Abstract The effects of restoration and certain elements, nitrogen, sulphur, calcium and Misch metal, on the hot ductility of austenitic, high-alloyed austenitic and duplex stainless steels have been investigated by means of hot rolling, hot tensile, hot bending and stress relaxation tests. The results of these different testing methods indicated that hot rolling experiments using stepped specimens is the most effective way to investigate the relationship between the softening and cracking phenomena under hot rolling conditions. For as-cast, high-alloyed and duplex stainless steels with a low impurity level, the cracking tendency was observed to increase with increasing pass strain and temperature, being minimal for the small strain of 0.1. No cracking occurred in these steels when rolled in the wrought condition. It could be concluded that the cracking problems are only exhibited by the cast structure with the hot ductility of even partially recrystallised steel being perfectly adequate. However, the recrystallisation kinetics of the high-alloyed austenitic stainless steels, determined by stress relaxation and double-pass rolling tests, were found to be so slow that only partial softening can be expected to occur between roughing passes under normal rolling conditions. In the duplex steel, the restoration is fairly fast so that complete softening can occur within typical interpass times in hot rolling, while certain changes in the phase structure take place as well. Sulphur was found to be an extremely harmful element in duplex stainless steel with regard to their hot ductility so that severe cracking can take place with sulphur content above 30 ppm. However, the effect of sulphur can be eliminated by reducing its content and by calcium or Misch metal treatments that significantly increase the number and decrease the average size of the inclusions. It seems that the desulphurisation capacity of an element is the most important property for assessing its usefulness in reducing the detrimental influence of sulphur. The hot ductility of type 316L stainless steel determined by tensile tests was found to be better for nitrogen content of 0.05 wt-% than 0.02%, while in double-hit tensile tests the hot ductility values were identical. The mechanism whereby nitrogen affects hot ductility remains unclear but a retarding effect on static recrystallisation was observed.

[pt] A exposição de aços a condições de geração de hidrogênio, como em ambientes que contenham H2S ou sob proteção catódica, pode provocar o aumento do teor de hidrogênio na sua superfície, fragilizando o material. Foi desenvolvido um estudo com o objetivo de comparar o comportamento das microestruturas de dois diferentes aços inoxidáveis, um austenítico de classe P550 e um duplex S31803, com relação à permeação e consequente fragilização pelo hidrogênio. Os aços foram testados nas condições com e sem solda autógena utilizando os mesmos parâmetros de soldagem. Foram realizados análise microestrutural por MO, MEV e MET, ensaios de tração, ensaios de BTD com os corpos de prova imersos em solução de água do mar sintética e sob aplicação de potencial catódico de -1200 mV SCE, com o objetivo de simular condições de serviço e fractografia por MEV dos corpos de prova ensaiados por BTD. Observou-se que ambas as classes de aços sofreram alguma fragilização, no entanto, com relação à perda de ductilidade em função da redução de área dos corpos de prova, esta ocorreu de forma mais pronunciada para o aço inoxidável duplex. Foi observado que o aço austenítico no metal de base continha maior densidade de maclas do que o metal de solda, resultando em maior fragilização. Além disto, no metal de base, observou-se mais alta densidade de discordâncias e de precipitados. O aço duplex, por outro lado, apresentou fragilização muito maior do que o austenítico em ambas as condições quando permeado pelo hidrogênio. No entanto, esta fragilização foi mais pronunciada na condição de como soldado. Atribuiu-se este comportamento à ferritização parcial da estrutura e à formação de austenita Widmanstätten. As análises fractográficas sugerem a alteração do mecanismo de fratura dos corpos de prova de dúctil, quando ensaiados ao ar, para frágil, na condição de ensaio com geração de hidrogênio. Esta observação fundamenta-se no fato de que as superfícies de fratura de todos os corpos de prova ensaiados ao ar são formadas predominantemente por dimples (dúctil), enquanto no ensaio com geração de hidrogênio, as superfícies de fratura se apresentam com aspecto frágil de diferentes formas para cada aço. / [en] Steel exposure to hydrogen generation conditions, such as in environments containing H2S or under cathodic protection, can cause an increase in the hydrogen content on the surface which leads to the material embrittlement. A comparative study was carried out on the structure behavior of two different stainless steels, an austenitic class P550 and a duplex S31803, concerning permeation and consequent hydrogen embrittlement. The steels were tested in conditions with and without autogenous welding using the same welding parameters. It was performed microstructural analysis by OM, SEM and TEM, tensile tests, BTD tests with the specimens immersed in a synthetic seawater solution and under application of a cathodic potential of -1200 mV SCE, in order to simulate service conditions, and surface fractography of these specimens by SEM. It was observed that both grades of steel suffered some fragility. However, the loss of ductility due to the reduction of the area of the specimens occurred in a more pronounced way for the duplex stainless steel. It was also observed that the austenitic steel in the base metal contained a higher density of twinnings than the weld metal, resulting in greater embrittlement. In addition, a higher density of dislocations and precipitates was observed in the base metal. On the other hand, duplex steel showed much more significant embrittlement than austenitic in both conditions when permeated by hydrogen. However, this weakness was more pronounced in the as welded condition. This behavior was attributed to the partial ferritization of the structure and the formation of Widmanstätten austenite. Fractographic analyzes suggest that the fracture mechanism morphology changed from ductile to brittle when the specimens were tested in the air and hydrogen condition respectively. This observation is based on the fact that the fracture surfaces of all samples tested in the air consisted predominantly of dimples (ductile). In contrast, in the test with hydrogen generation, the fracture surfaces appear with different brittle morphologies for each steel.

