Submitted by Maria Eneide de Souza Araujo (mearaujo@ipen.br) on 2015-09-01T18:02:01Z No. of bitstreams: 0 / Made available in DSpace on 2015-09-01T18:02:01Z (GMT). No. of bitstreams: 0 / As indústrias que desenvolvem motores de combustão interna têm como preocupação atual prover motores que sejam cada vez menos poluentes, uma vez que a preocupação com a preservação do meio ambiente é intensa em todo o mundo. No entanto, com o desenvolvimento de novas tecnologias destinadas à redução das emissões, a condensação dos gases, provenientes da combustão, está sendo promovida dentro das câmaras de combustão dos veículos. Ácidos, como sulfúrico e nítrico, são gerados pela condensação destes gases. Esta condensação está associada às altas taxas de recirculação de gases de escape, conhecido como EGR, (termo em inglês para Exhaust Gas Recirculation). Consequentemente, problemas de corrosão nos componentes do motor estão aumentando, especialmente em camisas de cilindro em ferro fundido. Neste estudo, foi investigada a resistência à corrosão de dois ferros fundidos, um de microestrutura perlítica e o outro com microestrutura bainítica, em soluções de condensado natural e sintético de motores movidos a diesel. Os resultados foram associados às microestruturas e as composições químicas dos materiais estudados. Resultados de testes de imersão e ensaios de espectroscopia de impedância eletroquímica, bem como de curvas de polarização potenciodinâmicas, indicaram que os dois materiais não apresentam resistência à corrosão nos meios de ensaio adotados. O ataque intenso da matriz observado em ambos os materiais observado pelos ensaios de imersão, mostraram a atuação do mecanismo eletroquímico de corrosão por grafitização. Este mecanismo causa o ataque localizado e destrutivo da matriz de ferrita (-Fe), que funciona como anodo enquanto as grafitas atuam como áreas catódicas. Enquanto em meio ácido não foi possível observar uma diferenciação entre os dois tipos de ferros fundidos estudados, em meio neutro e aerado, o ferro fundido bainítico mostrou resistência à corrosão superior à do ferro fundido perlítico. / Dissertação (Mestrado em Tecnologia Nuclear) / IPEN/D / Instituto de Pesquisas Energeticas e Nucleares - IPEN-CNEN/SP

Diploma thesis is worked-up as a part of master’s studies of engineering technology M2I – K, analyzing the possible methods of surface treatment of spherical pin, that increase its corrosion resistance. Spherical pin is mounted into several types of chassis components such as joints, control arms, etc. The current situation is inconvenient for manufacturers, insufficient corrosion protection increases the number of complaints and this may be reflected in the number of future orders. Via of suitable surface treatment is to achieve a higher corrosion resistance while maintaining the functional properties of pins used.

The goal of this work was to evaluate the electrochemical properties of TiZrNbTa thin films deposited by magnetron sputtering using an industrial physical vapor deposition system. Samples were deposited on both Si(001) and 316L stainless steel. The samples deposited on Si(001) were either crystalline (bcc reflections) or amorphous, depending on the sputtering parameters. The crystalline films were composed of thin films with two different layers. The upper layer was nanocolumnar composed of elongated nanocolumns, while the lower was dense. The amorphous films had only one nanocolumnar layer and higher porosity. Polarization curves revealed that all samples had low corrosion current densities, in the order of 10-8 A/cm2. The samples showed an extended passive region up to 3.0 V vs Ag/AgCl due to the growth of a passivating oxide. The surface of the samples consisted of Nb2O5, ZrO2, TiO2 and Ta2O5. The chronoamperometry tests showed current oscillations, related to a break-down and reformation of the passive film. Electrochemical impedance spectroscopy revealed that all samples behaved similarly in all three electrolytes, and the simulated electrical circuits were indicating no corrosion reactions. A decrease in capacitance values after polarization was observed and was related to the formed surface oxide. Samples deposited on 316L stainless steel showed a passive regime for a shorter potential window, probably related to surface defects of the films. Heat treatments at 400 and 800 Celsius for 20 hours could not trigger the phase transformation from single bcc to hcp or dual bcc, as predicted by the Thermo-Calc software.

3D printing, also known as additive manufacturing (AM), has got rapidly developed since 1987. Compared with conventional manufacturing methods, 3D printing provides some advantages such as increasing material utilization and less waste of material. Maraging steel provides good strength and toughness without losing ductility, which has been used for the 3D printing technique. Selective laser melting (SLM) is one of the 3D printing methods, which is mostly used for metal and alloy powder. In this thesis, selective laser melting will be used for maraging steel. 3D printing maraging steel is a new material, the research about the properties of 3D printing maraging steel is still ongoing. Corrosion resistance is one of the most important properties of maraging steel due to the high cost of corrosion. So this thesis will focus on the corrosion behavior of 3D printing maraging steel. The purpose of this thesis was to find the best heat treatment condition for high corrosion resistance and to find the relationship between microstructure and corrosion behavior of maraging steel. In this thesis, several kinds of maraging steel samples with different heat treatment conditions were used. SLM, SLM austenized&quenched, SLM aged, conventional austenized&quenched, and conventional aged. Besides, two kinds of solutions were produced, NaOH (pH=11.5) and Na2SO4 (pH=6.5). To observe the microstructure, an optical microscope was used. The grain size is different between SLM and conventional samples, and also different between the samples with different heat treatment conditions. The potentiodynamic polarization method was used to measuring the corrosion behavior. SLM samples have much lower current density, and the passivation potential and the corrosion rate are similar compared with conventional samples. But due to the lack of further experiments, the relationship between corrosion behavior could be affected by the combined effect of several factors.

