for operators with little computer experience. This makes GlassPanacea a very valuable tool for students, researchers and engineers who work on the development of ceramic materials using different synthesis techniques, such as melting, solid-state reaction, sintering and sol-gel processing. The archive containing the software, information for use and logo can be downloaded, free of charge, from http://www.certev.ufscar.br/research-1/glasspanacea-glass-and-ceramic-formulation-software.</p><p></p

A Survey on the Oxidation Behavior of a Nickel-Based Alloy Used in Natural Gas Engine Exhaust Valve Seats

This study reports the oxidation behavior of a Ni-based alloy used in the manufacture of valve seats for automotive engine exhaust systems. Isothermal thermogravimetric analyses were carried out at temperatures of 660, 740, 860, and 900 °C under an oxygen atmosphere for up to 1 h. At 660 and 740 °C, only one stage was observed during the whole time studied. At this stage, the oxide layer was formed mainly by NiO + Cr2O3, following a linear oxidation law with a rate constant (Kl) on the order of magnitude of 10−6 kg/m2s and an apparent activation energy (Ea) of ~47 kJ/mol. At 860 and 900 °C, an identical first stage was observed with a transition to a different stage. In the second stage, the oxidation layer was composed of Cr2O3, and a parabolic oxidation law was followed with a rate constant (Kp) on the order of 10−8 kg2/m4s and Ea of ~128 kJ/mol. Moreover, the Ni-based alloy formed a dense and compact oxide layer after oxidation, with no apparent cavities, pores, or microcracks. Characterization techniques such as Scanning Electron Microscopy (SEM), Energy-Dispersive X-ray Spectroscopy (EDS), Fourier Transform Infrared Spectroscopy (FTIR), and Raman Spectroscopy were carried out to characterize the formed oxide layer

A Simple and Complete Supercapacitor Characterization System Using a Programmable Sourcemeter

The development of new materials and systems capable of storing energy efficiently with a fast power delivering has been the subject of several studies. Many techniques and instruments are used for the characterization of these systems. Some involve the use of electrochemical techniques, such as cyclic voltammetry and electrochemical impedance spectroscopy, while others use instruments specially developed for this purpose. In this study, we provide a simple and functional supercapacitor characterization system using a programmable sourcemeter with an embedded scripting language. For the validation of the system, commercial capacitors and supercapacitors devices based on activated carbon and manganese dioxide were used. In a few steps, the system is capable of efficiently determine the main parameters used in the characterization of supercapacitors, for instance, specific capacitance, specific energy density, specific power density and equivalent series resistance. From the data collected, the system can also determine the stability and performance of supercapacitors, which are fundamental parameters used in the development of new electrodes for energy storage devices. DOI: http://dx.doi.org/10.17807/orbital.v11i2.1374 <br /

Corrosion Behavior of CW6MC Nickel Cast Alloy (Inconel 625) Welded by Shielded Metal Arc Welding

The aim of this study concerns the effect of multi-pass shielded metal arc welding (SMAW) on the corrosion behavior of CW6MC cast nickel alloy. Using optical and SEM techniques the welded joint is analyzed. Vickers microhardness mapping and potentiodynamic polarization in NaCl and H2SO4 solutions are also evaluated. Both the Laves phase and NbC-type carbides are identified in the base metal (BM) and weld metal (WM) regions. The main microstructural difference observed between these regions is the morphology aspect and fineness of the dendritic arrays. The welding process promotes the finer columnar grains formation with refined intermetallic particles in the WM than equiaxed grains of the BM, which in turn results in higher microhardness values in the former region. However, no substantial changes were observed in the corrosion behavior between the BM and WM regions, considering both acid and saline media. Nevertheless, during the multi-pass SMAW process, some non-metallic micrometric inclusions (Mo and S-rich regions) can be constituted to occur in the WM region. This is associated with a significant drop in the corrosion performance of this region when the electrochemical tests are evaluated