Additively manufactured (AM) 316L and 17-4PH stainless steel parts, concretely made by laser powder bed fusion (L-PBF), are characterized and micro-mechanical properties of those steels are analyzed. This study also explored and extended to proton irradiation and small-scale mechanical testing of those materials, to investigate how irradiation affects microstructural evolution and thus mechanical properties at the surface level, which could be detrimental in the long term in nuclear applications. In-depth anisotropy analysis of L-PBF 316L stainless steel parts with the variations of volumetric energy density, a combined study of nanoindentation with EBSD (electron backscatter diffraction) mapping is shown to be an alternative methodology for enriching qualification protocols. Each grain with a different crystallographic orientation was mapped successfully by proper indentation properties. <122> and <111> oriented grains displayed higher than average indentation modulus and hardness whereas, <001>, <101>, and <210> oriented grains were found to be weaker in terms of indentation properties. Based on an extensive nanoindentation study, L-PBF 17-4 PH stainless steels are found to be very sensitive to high load rates and irradiation further escalates that sensitivity, especially after a 0.25 s-1 strain rate. 3D porosity measurement via X-ray microscope ensures L-PBF stainless steel parts are of more than 99.7% density and could be promising for many industrial applications. High percentages of increment of nanohardness, maximum theoretical shear strength, and yield strength were observed due to proton irradiation of 5 um damage depth on the surface of 17-4 PH steel parts. Small-scale mechanical testing of irradiated AM nuclear stainless steels such as 17-4 PH was carried out and investigated by micro-compression of FIB fabricated pillars of different sizes of diameter. Irradiated 17-4 PH materials have never been investigated by this kind of testing procedure to asses the stress-strain characteristics of micro-scale volumes and to explore the structure-property relationship. Both as-built and irradiated AM 17-4 PH micropillars exhibited step-ups in the early stage of load-displacement curves with a varying number of slip bands intermittently formed throughout the pillar volume while compressed by the uniaxial load. As for the radiation-damaged zone, micropillars displayed lesser slip bands compared to as-built parts as irradiation damage creates an obstacle to dislocations movement and hence hardening. It requires higher loads to initiate plastic deformation as dislocation must overcome irradiation-induced obstacles for the slip to occur and localization of strain without increasing the load for a certain amount of time during the test. Proton irradiation effects on the compressive mechanical properties of AM 17-4 PH stainless steel parts depending on the volumetric energy density (VED) used during the parts' fabrication process. On as-built parts, compressive yield strength varied from 107.27 MPa to 150.70 MPa and it was in the range of 133.43 MPa to 244.57 MPa under irradiated conditions. All 2 μm pillars were fabricated as their height falls within the radiation damage depth of 5 μm. It was expected to generate the highest yield strength and tensile strength due to the radiation hardening effect as discussed earlier. Yield and tensile strength were found to be the highest as expected as of 244.57 MPa and 375.08 MPa in irradiated 17-4 PH sample 1 (VED = 54.76 J/mm3). Samples with lower VED exhibited better micro-mechanical compressive responses than higher VED AM 17-4 PH parts in both as-built and irradiated conditions.

