S. Skolianos

A study on pitting behaviour of AA2024/SiCp composites using the double cycle polarization technique.

Abstract A double cycle polarization technique was used to estimate the pitting potential ( E pit ) of AA2024/SiC particle composites. The volume fraction of SiC particles was 8, 14, 19 and 24%. The composites and the unreinforced alloy were potentiodynamically polarized in aerated 3.5 wt.% NaCl aqueous solution. Pitting potential was clearly visible in the first anodic part of the 2nd cycle and varied between −640 and −665 mV for all the composites, as well as for the monolithic alloy. However, significant differences were observed in the residual passivation current density ( I r ) (extracted from the 2nd polarization cycle) which decreased successively as the SiC p volume fraction of the composites increased. Metallographic examination of the composites and the unreinforced alloy polarized at 50 mV above E pit showed that pits were fewer as the SiC p content increased, whereas for the AA2024/19 vol.% SiC p , no pits were observed. Line scanning acquisition showed that Cu microsegregation in the a-phase fluctuates to a greater extent in composites than in the plain alloy and is more intense as the volume fraction of SiC p increases. As a result, more pitting initiation sites are created, in accordance with the protection potential values ( E prot ), which are shifted in the electronegative direction as the SiC p content increases.

Effect of applied pressure on the microstructure and mechanical properties of squeeze-cast aluminum AA6061 alloy

Abstract Aluminum AA6061 alloy was squeeze cast under different pressure levels. The microstructure and mechanical properties of these castings were examined in the as-cast and heat-treated conditions to determine the effect of the squeeze pressure on the former. It was found that squeeze casting decreased both the volume fraction porosity and the micropore size as well as the size of the dendrites and of the interdendritic areas. The ultimate tensile strength of the as-cast and heat-treated castings increased with the application of pressure. However, the increase appeared to be independent of the magnitude of the applied pressure. The elongation was seen to reach a maximum at a pressure of 60–80 MPa, for all the samples, as-cast and heat-treated. The strength of the casting was also found to increase with increasing volume fraction of Mg 2 Si, and decrease with increasing volume fraction of (FeCrMn) 3 SiAl 12 present in the casting.

Fabrication and examination of oxidation resistance of zinc coated copper and brass components by chemical deposition

Abstract In this work, the structure and the oxidation resistance of Zn deposited Cu and brass metallic components are examined. The deposition was accomplished with pack cementation chemical deposition. The examination of the samples was performed with electron microscopy and X-ray diffraction analysis. It was found that coatings on Cu substrate consist of two layers with different Zn concentrations, while coatings on brass were single layered with almost constant Zn concentration. The presence of distinct Zn–Cu phases was revealed in both cases. The subjection of the as coated samples together with the uncoated substrates in air at 400°C showed that both Zn coated samples have enhanced resistivity in such atmospheres, as most of the coating remained mostly unoxidised, and the substrates were fully protected. On the contrary, the bare substrates appear to have undergone severe damage as brittle oxides were formed on their surface.

Corrosion resistance of porous NiTi biomedical alloy in simulated body fluids

The corrosion performance of two porous NiTi in physiological and Hanks solutions was investigated by potentiodynamic polarization, cyclic polarization and impedance spectroscopy. Electric models simulating the corrosion mechanism at early stages of immersion were proposed, accounting for both microstructural observations and electrochemical results. Results indicate that both porous samples were susceptible to localized corrosion. The porosity increase (from 7% to 18%) resulted in larger and wider pore openings, thus favoring the corrosion resistance of 18% porous NiTi. Strengthening of corrosion resistance was observed in Hanks solution. The pore morphology and micro-galvanic corrosion phenomena were determining factors affecting the corrosion resistance.

Optimizing Ceramic Preform Properties for Liquid Metal Infiltration

In this paper, a methodology to optimize the production technique and properties of fly ash porous preforms, for liquid metal infiltration, is introduced. The preforms were produced by mixing fly ash with acid phosphate binder and acetone as liquid carrier, at varying compositions. The homogeneity of the green product was determined by micro computed tomography (μCT) and scanning electron microscopy. The effect of binder quantity, porosity, and compaction pressure of the preforms on the resulting bending strength was investigated and considered as one of the criterions for the production of crack-free metal matrix composites (MMCs). Finally the quality of the MMCs, resulting by liquid metal infiltration of the optimized preforms, was examined by μCT to locate micro cracks and uniaxial compression to determine variations in their compressive strength.

