Resumo A precipitação de fases intermetálicas nos aços inoxidáveis duplex constitui um problema no que diz respeito à utilização destes materiais em determinadas faixas de temperatura, desta forma, fazem-se necessários métodos de quantificar, de forma não destrutiva, a presença de alterações microestruturais decorrentes de aporte térmico. Neste trabalho, ensaios eletroquímicos de voltametria linear, de tração e de microdureza Vickers foram realizados no aço inoxidável duplex UNS S31803 para avaliação da presença da fase intermetálica sigma na microestrutura do material. Amostras deste aço foram envelhecidas isotermicamente a 870 °C, em tempos de até 3600 segundos para induzir a precipitação de diferentes teores desta fase. Os ensaios de voltametria linear foram conduzidos em solução eletrolítica de 5% KOH, em uma célula eletroquímica de 3,3.10–6 m2 de área, com eletrodo de referência do tipo Calomelano Saturado e eletrodo auxiliar de fio de platina. Os resultados desta técnica apresentaram sensibilidade para detectar teores de fase sigma maior que 1% no material viabilizando sua aplicação como um Ensaio Não Destrutivo (END).

Abstract Precipitation of intermetallic phases in duplex stainless steels constitute a problem regarding the use of such materials in certain temperature ranges, in this way, are necessary methods to quantify, non-destructively, the presence of microstructural changes due to thermal contribution. In this work, electrochemical tests of linear voltammetry, tensile and Vickers hardness were performed in duplex stainless steel UNS S31803 for evaluation of the intermetallic sigma phase presence in the microstructure of the material. Samples of this steel were isothermally aged at 870 °C for times up to 3600 seconds to induce the precipitation of different grades of this phase. Linear voltametry tests were conducted in 5% KOH electrolyte solution in an electrochemical cell 3.3.10-6 m2, with a reference electrode of the type Saturated Calomel and auxiliary electrode of platinum wire. The results of this technique showed sensitivity to detect sigma phase content greater than 1% in the material enabling its application as a Non-Destructive Testing (NDT).

To improve the implants biocompatibility many surface modifications were proposed. Investigations about the surface modification on Ti alloys by anodic oxidation are reported. This research presents a study on the stability of thin titanium dioxide grown by potentiodynamic method on Ti6Al4V surfaces up to 5.0 V. Its bioactive surface in phosphate buffer solution (PBS) and the oxide stability after immersion in artificial blood media were measured by Electrochemical Impedance Spectroscopy (EIS). Hydroxyapatite (HAP) presence was evaluated using simulated body fluid (SBF) with different immersion times. The oxides and HAP presence were analyzed by Scanning Electron Microscopy (SEM) and X-ray Photoelectron Spectroscopy (XPS). The oxide stability was confirmed with low dissolution rates where the Rp was around 106Ω.cm2. The results showed the TiO2 was compact and thin oxide that could prevent the severe corrosion processes and improve in few days the physical-chemical interaction of the Ti alloys with bone in physiological media.

Ensaios de tração a quente e fluência foram realizados na liga Kanthal A1 na faixa de temperatura de 600º a 800ºC. Cada um desses conjuntos de dados foram analisados separadamente de acordo com suas próprias metodologias, mas, neste trabalho, uma tentativa foi realizada para se estabelecer uma correlação entre os mesmos. Um novo critério proposto para converter dados de tração para fluência torna possível a análise conjunta desses dois grupos de resultados de acordo com relações usuais de fluência como: Norton, Monkman-Grant, Larson-Miller e outros. A notável compatibilidade entre os dois conjuntos por esse procedimento sugere que dados de tração a quente podem ser convertidos para dados de fluência para a liga Kanthal A1, como verificado anteriormente para outros materiais metálicos.

Hot tensile and creep tests were carried out on Kanthal A1 alloy in the temperature range from 600 to 800ºC. Each of these sets of data were analyzed separately according to their own methodologies, but an attempt was made to find a correlation between them. A new criterion proposed for converting hot tensile data to creep data, makes possible the analysis of the two kinds of results according to usual creep relations like: Norton, Monkman-Grant, Larson-Miller and others. The remarkable compatibility verified between both sets of data by this procedure strongly suggests that hot tensile data can be converted to creep data and vice-versa for Kanthal A1 alloy, as verified previously for other metallic materials.

This work refers to the application of a Voltammetric Anodic Dissolution (VAD) Technique in the analysis of coating discontinuities, focusing on pores and cracks that exposed the substrate. An evaluation was made of the influence of several parameters, such as the concentration of the passivation solution and sweep rate (SR), on the substrate passivation process and on the porosity indexes of tricationic phosphate coatings of Fe/Zn/Mn. The phosphatization process used was a commercial tricationic Fe/Zn/Mn phosphate bath applied on a galvanized steel (GS) substrate. Once the best experimental conditions for the use of the VAD technique had been defined, the grain size and layer weight were related to porosity indexes. The porosity was found to show a tendency to decrease with increasing grain size. The VAD technique consists of the anodic polarization of the substrate/coating system and measurement of the charge density involved in the substrate passivation process. A quantitative porosity index was obtained by comparing the passivation charge density of the substrate without coating (standard passivation charge density) and the passivation charge of the coated substrate.