Duplex stainless steels (DSSs) are important engineering materials due to their combination of good mechanical properties and corrosion resistance. However, as a consequence of their ferrite content, DSSs are sensitive to the so-called ‘475°C embrittlement’, which is induced by phase separation, namely, the ferrite decomposed into Fe-rich ferrite (α) and Cr-rich ferrite (α'), respectively. The phase separation is accompanied with a severe loss of toughness. Thus, the ‘475°C embrittlement’ phenomenon limits DSSs’ upper service temperature to around 250°C. In the present work, Fe-Cr binary model alloys and commercial DSSs from weldments were investigated for the study of phase separation in ferrite. Different techniques were employed to study the phase separation in model alloys and commercial DSSs, including atom probe tomography, transmission electron microscopy and micro-hardness test. Three different model alloys, Fe-25Cr, Fe-30Cr and Fe-35Cr (wt. %) were analyzed by atom probe tomography after different aging times. A new method based on radial distribution function was developed to evaluate the wavelength and amplitude of phase separation in these Fe-Cr binary alloys. The results were compared with the wavelengths obtained from 1D auto-correlation function and amplitudes from Langer-Bar-On-Miller method. It was found that the wavelengths from 1D auto-correlation function cannot reflect the 3D nano-scaled structures as accurate as those obtained by radial distribution function. Furthermore, the Langer-Bar-On-Miller method underestimates the amplitudes of phase separation. Commercial DSSs of SAF2205, 2304, 2507 and 25.10.4L were employed to investigate the connections between phase separation and mechanical properties from different microstructures (base metal, heat-affected-zone and welding bead) in welding. Moreover, the effect of external tensile stress during aging on phase separation of ferrite was also investigated. It was found that atom probe tomography is very useful for the analysis of phase separation in ferrite and the radial distribution function (RDF) is an effective method to compare the extent of phase separation at the very early stages. RDF is even more sensitive than frequency diagrams. In addition, the results indicate that the mechanical properties are highly connected with the phase separation in ferrite and other phenomena, such as Ni-Mn-Si-Cu clusters, that can also deteriorate the mechanical properties. / <p>QC 20130308</p>

This report aims to increase the scientific knowledge about long-term prospects for the adhesive and adhesive joints for bonding of stainless steels. The effects of water, temperature and chemicals on the adhesive and adhesive joints are investigated. Stainless steel plates are pretreated with a primer and isopropanol, there after joined together with single lap modeling. The strength of the joint is tested with a tensile test and additionally a watertightness test is performed to determine if the joints are watertight. For this project three versions of stainless steels is used and two different technologies of two- part adhesives, silicone and silane-modified polymer and one technology of tape, a double coated acrylic foam tape are tested. The result shows that all the adhesives fails cohesively and that tape fails partly adhesively. Result shows that all tests are affected by water, temperature and chemicals on different levels but tape is affected the most with a minimum of 40% loss in shear strength. Watertightness test shows that aged tape joints are not watertight. The polymer shows no signs of decreasing in shear strength and is watertight, but does become more viscous by aging. The report shows that a possible combination of adhesive and pretreatment that can withstand the effects of water, temperature and chemicals is found. The polymer presents a possibility to bond stainless steel with a simple pretreatment. Tape didn’t pass the test in a suitable way but presents opportunities if a sufficient pretreatment can be found.

