Materials Science Forum Vol. 941

Abstract: Alloy 617 OCC, a variant of INCONEL 617 with optimised chemical composition, has been produced in India for manufacture of superheater and reheater tubing in boilers operating in advanced ultrasupercritical (A-USC) power plants. The tubing encounters intense hot corrosion conditions during service. The present study deals with hot corrosion behaviour of 617 OCC in A-USC environment. The environment occurring in A-USC plants was simulated in the laboratory by exposing the material coated with a mixture of salts at 700°C in a flowing gas mixture. For use in A-USC boiler technology, the metal loss due to fireside corrosion of the material should be less than 2 mm in 200,000 hours. The loss obtained in the present study was nearly 5 times this value. The corrosion processes were studied using SEM/EDAX, XRD and thermogravimetry. The degradation mechanisms coming into play, disqualifying the material for use in A-USC plants, would be discussed.

Abstract: The intergranular corrosion behavior of friction-stir-welded 7N01-T5 aluminum alloy joints was investigated by using sliced samples extract from the top, middle and bottom layers of the joint, with the sliced direction parallel to the welding direction. The relationship between microstructure and local corrosion property of the welded joints was analyzed. The results show that the heat-affected zone (HAZ) of the weld exhibit the highest susceptibility to intergranular corrosion, and the corrosion degree of the top and bottom surface layers were more serious than the central layer in the HAZ, continuously dispersed precipitates on grain boundaries and the grains size may be the main factor which caused the different corrosion severity of HAZ. The thermo-mechanically affected zone (TMAZ) is characterized by a highly deformed structure, and the temperature reached in this local zone during welding is high enough for the strengthening precipitates, especially the precipitates on the grain boundary to be partially re-dissolved. The temperature gradient caused resistance to intergranular corrosion of TMAZ reduced from top to bottom surface. The stirred zone (SZ) experienced heavily plastic deformation and temperature cycling, the sensibility of intergranular corrosion was low, which is attributed to the sufficient re-dissolve of the strengthening precipitates during welding.

Abstract: Sintered magnesium alloys, which were fabricated by Spark Plasma Sintering (SPS) method, were examined to study corrosion characteristics by electrochemical method, XRD and EPMA. The binary mixtures alloys of a low-melting-point metal powder (Sn, Bi, Sb) of1.0 vol.% and the pure magnesium powder were prepared. In the Mg-1.0vol.%Bi and Mg-1.0vol.%Sn, Mg₃Bi₂ and Mg₂Sn precipitates was recognized by XRD, respectively. In addition, formation of oxide along powder particle boundaries was observed by EPMA elemental mapping in all specimens. In the case of Mg-1.0vol.%Zn, precipitation of metallic compounds was not recognized by these experiments. According to the results of polarization curve measurements, the Mg-1.0vol.%Bi shows highest corrosion potential. However, corrosion rate which was estimated by Tafel method is relatively larger than other alloys due to Mg₃Bi₂ precipitation. This result suggests that Mg₃Bi₂ acts as cathode site. The Mg-1.0vol.%Sn shows superior corrosion rale in these alloys.

Abstract: The current medical technology necessitates the usage of biodegradable metals like Magnesium (Mg) as the future implant material due to the numerous benefits it can provide. Therefore, new Magnesium-based rare earth alloys targeting biomedical applications were synthesized using Disintegrated Melt Deposition (DMD) technique followed by hot-extrusion. In this investigation, Zinc (Zn) and Erbium (Er) were chosen as alloying elements to provide suitable strengthening effect and Mg-2Zn, Mg-2Zn-2Er alloys were synthesized. With the addition of alloying elements, the grain size was reduced and several MgZn intermetallics were formed. Corrosion studies of as-extruded materials were done in 0.5 wt.% NaCl solution to elucidate the microstructure-corrosion relationship. Improved corrosion resistance is seen in the alloys in comparison to pure Magnesium. Addition of Erbium is seen to improve the protectiveness of the surface film formed during immersion. Both these elements have proven to increase the corrosion potential of Mg in NaCl solution.

Abstract: Hot-dip galvanized steel surfaces are cleaned, pre-treated with titanium-containing solution and then painted for use. Removal of the alumina layer from galvanized steel surfaces during the cleaning process is essential before effective paint application. Bad adhesion results if the alumina layer is not completely removed, and an insufficient concentration and uneven distribution of titanium oxide is formed across the galvanized surface during pre-treatment. The alkaline cleaner concentration must be optimized to ensure effective removal of the alumina layer. Hot-dip galvanized steel samples were cleaned using typical line conditions and cleaning solutions with varying free alkalinities. The alumina layer was then measured on each sample by glow-discharge optical emission spectroscopy. Thereafter, the samples were treated with titanium-containing pre-treatment solution. The titanium oxide concentration was measured by inductively coupled plasma optical emission spectroscopy. It was found that a free alkalinity of at least 3.2 mEq/L is required to fully remove the alumina layer. The alumina-free samples also gave a titanium oxide layer after pre-treatment within the target concentration of 4–8 mg/cm². A free alkalinity of 3.5 ml was thereafter implemented commercially: analysis of samples from two galvanizing lines showed no presence of alumina, a uniform titanium oxide distribution across the widths of the line samples and acceptable titanium concentrations.