Corrosion behaviour of 304 stainless steel in simulated oilfield produced water

Corrosion behaviour of 304 austenitic stainless steel in simulated oilfield produced water at ∼25°C was investigated. The electrochemical properties of 304 stainless steel were assessed experimentally employing potentiodynamic polarisation and electrochemical impedance spectroscopy measurements. The corrosion mechanism of 304 stainless steel in simulated oilfield produced water along with appropriate modelling equivalent electrical circuits were deducted. Results showed that the corrosion rate is reduced with time. This behaviour does not indicate protection of 304 stainless steel. Instead, pits were formed on the surface of the specimens, and their evolution continued with time.

EXAMINATION OF THE OXIDATION PROTECTION OF ZINC COATINGS FORMED ON COPPER ALLOYS AND STEEL SUBSTRATES

The exposure of metallic components at aggressive high temperature environments, usually limit their usage at similar application because they suffer from severe oxidation attack. Copper alloys are used in a wide range of high‐quality indoor and outdoor applications, statue parts, art hardware, high strength and high thermal conductivity applications. On the other hand, steel is commonly used as mechanical part of industrial set outs or in the construction sector due to its high mechanical properties. The aim of the present work is the examination of the oxidation resistance of pack cementation zinc coatings deposited on copper, leaded brass and steel substrates at elevated temperature conditions. Furthermore, an effort made to make a long‐term evaluation of the coated samples durability. The oxidation results showed that bare substrates appear to have undergone severe damage comparing with the coated ones. Furthermore, the mass gain of the uncoated samples was higher than this of the zinc covered ones. P...

Microstructural Evolution and Corrosion Behavior of Carburized α-Fe Plates by Glucose

The aim of this paper is to obtain an experimental characterization of glucose-carburized substrates of α-Fe. The carburization process was achieved under vacuum condition using glucose as a carburizing medium. The process was carried out at several temperatures keeping the duration constant at 2h. The samples were treated at 400°C, 650°C and 900°C. The microstructure of the as produced coatings was observed by Scanning Electron Microscopy (SEM) and the formed phases were analyzed by X-Ray Diffraction (XRD). To investigate the corrosion properties of the carburized iron specimens, electrochemical tests were conducted. The samples were exposed to a solution of 3.5% wt. NaCl electrolyte, under quiescent conditions at room temperature and open to the air. The corroded samples were observed with use of Optical Microscopy in order to evaluate the corrosion effect on their surfaces. Carburization of iron samples at temperatures up to 650°C resulted in a shift of the polarization curves to lower current densities. Smaller corrosion rates were measured indicating higher corrosion resistance for these specimens.

Evaluation of the Corrosion Behavior and Hardness of Glucose-Carburized Steel in Comparison with Fe2B Borided Layers

The paper aims to compare the hardness and corrosion behavior of carburized and borided Iron-Based Austenitic Steels (IBAS) which were treated at the same temperature and time under vacuum conditions. Boronizing was performed by pack cementation in Ekabor II powder and carburization was carried out by annealing under vacuum conditions using glucose as a carburizing medium. Both boronizing and carburizing procedures were performed at 900°C for 3h. The characterization of the treated materials was assessed by using X-Ray Diffraction (XRD) analysis, optical observations, Scanning Electron Microscopy (SEM), Energy-Dispersive X-ray Spectroscopy (EDS) measurements and micro-Vickers hardness tests. Potentiodynamic polarization curves were also used to determine electrochemical characteristics such as the corrosion potential (mV), corrosion current density (μΑ/cm2) and corrosion rate (μm/year). The results showed that borided samples with a single phase layer, exclusively Fe2B strongly toothed, exhibited better hardness behavior towards carburized samples but lesser corrosion resistance instead.

Microstructure and Growth Mechanism of Glucose-Carburized Nickel Substrates

In this research, efforts were made to study the modification of microstructure of pure Ni matrices. Modification was attempted using glucose as carburizing medium under a control of heat treatment conditions. Nickel plates were carburized under vacuum conditions at 380°C and 650°C for 3 hours. In order to determine the parameters of the carburizing, thermal properties of glucose along with the thermochemical behavior were examined by Thermogravimetric Analysis (TGA) and Differential Thermal Analysis (DTA). The characterization of the microstructure of the carburized specimens was investigated by X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM) and Energy-Dispersive X-ray Spectroscopy (EDS). For analyzing the effect of temperature treatment on corrosion resistance, electrochemical corrosion tests were conducted. It was observed that the polarization curves for carburized samples at 380°C were shifted to lower corrosion current densities. Consequently, lower corrosion rates were achieved for these samples preventing the formation of extensive corrosion over their surfaces comparing with carburized Nickel substrates at 650°C.