Os aços inoxidáveis ferríticos são naturalmente ferromagnéticos, o que impossibilita sua utilização em próteses ortopédicas. Apesar disso, em algumas aplicações específicas, faz-se necessário o uso de um biomaterial ferromagnético, como nas próteses odontológicas e faciais com conectores magnéticos. Este trabalho apresenta o estudo da resistência à corrosão e citotoxicidade do aço inoxidável ferrítico AISI 444, para avaliar seu potencial de uso como um biomaterial. O aço AISI 444 possui baixo teor de níquel, teores extrabaixos de intersticiais (C e N) e é estabilizado com Ti e Nb. Como materiais de referência foram utilizados o aço inoxidável austenítico ISO 5832-1 (ASTM F-138), por ser o biomaterial metálico mais empregado na fabricação de próteses ortopédicas, e uma base ferromagnética do sistema de fixação de próteses odontológicas, feita em aço inoxidável ferrítico (NeoM). O ensaio de citotoxicidade in vitro, pelo método de incorporação do corante vermelho neutro, revelou que o aço inoxidável AISI 444 não apresentou citotoxicidade. O comportamento frente à corrosão foi estudado por meio de curvas de polarização anódica potenciodinâmicas e espectroscopia de impedância eletroquímica (EIE). O meio de ensaio foi uma solução salina tamponada de fosfato (PBS), em condição naturalmente aerada e em fresta, a temperatura de 37 ºC. Para simular a condição de fresta dos ensaios eletroquímicos foi desenvolvido um novo equipamento. As propriedades eletrônicas do filme passivo foram avaliadas pela técnica de Mott-Schottky. Em meio aerado, os resultados de EIE indicaram que todos os materiais se mostraram passivos. As curvas de polarização indicaram que a resistência à corrosão por pite do aço AISI 444 foi equivalente à do aço ISO 5832-1, porém superior à do NeoM. Pelos diagramas de Mott-Schottky, conclui-se que o filme óxido no aço AISI 444 possui menor concentração de dopantes que o aço NeoM. Isto sugere que o aço AISI 444 apresenta maior resistência à transferência de carga através do filme passivo. Em condição de fresta, as polarizações indicaram taxas de corrosão baixas para ambos os aços, porém superiores para o aço ISO 5832-1, em comparação ao aço AISI 444. As micrografias das superfícies dos aços, após polarização, revelaram um maior ataque corrosivo no aço ISO 5832-1 do que no aço AISI 444. O aço NeoM apresentou composição química fora da especificação da norma. Os baixos teores de Cr e de Mo, além das altas concentrações de sulfetos, foram as prováveis causas da menor resistência à corrosão do NeoM, indicada pelos ensaios eletroquímicos. O equipamento proposto para avaliação da resistência à corrosão, em condição de fresta, mostrou boa reprodutibilidade de resultados. O aço inoxidável AISI 444 apresentou alta potencialidade para uso como biomaterial, especialmente na fabricação de componentes protéticos com fixação magnética. / Ferritic stainless steels are ferromagnetic materials. This property does not allow their use in orthopedic prosthesis. Nevertheless, in some specific applications, this characteristic is very useful, such as, for fixing dental and facial prostheses by using magnetic attachments. In this study, the corrosion resistance and cytotoxicity of the AISI 444 ferritic stainless steel, with low nickel content, extra-low interstitial levels (C and N) and with the addition of Ti and Nb as stabilizers, were investigated to evaluate its potentiality for biomaterials fabrication. The ISO 5832 austenitic stainless steel (SS) and a commercial universal keeper for dental attachment (Neo-magnet System) were evaluated for comparison reasons. The first stainless steel is the most used metallic material for orthopedic prostheses fabrication, and the second one, is a ferromagnetic keeper for dental prostheses (NeoM). In vitro cytotoxicity analysis was performed by the red neutral incorporation method. The results showed that the AISI 444 stainless steel is non cytotoxic. The corrosion resistance was studied by anodic polarization methods and electrochemical impedance spectroscopy (EIS), in a saline phosphate buffered solution (PBS) at 37 °C, either naturally aerated or under crevice condition. A new device was developed to simulate the crevice condition on electrochemical tests. The electronic properties of the passive film formed on AISI 444 SS were evaluated by the Mott-Schottky approach. All tested materials showed passivity in the PBS medium and the passive oxide film presented a duplex nature. In aerated condition, the resistance to pitting corrosion associated to AISI 444 SS was similar to that of the ISO 5832 SS and both were superior to that of the NeoM SS. The 444 SS oxide film showed lower dopants concentration than the NeoM SS, suggesting that the 444 SS film presents a higher resistance to charge transfer through it than the fim on the NeoM SS. Under crevice conditions, the potentiodinamic polarization tests indicated low corrosion rates for both steels, but slightly higher for the ISO 5832 SS when compared to the AISI 444 SS tested. Observation of the surface, after crevice polarization, indicated a larger area of corrosive attack on the ISO 5832-1 SS than on the AISI 444 SS.The chemical composition of the NeoM SS was out of the standard specification. The low levels of Cr and Mo in the NeoM, and the high concentration of MnS precipitates, are the probable cause of its lower corrosion resistance. The new devide proposed for crevice corrosion resistance evaluation, showed good reproducibility of results.The AISI 444 stainless steel showed a high potential for use as a biomaterial, especially for the manufacture of prosthetic components with magnetic attachment.