No description available.

In recent years, the automotive industry has turned to press hardened steels (PHS) to improve passenger safety while enabling vehicle weight reduction. To form the complex shapes required for this purpose, they are often direct hot press formed. It is possible to provide corrosion resistance to these parts by galvanizing the PHS sheets prior to direct hot press forming (DHPF). However, the austenitization of the galvanized steel causes the Zn-based coating to transform into two intermetallic phases. These are iron-rich α-Fe(Zn) and zinc-rich Г-Fe3Zn10. The Г-Fe3Zn10 is liquid during traditional DHPF, and the applied stress can result in liquid metal embrittlement (LME). Recently, two new grades of PHS have been developed, which allow for DHPF at 600-700°C, below the Fe-Zn peritectic temperature at 782°C, thus avoiding LME. These prototype PHS grades are designated 2%Mn (0.2C-2Mn-0.25Si-0.005B (wt%)) and 2.5%Mn (0.2C-2.5Mn-0.25Si-0.005B (wt%)). The objective of this work is to determine the effect of DHPF on the ability of a Zn-based coating to provide robust cathodic protection to the two prototype PHS. Galvanized panels of both the 2%Mn and 2.5%Mn steel were DHPF with a U-shape die at 700°C. The surface and cross-section of the coating were examined to determine the effects of DHPF on the coating surface. Die friction during DHPF resulted in die wiping on the wall of the part, leading to removal of surface Г-Fe3Zn10. In cross-section, coating cracks were present at the wall and corner of the U-shape part due to the deformation during DHPF. Potentiodynamic polarization scans were used to determine the corrosion potential of the coating, and this was used to calculate the driving force for cathodic protection using the difference in corrosion potential between the coating phases and the substrate. It was found that only Γ-Fe3Zn10 provided robust cathodic protection to both steel substrates, and the driving force for cathodic protection was lower for the coated DHPF 2.5%Mn steel. Galvanostatic scans were used to evaluate dissolution kinetics of coating phases. Robust cathodic protection was provided by the galvanized coating for austenitization times of 30 - 120 s for the 2%Mn substrate and 30 - 60 s for the 2.5%Mn substrate. The duration that robust cathodic protection was provided was shortest at the wall of the U-shape part. This result was attributed to die wiping caused by DHPF, where the surface is smoothed by die friction. When there is less Г-Fe3Zn10 in the coating, such as at longer austenization times, surface Г-Fe3Zn10 was removed and an increased amount of α-Fe(Zn) is exposed, which does not provide robust cathodic protection. In addition, coating cracks form along α-Fe(Zn) grain boundaries after austenitization for 180 s on all examined regions of the U-shape part, allowing a greater surface area of the coating exposed to electrolyte, further increasing dissolution of the coating. / Thesis / Master of Applied Science (MASc)

The aim of this work was to rationalize the corrosion resistance of candidate austenitic iron-chromium-nickel (Fe-Cr-Ni) alloys in supercritical water (SCW) for use as fuel claddings within the in-core structure of the Canadian supercritical water-cooled reactor (SCWR) concept. High chromium (Cr)-containing alloys (Alloy 800HT with 20.6 wt.% Cr and 30.7 wt.% Ni and Alloy 33 with 33.4 wt.% Cr and 31.9 wt.% Ni) in the mill annealed condition were chosen for this purpose. Coupons were exposed on a short-term basis (500 h) in a static autoclave containing 25 MPa SCW at 550 °C and 625 °C. Gravimetric measurements and electron microscopy techniques were then used to study the oxidation/corrosion resistance of two alloys. Alloy 33 was found to exhibit the higher corrosion resistance at both temperatures. The improved corrosion resistance of Alloy 33 was attributed to two factors: (i) the formation of a continuous Cr-rich corundum-type M2O3 (M= Cr and Fe) oxide layer that prevented the diffusion of Fe and the formation of a less-protective Fe/Mn-Cr spinel ([Fe,Mn]Cr2O4) outer layer, (ii) a sufficient residual bulk Cr in the Cr-depleted layer adjacent to the alloy/scale interface that prevented any localized internal oxidation from occurring. A mass balance conducted on the corroded Alloy 33 material suggested that volatilization of the corundum-type oxide layer did not occur, at least not within the short-term exposure in the essentially deaerated SCW. A key issue requiring further study was the observation of intermetallic precipitates that formed below the Cr-depleted zone adjacent to the alloy/scale interface in both alloys when exposed for 500 h at 625 °C and their possible influence on the in-service mechanical integrity. / Thesis / Master of Applied Science (MASc) / The supercritical water-cooled reactor (SCWR) is one of the six reactor design concepts developed by the Generation-IV International Forum (GIF). Canada is planning to build the SCWR within the next decades. However, selection of proper materials that perform well within such high pressure high temperature circumstances inside the reactor core with minimum degradation is a very imperative challenge. The current work has addressed this issue by studying the corrosion behaviour of Fe-Cr-Ni alloys in similar environment using electron microscopy techniques.

