The pitting corrosion of supermartensitic stainless steel (SMSS) was studied in an aqueous solution of 0.1 mol/L NaCl using the scanning vibrating electrode technique (SVET) and cyclic voltammetry. The SMSS showed 0.9% of retained austenite, as verified by Mössbauer spectroscopy. Cuboidal TiN particles were identified on the steel surfaces, by using optical, scanning electron, and transmission microscopy, associated to Mg-Ca-Al rich precipitates. Pit nucleation was slow and only verified after 24 h, associated to TiN/CaO·MgO·Al2O3 complex inclusions, thus on original interdendritic spaces, where lower Cr content and the presence of inclusions are expected.

ABSTRACT Austenitic stainless steel and nickel-base alloys welds are widely used in nuclear reactor components, plants of energy generation, chemical, and petrochemical industries, due to their high corrosion resistance. The post weld heat treatments (PWHT) are generally applied to welding in order to relieve the welding residual stress. The aim of this work was to evaluate the influence of different PWHT on corrosion behavior of a dissimilar weld joint of two AISI 316L austenitic stainless steel plates with a nickel-base alloy as filler material in saline environments. The material was submitted to heat treatments for three hours at 600, 700 and 800 °C. The weld joint was examined by optical microscopy to determine the effects of PWHT in the microstructure. The corrosion behavior of the samples before and after heat treatment was evaluated using cyclic potentiodynamic polarization (CPP) in sodium chloride solutions (19% v/v) and pH 4.0 at room temperature. Metallographic analyses showed that delta ferrite dissolute as PWHT temperature increased. CPP curves demonstrated an increase of pitting corrosion resistance as the PWHT temperature rises, although the pit size has been increased. The heat treated weld joint at 600 °C showed a similar corrosion resistance compared to as-welded material.

Resumo Neste estudo, a microestrutura e a resistência à corrosão intergranular de aços inoxidáveis ferríticos com adições de Nb, Ti e (Ti+Nb) submetidos à soldagem TIG foram investigadas. A caracterização estrutural dos cordões de solda foi realizada através de microscopias óptica (MO) e eletrônica de varredura (MEV) e espectroscopia por energia dispersiva (EDS). A susceptibilidade à sensitização foi avaliada qualitativamente através da Prática W da norma ASTM 763 e quantitativamente através da técnica DL-EPR. Observou-se uma rede de martensita nos contornos de grão na região da solda do aço não estabilizado e uma microestrutura completamente ferrítica nas amostras dos aços com adições de estabilizantes. A Prática W revelou corrosão generalizada no interior dos grãos do aço AISI 430 e a presença de valas no AISI 430 Ti e concomitantemente com a técnica DL-EPR constatou-se que esses aços foram sensitizados pela soldagem. Já as amostras dos aços com adições de Nb e (Ti+Nb) mantiveram boa resistência à corrosão, estando livres de sensitização após a soldagem.

Abstract In this study the microstructure and the resistance to intergranular corrosion of ferritic stainless steels with additions of Nb, Ti and (Ti+Nb) submitted to the TIG welding were investigated. The microstructure of the weld beads was characterized by optical (MO) and scanning electron (SEM) microscopies and by energy dispersive x-ray spectroscopy (EDS). The susceptibility to sensitization was assessed qualitatively by Practice W of ASTM 763 and quantitatively by the DL-EPR technique. Martensite was observed at the ferrite grain boundaries in the fusion and heat affected zones of the non-stabilized steel and a completely ferritic microstructure in the steels samples with addition of stabilizers. Practice W revealed general corrosion inside the grains of the AISI 430 steel and the presence of ditches in the AISI 430 Ti. Concomitantly, with the DL-EPR technique, it was found that the weld region of these steels was sensitized. On the other hand, the samples of the steels with Nb and (Ti + Nb) additions maintained good resistance to corrosion and presented no sensitization after welding.