O presente estudo objetivou caracterizar as juntas soldadas de uma chapa de aço inoxidável duplex UNS S32205 soldada utilizando diferentes energias (0,5 kJ/mm, 1,0 kJ/mm e 3,5 kJ/mm) e avaliar a susceptibilidade dessas juntas à corrosão sob tensão (CST) em ambiente contendo cloreto. O ensaio por gotejamento \"drop evaporation test\" (DET) de água do mar sintética foi utilizado para avaliar a susceptibilidade à CST das juntas soldadas e investigar os efeitos da temperatura (70 ºC, 90 ºC e 110 ºC) e da tensão de tração (70%, 90% e 100 % do limite de escoamento do metal base) na resistência à CST. Os resultados da caracterização mecânica, suscetibilidade à corrosão intergranular e caracterização microestrutural das juntas soldadas não evidenciaram a presença de fases deletérias, como as fases ? e ?. A proporção da fase ferrita na zona fundida e na zona afetada pelo calor (ZAC) foi inversamente proporcional à energia de soldagem. Por exemplo, a ferrita na ZAC variou de 68% para 0,5 kJ/mm a 54% para 3,5 kJ/mm. Os resultados dos ensaios de CST mostraram houve trincamento e fratura para todos os corpos de prova ensaiados a 110 ºC (mesmo com 70% do limite de escoamento), enquanto que nenhuma trinca foi observada nos corpos de prova ensaiados a 70 ºC e 90 ºC. Os ensaios de CST realizados a 90 ºC apresentaram corrosão localizada e preferencial da fase de ferrita longe do cordão de solda, enquanto os ensaios realizados a 70 ºC não apresentaram sinais significativos de corrosão. Todos os corpos de prova ensaiados a 110 ºC apresentaram fratura abaixo do deposito de sal com aspecto de vulcão formado pela evaporação da solução gotejada. As trincas propagaram pela interface ?/? ou clivagem transgranular nas fases ferrita e austenita. Para menores tensões ensaiadas, a proporção de clivagem transgranular da fase de ferrita foi mais significativa que a fase de austenita. O ensaio de DET não foi eficiente para investigar o efeito das alterações microestruturais na ZAC na susceptibilidade à CST. A análise de distribuição de temperatura na superfície dos corpos de prova indicou que o gotejamento da água do mar sintética teve um efeito de resfriamento e a temperatura na região de gotejamento foi pelo menos 10 ºC mais baixa quando comparada a outras regiões do corpo de prova. Além disso, o pH na região gotejante foi mais básico (pH = 11) do que a solução de água do mar sintética (pH = 8,2). / The present investigation characterized the welded joints of a UNS S32205 duplex stainless steel plate using different welding energies (0.5 kJ/mm, 1.0 kJ/mm and 3.5 kJ/mm) in order to evaluate the susceptibility of these joints to stress corrosion cracking (SCC) in Cl- environments. Drop evaporation test (DET) of synthetic seawater was used to assess the SCC behavior of the welded joints and to investigate the effects of the temperature (70 ºC, 90 ºC and 110 ºC) and the tensile strength (70%, 90% and 100% of the base metal\'s yield strength) on their SCC resistance. The results of the mechanical, intergranular corrosion susceptibility and microstructural characterization of the welded joints did not show the presence of deleterious phases, such as ? and ? phases. Additionally, the proportion of ferrite phase in the molten zone and in the heat-affected zone was inversely proportional to the welding energy. For instance, the ferrite in the HAZ varied from 68% for 0.5 kJ/mm to 54% for 3.5 kJ/mm. The results of SCC testing showed the cracking and fracture for all specimens tested at 110 ºC (even at 70% of the yield strength), while no cracking was observed at 70 ºC and 90 ºC. DET performed at 90 ºC showed that there was a localized and preferential corrosion of the ferrite phase far away from the weld bead, while DET performed at 70 ºC did not show any significant sign of corrosion. All samples tested at 110 ºC presented a noteworthy salt deposition in the shape of a volcano and their cracks were formed underneath the salt layer by the pit corrosion of the ferrite phase. The cracks propagated by either ? / ? interfacial or transgranular cleavage at ferrite and austenite phases. For lower stresses, the proportion of transgranular cleavage of the ferrite phase was more significant than the austenite phase. DET was not an efficient test to investigate the effect of the microstructural changes in the HAZ on the SCC susceptibly of the welded joints as the crack took place below the salt deposit and away from the weld bead. The temperature distribution analysis on the surface of the DET samples indicated that the seawater drip has a significant cooling effect and the temperature of this region was at least 10 ºC lower when compared to other regions of the samples. Additionally, the pH on the drip region was more basic (pH = 11) than the synthetic seawater solution (pH = 8.2). Keywords: Duplex stainless steels; stress corrosion cracking; synthetic seawater; welding.