Abstract: Various aging treatments were conducted on AISI 630 martensitic precipitation hardening stainless steel in order to optimize aging condition. Aging treatment was carried out with Ar gas in the chamber at temperature of 380°C, 400°C and 430°C and times of 2, 4 and 8 h. After obtaining the optimized aging condition, several nitrocarburizing treatments were done with and without the aging treatment. Nitrocarburizing was performed on the samples at 400V with a gas mixture of H₂, N₂ and CH₄ for 15 hrs at 4.0 Torr vacuum pressure. The aging condition was optimized as 400°C temperature for 4 h. Better results were found when the samples were experimented with aging + nitrocarburizing treatment. In this combined process, the corrosion resistance improved noticeably which is confirmed by potentiodynamic polarization test and by observing the surface of the samples after corrosion test. Moreover, the surface hardness increases up to 1300 HV_0.05 approximately and α'_N layer thickness increases up to 11 μm (approximately).

Abstract: The effect of gas content and treatment temperature on the surface characteristics of hardened layers of low temperature plasma nitrided 316L Austenitic Stainless Steel was investigated. The process was performed at fixed processing time at 15Hr and changing the N₂ content from 10% to 25%, changing the treatment temperature from 370 °C to 430 °C. The surface hardness and the thickness of expanded austenite (S-Phase) layer increase with increasing both temperature and nitrogen content. At 430°C processing temperature and 25% N₂ content, the S-phase layer thickness increased up to 13 μm and surface hardness increased about 3.5 times of that of untreated sample which is 880 HV_0.1 (before treatment 250HV_0.1 hardness). However, the XRD pattern of the this sample shows the presence of γ'-(Fe,Cr)₄N precipitation on the surface which is the reason for the increased corrosion current density that results in increased corrosion rate. Thus the corrosion resistance degrades and presents even worst behavior comparing that of before treatment. Therefore, for increasing the corrosion behavior, further research was conducted by fixing the N₂ content at 10% with changing the CH₄ content from 0% to 5%. The best results were found when treated at 430°C with 5% CH₄ content. At this condition the S-phase thickness increase to around 17μm and surface hardness (980 HV_0.1) is improved about 3.9 times of the hardness of untreated one. This sample also showed superior corrosion resistance than the other treated samples and the untreated one due to much higher pitting potential.

Abstract: Along with high tensile strength, high carbon steel wires must possess sufficient torsional ductility is to avoid longitudinal splitting along the wire axis, known as delamination. Often, wires ductile in the bright (asdrawn) surface condition exhibit delamination failure after undergoing a post-drawing surface treatment such as hot-dip galvanizing. The objectives of this study were to examine the influence of postdrawing heat treatment time and temperature on drawn wire mechanical properties, and to identify possible treatment conditions that suppress delamination. Stelmor cooling or lead patenting prior to drawing were used to develop a pearlitic microstructure. Salt pot heat treatments simulated postdrawing heat treatments; experimental heat treatments were conducted between 325 °C and 475 °C for 20 s or at 450 °C for immersion times between 1 s and 20 s. Tension and torsion tests quantified the changes in mechanical properties due to aging. Lead patented wires experienced greater tensile strength and torsional ductility changes with aging time and temperature when compared to the Stelmor cooled wires. This increased sensitivity was attributed to greater dislocation recovery in the patented wire after drawing.

Abstract: Using density functional theory (DFT) we investigated the adsorption of chlorine atom on aluminum surface in the orientations, (100), (110), and (111). The structural and thermodynamic properties of chlorine atoms adsorbed on the Al (100), Al (110), and Al (111) surface for chlorine coverages of 1/9, 1/4, 1/3, 1/2, 3/4, and 1 monolayer (ML) are calculated. The largest bond strength is observed for a top, fcc, and hcp mixed structure at 3/4 ML coverage on Al (111). Adsorption free energy analysis reveals that the chlorine coverage of 3/4 ML of Al (111) is the most thermodynamically stable over the widest range of chlorine chemical potential and 1 ML of Al (111) is thermodynamically unstable, whereas various chlorine coverages of Al (100) and Al (110) take stable phase for a range of chlorine chemical potential.

Abstract: A facile method is introduced for production of micro-nanostructured silicone rubber surfaces by means of direct replication using a compression molding system. The fabricated samples possessing surface roughness display water contact angle of more than 160o and contact angle hysteresis (CAH) and sliding angle of less than 5o. Such low surface wettability of silicone specimens verifies the induced superhydrophobic property. Chemically etched aluminum surfaces could work excellently as templates whose patterns were replicated on the rubber surfaces successfully. Various etching conditions were examined. Surface characterization techniques revealed the presence of micro-nanostructures on the produced silicone surfaces.