Corrosion and metal release mechanisms of CoCrMo alloys are at human biological conditions not fully understood. The main objective of this master thesis was to investigate whether the Vroman effect influences the extent of metal release from CoCrMo alloy in mixed protein solutions. The project focuses on the corrosion properties and release of cobalt (Co), chromium (Cr) and molybdenum (Mo) from a CoCrMo alloy into simulated physiological solutions of pH 7.2-7.4 in the presence of proteins. The metal release study was performed in phosphate buffered saline (PBS) for 4 and 24 h at 37 °C with and without different concentration of proteins (bovine serum albumin-BSA and fibrinogen-Fbn from bovine plasma). In order to investigate whether any Vroman effect could affect the extent of released metals in solutions, sequential tests were performed by sampling after 1, 4, 6 and 24 h in solutions that were partially replenished after 5 h. Significant metal-induced protein aggregation and precipitation were observed in solutions of physiologically-relevant protein concentrations (40 g/L BSA and 2.67 g/L Fbn). Cr was strongly enriched in the surface oxide of CoCrMo after exposure in all solutions. This was for all solutions accompanied by metal release processes dominated by Co. Based on electrochemical investigations, the electrochemical activity did not increase, but rather decreased, in protein-containing solutions as compared to PBS alone. This could possibly be explained by blocking of cathodic areas as a result of protein adsorption.

Yes / Titanium alloys are known for their excellent corrosion resistance; however, low surface hardness results in poor wear resistance, which limits its potential application. This study employs a novel two-step process to embed a hard Ni coating containing a mixture of nanosized particles (Al2O3 and TiO2) into the surface of the Ti-6Al-4V alloy using an electric arc produced during the inert tungsten gas welding process. The surface of the sample was evaluated using Vickers Microhardness, Scanning electron microscopy, Energy dispersive spectroscopy and pin-on-plate wear testing. Microstructural analysis showed that impregnating the titanium surface with Ni/(Al2O3 and TiO2) nanomaterials resulted in the formation of a hard martensitic structure to a depth of approximately 2 mm below the surface. The changes observed are driven by modification of the surface chemistry and the presence of nickel, causing grain size reduction, solid solution strengthening and dispersion strengthening of the treated layer by the nanoparticles. The hardness of the treated layer increased by more than 180% when 40 nm Al2O3 and 30 nm TiO2 particles were embedded into the surface. Similarly, the wear resistance of the treated surface improved by 100%.

The microstructure of steel is well known to affect hydrogen-induced cracking (HIC) susceptibility by having certain heterogeneities serving as effective hydrogen trap sites. A consensus on whether or not fine-scale niobium carbide (NbC), nitride (NbN) and carbonitride (Nb(C,N)) precipitates can behave as effective hydrogen traps has yet to be established. The H-trapping capacity of Nb precipitates in a Fe-C-Mn-Nb model steel was investigated with the goal of minimizing embrittlement effects and improving the design of X70 pipeline grade steel for sour service oil and gas applications. First, a heat treatment was applied to the model steel to change the Nb-based precipitate size distribution, which was subsequently characterized via transmission electron microscopy, electron energy loss spectroscopy, and atom probe tomography. The experimental heat treatment increased the number of fine-scale precipitates (<15 nm) that are ideal for APT characterization. NbN and NbC precipitates of various stoichiometries were confirmed within the steel. Further, a custom electrolytic H-charging device was designed, fabricated, and validated using thermal desorption spectroscopy. Additionally, the extent of galvanic corrosion between NbC and NbN and the steel matrix was determined using custom scaled-up particle matrix specimens. Potentiodynamic polarizations conducted using active and passivating electrolytes revealed the relative nobility of the materials. Both NbC and NbN particles were more noble than the steel matrix; thus, possessing driving force for galvanic corrosion, with the particles serving as cathodes. Future studies involving electrolytic charging of the steel in a D-based electrolyte coupled with atom probe tomography will facilitate the direct observation of H-trapping sites relative to various Nb-based precipitates and contribute to an improved understanding of the mechanisms governing HIC. / Thesis / Master of Science in Materials Science and Engineering (MSMSE)