Aços inoxidáveis dúplex apresentam uma estrutura composta por ferrita e austenita. O fato de a austenita ser estável à temperatura ambiente possibilita que esses aços sejam usados no estudo da reação de formação da austenita a partir da ferrita, podendo os resultados ser aplicados a aços de baixa liga, em que a austenita se transforma em martensita no resfriamento brusco, e a outros sistemas com transformações entre fases cúbicas de corpo centrado (CCC) e cúbicas de face centrada (CFC). Foram realizados estudos em um aço inoxidável dúplex do tipo DIN W.Nr. 1.4462 (UNS 31803). As amostras foram solubilizadas no campo ferrítico a 1325°C e resfriadas em água. As amostras foram ainda tratadas isotermicamente em temperaturas entre 700°C e 1100°C por tempos entre 5.000 e 30.000 segundos. Deste modo, a formação de austenita a partir da ferrita foi estudada em seu estágio inicial e em estágios avançados da reação, com relação aos seus aspectos morfológicos e cristalográficos. As morfologias encontradas apresentaram variações dependentes dos segmentos de contorno de grão em que as partículas se formam. As partículas nucleadas nos contornos de grão podem ser adequadamente descritas pela classificação morfológica de Dubé. Além disso, essas partículas apresentaram, em geral, relações de orientação do tipo Kurdjumov-Sachs e Nishyiama-Wassermann com pelo menos um dos dois grãos, podendo ser encontradas relações intermediárias entre essas duas. . Em alguns casos, as partículas mantêm relações de orientação com os dois grãos adjacentes, apresentando pequenos desvios das relações exatas relatadas na literatura. As partículas de austenita intragranulares apresentam desvios em relação à relações exatas maiores do que os encontrados nas partículas de contornos de grão. Em alguns casos, as partículas intragranulares aparentam não apresentar relações de orientação com a matriz ferrítica. / Duplex Stainless Steels have a structure composed by ferrite and austenite. The fact that austenite, in these steels, is stable at low temperatures, allows the use of these steels in studies of austenite formation from ferrite, in such way that the results can be applied to low alloy steels, in which austenite transforms to martensite upon rapid cooling, and to other systems containing transformations between body-centred cubic (BCC) and face-centred cubic (FCC) phases. Studies were performed on a DIN W.Nr. 1.4462 (UNS 31803) duplex stainless steel. The samples were solution treated in the ferrite region at 1325°C and water cooled. Samples were then submitted to isothermal treatments at temperatures between 700°C and 1100°C for up to 30.000 seconds. In this way, austenite formation from ferrite was studied on initial and advanced stages of the reaction, concerning morphological and crystallographic aspects. The morphologies observed varied with the grain boundary segment in which the particles were formed. The grain boundary particles may be adequately described by the Dubé classification. Moreover, these particles presented Kurdjumov-Sachs and Nishyiama-Wassermann orientation relationships with at least one of the adjacent grains, with possibilities of occurrence of intermediate relationships between K-S and N-W relationships. In some cases, the particles show orientation relationships with both adjacent grains, with small deviations form the exact relationships reported in literature. Intragranular austenite particles show higher deviations from the exact relationships than grain boundary particles. In some cases, intragranular particles have no apparent orientation relationships with the ferrite matrix.

