Estudo do eletropolimento galvanostático de aço inoxidável AISI 304 utilizando soluções ácidas concentradas

• Ano de defesa: 2011
• Autor(a) principal: Veroli, Alyne Bernardes
• Orientador(a): Bocchi, Nerilso
• Tipo de documento: Dissertação
• Tipo de acesso: Acesso aberto
• Idioma: por
• Instituição de defesa: Universidade Federal de São Carlos
• Programa de Pós-Graduação: Programa de Pós-Graduação em Química - PPGQ
• País: BR
• Palavras-chave em Português: Química; Planejamento experimental; Otimização
• Área do conhecimento CNPq: CIENCIAS EXATAS E DA TERRA::QUIMICA
• Link de acesso: https://repositorio.ufscar.br/handle/ufscar/6511

Stainless steel has been the subject of numerous studies due to technological and economic importance. Features such as durability, corrosion and high temperatures resistances confer to the steel relevant applications in several industrial sectors. Thus, the study of methodologies for chemical and electrochemical polishing, instead of mechanical polishing, has been increasing due to improved surface quality of these steels by not introducing contaminants and/or tension in the superficial layers of the metal, making them microscopically smooth, passive and anticorrosive. Therefore, in this work, the process of electrochemical polishing of AISI 304 stainless steel in solutions of concentrated sulfuric acid (H2SO4 also used in the coloring process of these steels) and phosphoric-sulfuric mixed acids (H3PO4 and H2SO4 2:1) containing glycerol was studied using the galvanostatic method (constant current). Factorial design was used to optimize the main process parameters such as concentration of the acid solution, current density, temperature, electrolysis time and additive content. The surface quality of steel samples was evaluated by spectral reflectance. Some steel samples electropolished or not (steel samples as received) were also characterized by atomic force microscopy (AFM) and scanning electron microscopy (SEM). The statistical model generated for the electropolishing process of steel in solution of H2SO4 did not fit well (R2 = 0.75) to the experimental results. Among all the electrolysis, the one that produced a steel surface with the highest percentage value of reflectance (R = 71%) was performed in the following experimental conditions: [H2SO4] = 9 mol L-1, i = 0.6 A cm-2, θ = 55 ° C and t = 7.5 min. In these conditions the corrosion rate of steel samples was 0.33 mm h-1 and the process energy consumption was 3.0 kW h m-2 for an average cell potential of 4.0 V. On the other hand, the statistical model generated for the electropolishing process of steel in solution of H3PO4 and H2SO4 2:1 containing glycerol fitted reasonable (R2 = 0.93) to the experimental results. The punctual optimal conditions were: H2SO4 and H3PO4 2:1 containing 25% glycerol and kept at 30 ° C, i = 1.0 A cm-2 and t = 8 min. In these conditions, the surfaces of the steel samples showed R values of 70 %. The corrosion rate of steel samples was 0.19 mm h-1 and the process energy consumption was 23 kW h m-2 for an average cell potential of 17 V. From AFM images, the calculated values of average surface roughness factor (Ra) were 139 ± 20 nm for steel samples as received (R = 58%) and between 20 and 75 nm for steel samples electropolished. The SEM images for these latter samples revealed the different crystallographic orientations of the AISI 304 austenitic stainless steel.