Secondary caries is still the main cause of restoration replacement, especially on the root surface OBJECTIVE: This in vitro study evaluated the cariostatic effects of fluoride-containing restorative materials associated with fluoride gels, on root dentin. MATERIALS AND METHODS: A randomized complete block design was used to test the effects of the restorative systems, fluoride regimes and the interactions among them at different distances from restoration margins. Standardized cavities were prepared on 240 bovine root specimens and randomly assigned to 15 groups of treatments (n=16). Cavities were filled with the following restorative materials: Ketac-Fil (3M-ESPE); Vitremer (3M-ESPE); Dyract/Prime & Bond NT (Dentsply); Charisma/Gluma One Bond (Heraeus Kulzer) and the control, Z250/Single Bond (3M-ESPE). The specimens were subjected to a pH-cycling model designed to simulate high-caries activity. During the cycles, 1.23% acidulated phosphate fluoride, 2.0% neutral sodium fluoride or deionized/distilled water (control) was applied to the specimens for 4 min. The surface Knoop microhardness test was performed before (KHNi) and after (KHNf) the pH cycles at 100, 200 and 300 mm from the margins. Dentin microhardness loss was represented by the difference in initial and final values (KHNi - KHNf). Data were analyzed by Friedman's and Wilcoxon's tests, ANOVA and Tukey's test (α=5%). RESULTS: The interaction of restorative systems and topical treatments was not significant (p=0.102). Dentin microhardness loss was lowest closer to the restoration. Ketac-fil presented the highest cariostatic effect. Vitremer presented a moderate effect, while Dyract and Charisma did not differ from the control, Z250. The effects of neutral and acidulated fluoride gels were similar to each other and higher than the control. CONCLUSION: Conventional and resin-modified glass ionomer cements as well as neutral and acidulated fluoride gels inhibit the progression of artificial caries adjacent to restorations. The associated effect of fluoride-containing restorative materials and gels could not be demonstrated.

Aços carbono de baixa liga e aços inoxidáveis são amplamente utilizados nos circuitos primários de reatores nucleares do tipo PWR (Pressurized Water Reactor). Ligas de níquel são empregadas na soldagem destes materiais devido a características como elevadas resistências mecânica e à corrosão, coeficiente de expansão térmica adequado, etc. Nos últimos 30 anos, a corrosão sob tensão (CST) tem sido observada principalmente nas regiões das soldas entre materiais dissimilares existentes nestes reatores. Este trabalho teve como objetivo avaliar, por comparação, a suscetibilidade à corrosão sob tensão da zona afetada pelo calor (ZAC) do aço inoxidável austenítico AISI 316L quando submetida a um ambiente similar ao do circuito primário de um reator nuclear PWR nas temperaturas de 303ºC e 325ºC. Para esta avaliação empregou-se o ensaio de taxa de deformação lenta - SSRT (Slow Strain Rate Test). Os resultados indicaram que a CST é ativada termicamente e que a 325ºC pode-se observar a presença mais significativa de fratura frágil decorrente do processo de corrosão sob tensão.

In pressurized water reactors (PWRs), low alloy carbon steels and stainless steel are widely used in the primary water circuits. In most cases, Ni alloys are used to joint these materials and form dissimilar welds. These alloys are known to accommodate the differences in composition and thermal expansion of the two materials. Stress corrosion cracking of metals and alloys is caused by synergistic effects of environment, material condition and stress. Over the last thirty years, CST has been observed in dissimilar metal welds. This study presents a comparative work between the CST in the HAZ (Heat Affected Zone) of the AISI 316L in two different temperatures (303ºC and 325ºC). The susceptibility to stress corrosion cracking was assessed using the slow strain rate tensile (SSRT) test. The results of the SSRT tests indicated that CST is a thermally-activated mechanism and that brittle fracture caused by the corrosion process was observed at 325ºC.