Este trabalho tem por objetivo estudar a eficiência dos oxiânions molibdato (MoO4 2-) e tungstato (WO4 2-) na inibição da corrosão localizada em aços inoxidáveis austeníticos (347 e 304L) em sistemas de água com alto grau de pureza na presença de íons cloreto (Cl-). Para avaliar a ação dos oxiânions estudados foram empregadas técnicas eletroquímicas de monitoramento do potencial de circuito aberto (Eoc), espectroscopia de impedância eletroquímica (EIS), polarização cíclica (PC) e cronoamperometria (CA). Para caracterização superficial foram utilizadas microscopia óptica (MO) e microscopia eletrônica de varredura (SEM). Os ensaios foram realizados em meios contendo somente água pura desoxigenada e água pura desoxigenada contendo íons cloreto (50, 100, 200, 300 e 500 ppm) variando a concentração (0, 10-4, 10-3 e 10-2 M) dos inibidores molibdato e tungstato de sódio. Todos os experimentos foram realizados à temperatura ambiente e em condições hidrodinâmicas controladas, empregando o sistema de eletrodo de disco rotativo (RDE) com velocidade constante de1200 rpm. Os resultados mostraram que os aços inoxidáveis 347 e 304L permanecem passivos em água pura, uma vez que o meio não é agressivo. Entretanto, a adição de íons cloreto tornou-os susceptíveis à corrosão por pites, pois os cloretos atuam como desapassivadores, fragilizando o filme passivo que fornece proteção aos aços inoxidáveis. O aumento da concentração dos íons cloreto no eletrólito aumentou a sua agressividade para os dois aços estudados, sendo observado o precoce aparecimento da corrosão. No entanto, a adição dos inibidores molibdato e tungstato de sódio melhorou a qualidade da camada passiva, aumentando os valores de potencial de pite (Ep) e de módulo de impedância, mantendo a passividade dos aços num amplo intervalo de potenciais. Ensaios cronoamperométricos mostraram que na presença dos oxiânions os valores medidos de densidade de corrente de resposta para os aços inoxidáveis 347 e 304L foram acentuadamente menores em relação aos valores obtidos na ausência dos mesmos, confirmando que os inibidores utilizados atuam na melhora da camada passiva, diminuindo a sensibilidade aos cloretos e conseqüentemente aumentando a resistência contra a corrosão por pites dos aços. O estudo mostrou que os melhores resultados para ambos os aços foram obtidos na presença de 10-2 M de inibidor, tanto para o tungstato quanto para o molibdato, sendo este com eficiência levemente melhor. Além disso, o aço 304L sem partículas incrustadas de alumina mostrou-se menos susceptível aos ataques dos cloretos do que o aço 347. Nas amostras que tiveram a superfície tratada com lixas de abrasivo de alumina, essas partículas foram incrustadas nos aços e isso levou ao desaparecimento de um potencial de pite bem definido. Na presença de alta concentração de cloretos, essas partículas foram arrancadas pelo ataque às regiões deformadas mecanicamente, localizadas ao redor das partículas incrustadas e as propriedades do aço em si puderam ser verificadas. / The aim of this work is to study the efficiency of molibdate (MoO4 2-) and tungstate (WO4 2-) oxianions in the inhibition of the localized corrosion in austenitic stainless steels (347 and 304L) in high purity water systems in the presence of chloride ions (Cl-). In order to evaluate the action of the oxianions electrochemical techniques as open circuit potential (OCP), electrochemical impedance spectroscopy (EIS), cyclic polarization (CP) and chronoamperometry (CA) were accomplished. For surface characterization optical microscopy (OM) and scanning electron microscopy (SEM) were used. The experiments were carried out in deoxygenated pure water and deoxygenated pure water containing chloride ions (50, 100, 200, 300 and 500 ppm) adding different concentrations (0, 10-4, 10-3 and 10-2 M) of molibdate and tungstate ions. All the experiments were performed at room temperature under controlled hydrodynamic conditions using a rotating disk electrode (RDE) at constant rotation speed of 1200 rpm. The results showed that 347 and 304L stainless steels remain passive in pure water, since the environment is not aggressive. However, in the presence of chloride ions they become susceptive to pitting corrosion, with chloride ions acting depassivity agents weakening the passive film that supplies protection to the stainless steels. The increase of chloride ions concentration in the electrolyte enhanced the aggressiveness for both steels evidenced by a precocious accurance of pitting corrosion. On the other hand, the addition of sodium molibdate and tungstate inhibitors improved the quality of the passive layer, increasing the pitting potentials (Ep) and the impedance modulus, improving the the passivity of steels a large of potential range. Chronoamperometric measurements showed that in the presence of oxianions response current density for 347 and 304L stainless steels was strongly diminished in relation to the values obtained in the absence of the inhibitors, confirming that the inhibitors act in the improvement of the passive layer, decreasing its sensitivity to chlorides and consequently increasing the steels resistance against pitting corrosion. Results showed that the best results for both steels were obtained in the presence of 10- 2 M tungstate or molibdate, being this latter with efficiency lightly better. Moreover, the 304L stainless steel showed to be less susceptive to pitting corrosion compared to 347 stainless steel. For stainless steel specimens previously treated with Al2O3 emery papers, Al2O3 particles become embedded in the steel surface and led to the indefinition of the pitting potential values. In the presence of high chloride concentrations, the mechanically affected region of steel in the surrounding of Al2O3 particles was attacked and Al2O3 particles were removed reveling the stainless steel own resistance to chlorides.