O objetivo deste estudo in vitro foi determinar o tempo gasto para a remoção de dentina cariada (eficiência) e a microdureza Knoop (KHN) da dentina remanescente (eficiência), usando o método químico-mecânico (Carisolv) ou escavação manual. Trinta molares humanos foram seccionados através da lesão de cárie oclusal, produzindo duas metades iguais. Cada metade foi escavada, aleatoriamente, utilizando um escavador manual ou o kit Carisolv gel, seguindo as instruções do fabricante. O tempo gasto para a remoção da dentina cariada foi registrado. As hemi-secções foram incluídas em resina, planificadas e polidas. KHN da dentina remanescente foi determinada nas distâncias de 100, 200, 300, 400 e 500 mim, a partir do assoalho cavitário. Os dados foram analisados pelos testes de Wilcoxon (alfa=0,01), ANOVA e t de Student (alfa=0,05). O tempo gasto na escavação químico-mecânica foi significativamente maior que na escavação manual. As médias e os desvios-padrão de KHN a 100, 200, 300, 400, 500 mim foram, respectivamente, para o método químico-mecânico: 15,6 (±4,96), 18,0 (±6,22), 21,3 (±9,30), 24,3 (±9,25), 28,5 (±11,80); e para a escavação manual: 21,2 (±10,26); 23,4 (±9,49); 28,2 (±11,62); 31,0 (±12,17); 34,3 (±11,95). Concluiu-se que a escavação manual apresentou maior efetividade e eficiência que a escavação assistida pelo Carisolv.

The aims of this in vitro study were both to determine the time necessary for removal of carious dentin (efficiency) and the Knoop Hardness Number (KHN) of the remaining dentin (effectiveness), using a chemomechanical method (Carisolv) or hand excavation. Thirty human molars were bisected through occlusal carious lesions into two equal halves. Each half was randomly excavated by hand in circular movements with a spoon excavator or using Carisolv gel according to the manufacturer's instructions. The duration of carious dentin removal was recorded. Tooth sections were resin-embedded, ground flat and polished. Dentin KHN was determined at distances of 100, 200, 300, 400 and 500 mum from the cavity floor. Data were analyzed by Wilcoxon's test (alpha=0.01), ANOVA and Student's t test (alpha= 0.05). The median of the time necessary for chemomechanical excavation was significantly greater than for hand excavation. KHN means (± SD) at 100, 200, 300, 400, 500 µm for chemomechanical method were, respectively: 15.6 (±4.96), 18.0 (±6.22), 21.3 (±9.30), 24.3 (±9.25), 28.5 (±11.80); and for hand excavation were: 21.2 (±10.26), 23.4 (±9.49), 28.2 (±11.62), 31.0 (±12.17), 34.3 (±11.95). It may be concluded that hand excavation presented higher efficiency and effectiveness than chemomechanical excavation.

A caracterização de meios porosos é muito importante em áreas científicas e tecnológicas envolvendo materiais e processos químicos. Processos de transferência de calor e massa em meios porosos dependem, além de suas propriedades métricas, das propriedades topológicas e topográficas. No entanto, essas propriedades são de difícil acesso experimental, razão pela qual torna-se necessária, algumas vezes, a utilização de técnicas de simulação para auxiliar nas suas determinações. Este trabalho apresenta os resultados de simulação de leitos adsorvedores utilizando zeólitas 5A comerciais. Os modelos de compactação de partículas esféricas dos leitos adsorvedores são ajustados para que suas propriedades métricas, como fração volumétrica e área superficial de poros, se aproximem daquelas determinadas para os leitos reais. A partir dos modelos ajustados, os parâmetros topográficos e topológicos, como a distribuição radial da porosidade e a conectividade de poros, são estimados e comparados com valores obtidos para os leitos reais através de métodos indiretos.

The characterization of porous bed is of crucial importance in many scientific and technological areas related to materials and chemical processes. Mass and heat transfer in a porous media depend on metric, topological and topographical properties. Because the difficulties of experimental access to topological and topographical properties sometimes it is practical the use of computer simulation techniques to evaluate those parameters. This paper presents the simulation results of adsorbing beds using commercial 5A zeolites. The sphere packing models applied to adsorbing beds are adjusted through the comparison of metric properties, such as volumetric fraction and specific area of pores, evaluated for simulated and real beds. Starting from adjusted models, the topological and topographical properties, like radial distribution of porosity and pore connectivity are estimated and compared with those parameters obtained for real beds through the use of indirect methods.