A produção de superfícies funcionais sobre componentes, para a obtenção de melhores resistências ao desgaste, à corrosão e à fadiga constitui-se num persistente desafio tecnológico. Os processos termoquímicos de nitretação e nitrocementação por plasma são técnicas de engenharia de superfície usadas para aumentar a dureza superficial e resistência ao desgaste de aços inoxidáveis, sem deteriorar suas resistências à corrosão. O presente trabalho teve como objetivo a avaliação da influência das temperaturas de nitretação e nitrocementação por plasma na estrutura das camadas produzidas nos aços inoxidáveis UNS S31603 (austenítico), S31254 (superaustenítico) e S41425 (supermartensítico) e seus desempenhos quanto ao desgaste, à corrosão e à fadiga. Verificou-se que os tratamentos produziram camadas homogêneas e contínuas, sendo mais espessas, para os aços UNS S31603 e S31254 nitrocementados, e para o aço UNS S41425 nitretado, em uma dada temperatura. As microdurezas das camadas cresceram com o aumento da temperatura, para ambos os tratamentos e para os três aços estudados. A difração de raios X indicou que as fases expandidas, fase-S ou &alpha;\'N, foram obtidas nas temperaturas de tratamento mais baixas (400 e 450ºC). O aumento na temperatura de tratamento promoveu a formação de carbonetos e/ou nitretos, para a nitrocementação e nitretação, respectivamente. Para os aços UNS S31603 e S31254, isto ocorreu devido a decomposição da fase-S, em uma microestrutura tipicamente lamelar composta por ferrita e nitretos de cromo. Já no caso do aço UNS S41425, o aumento na temperatura de tratamento proporcionou um aumento na quantidade de carbonetos e/ou nitretos. Tal aumento na temperatura de tratamento também promoveu um decréscimo nas resistências ao desgaste das camadas dos aços UNS S31603 e S31254. A resistência ao desgaste aumentou com a temperatura de tratamento, para o aço UNS S41425, para ambos os tratamentos. A resistência à corrosão em solução de NaCl diminuiu com o aumento da temperatura de tratamento, para os três aços estudados, devido a presença dos carbonetos e/ou nitretos. Os ensaios de fadiga de contato indicaram que nos aços UNS S31603 e S31254 o aumento na temperatura de tratamento não causou mudanças significativas nas tensões de ruptura das camadas. No aço UNS S41425, tal tensão diminuiu com o aumento da temperatura. / The production of functional surfaces on engineering components, in order to obtain improved wear, corrosion and fatigue resistance is a persistent technological challenge. The plasma nitriding and nitrocarburizing thermochemical processes are surface engineering techniques used to improve surface hardness and wear resistance of stainless steels, without compromising its corrosion resistance. The main goal of this study is to evaluate the influence of plasma nitriding and nitrocarburizing temperature on the structure of the layers produced on UNS S31603 (austenitic), S31254 (superaustenitic) and S41425 (supermartensitic) stainless steels and in addition their wear, corrosion and fatigue performance. It was found that both treatments produced homogeneous and continuous layers. Of all the samples in this work, the nitrocarburized UNS S31603 and S31254 steels and the nitrided UNS S41425 steel presented the thickest layers at a given temperature. Regardless of the treatment used, the microhardness of the layers increased with the raising of the temperature for all the samples. The X-ray diffraction indicated that expanded phases, either S-phase or &alpha;\'N, were obtained at lower treatment temperatures (400 and 450°C). The increase in treatment temperature promoted the formation of carbides and/or nitrides for nitrocarburizing and nitriding, respectively. For the samples of UNS S31603 and S31254 steels, this occurred due to the decomposition of S-phase in a typical lamellar microstructure consisting of ferrite and chromium nitride. In the case of UNS S41425 steel, the increase in treatment temperature caused an increase on the amount of carbides and/or nitrides. This increase in treatment temperature also promoted a decrease of the wear resistance for the layers produced on the UNS S31603 and S31254 steels samples. On the other hand, the wear resistance increased with treatment temperature for the UNS S41425 steel for both treatments. The corrosion resistance in NaCl solution decreased with increasing treatment temperature for all the samples, due to the presence of carbides and/or nitrides. The contact fatigue tests on UNS S31603 and S31254 steels indicated that an increase on treatment temperature did not cause significant changes on rupture stress of the layers. In UNS S41425 steel, such critical stress decreased with increasing temperature.