Materials Science Forum Vols. 830-831

Abstract: The electrochemical corrosion and mechanical properties of ceramic coatings fabricated by plasma electrolytic coating (PEO) and hard anodizing (HA) methods comparatively examined for AA2219. Potentiodynamic polarization results revealed that the corrosion resistance of both coatings are comparable to each other. However, the indentation and scratch testing indicated that the hardness and modulus of the PEO coating was significantly higher when compared to HA coating. The critical load (Lc2) causing adhesive failure of the PEO coating was much high (19N) when compared to HA coating (10N) showing better adhesive strength of the PEO coating.Key words: Plasma electrolytic oxidation (PEO), potentiodynamic polarization, nanoindentation, hard anodizing

Abstract: In this study, two Mg-Zn-Gd alloys (Mg-5Zn-5Gd and Mg-5Zn-3Gd) are fabricated using high purity raw materials and by a clean melting process and subjected to immersion testing. The results indicate an increase in corrosion rate for increase in Gd content and the results are correlated using SEM, XRD and EDS analyses. It is seen that RE addition provides a protective oxide coating over the sample surface thereby assisting the corrosion resistance.

Abstract: The coating quality of nickel is important factor in improvement of wettability of carbon fibers to be used as reinforcing material in the production of carbon fiber reinforced metal matrix composites. In this research work, Polyacrylonitile (PAN) based carbon fibers have been Ni coated in Sodium hypophosphite reduced acidic bath by electroless plating method. These carbon fibers are coated using 4, 4.5, 5 and 5.5 pH values for 5, 10, 15, 20, 25 and 30minutes. Coating thickness is found to increase with time linearly. Nickel deposition rate per unit time increases with pH, however it reaches a maxima and then declines. The surface condition of fibers reveals that coating becomes more and more rough due to non uniform coating, as coating time and pH goes on increasing.

Abstract: Alumina coating was formed on AA7020 aluminum alloy by micro arc oxidation (MAO) method and its corrosion and stress corrosion cracking (SCC) behaviors were examined in 3.5 wt. % NaCl solution. Potentiodynamic polarization (PP) was used to evaluate the corrosion resistance of the coating and slow strain rate test (SSRT) was used for evaluating the environmental cracking resistance in 3.5% NaCl solution. Results indicated that MAO coating on AA7020 alloy significantly improved the corrosion resistance. However the environmental cracking resistance was found to be only marginal. Key words: aluminum, micro arc oxidation, x-ray diffraction, stress corrosion cracking

Abstract: The influence of the major process parameters for anodizing AA7075 is investigated to obtain an anodized layer with minimum porosity with approximately 15μm thickness. Anodization was carried out in a sulphuric acid bath with well-defined process parameters and the anodized layer thickness & Inter pore distance were measured for the chosen process parameter combinations. Porosity index was calculated and an optimized parameter combination was obtained. Voltage was found out to be most influencing parameter in the determination of porosity and thickness. In addition to this, surface finish of the non-anodized and anodized specimens was also compared. It was found out that surface finish of the specimen decreased after anodization. A statistical data model was prepared for thickness and porosity using the data obtained from experiments.

Abstract: Mg alloy shows higher specific strength and ductility relative to aluminum and also lower machining and casting costs over to steels. However, it also shows limitation to use in many industrial applications due to the relatively poor corrosion resistance in many aqueous solutions.Corrosion and passivation study of magnesium alloy (AZ91D) in various Cl ^̄ ions concentrations and temperatures were carried out using electrochemical impedance spectroscopy (EIS) in NaCl solutions of 0.4, 0.6, 0.8 & 1.0 molar (M) concentrations. The passivation behavior was also observed at temperature of 45°C, 55°C and 65 °C in 3.5 wt% NaCl solutions. The total polarization resistance was observed about 5530, 4030, 2465 and 2000 Ohms.cm² in solutions of 0.4, 0.6, 0.8 & 1.0 M NaCl respectively indicate reduction of film stability at higher chlorides concentration. A similar trend was found on increasing temperatures of 3.5 wt% NaCl solutions at 45°C, 55°C and 65 °C and noticed about 970, 600 and 300 Ohms.cm² respectively showed significant decline of passivity and more pitting tendency.

Abstract: Electrocatalytically active Ni-W alloy coatings have been developed through compositionally versatile electrodeposition method on copper substrate from tri-sodium citrate bath, using glycerol as the additive. The deposition conditions have been optimized for peak performance of their electrocatalytic behavior, like hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in 1M KOH. Corrosion behaviors of the coatings have also been tested under study conditions of electrocatalysis. Electrocatalytic behaviors were tested by cyclic voltammetry (CV) and chronopotentiometry techniques. Experimental results demonstrated that Ni-W alloy coatings, deposited at low and high current densities (c. d.) were showing superior performance for OER and HER, respectively. Better electrocatalytic activity for HER with increase of deposition c. d. was attributed by the unique phase structure, surface morphology and chemical composition of the coatings, confirmed by XRD, SEM and EDX analysis. The dependency of coating thickness and hardness on HER and OER were analyzed, and results are discussed.

Abstract: A new alkaline bath has been proposed for deposition of bright Sn-Ni alloy coatings on mild steel (MS). Depositions were carried out at different current densities (c.d.) and their corrosion behaviors were studied in 5 % NaCl solution by electrochemical AC and DC methods. Sn-Ni coating, deposited at low c.d. i.e. at 1.0 A/dm² was found to be the most corrosion resistant compared to those at other higher c.d., even up to 4.0 A/dm². This least corrosion rate (CR) is attributed to high wt. % Sn in the deposit. Increase of CR at high c.d. range is due to decrease of wt. % Sn, explained by the observed anomalous type of codeposition, followed by the bath. Regardless of the deposition c.d., the bath developed bright coatings, inherent of Sn-Ni alloy. Experimental results are discussed taking in account of the phase structure, composition and surface morphology of the coatings, evidenced by X-ray diffraction (XRD), Energy Dispersive X-ray (EDX) and Scanning Electron Microscopy (SEM) analysis.

Abstract: Hastelloy C-276 is a nickel-based super alloy which has inbuilt corrosion resistance and exhibits low wear resistance. In this work, an attempt has been made to improve the tribological properties of this alloy without sacrificing the corrosion properties by laser surface melting in argon atmosphere. The results indicated better grain refinement at optimum laser parameters of 1.5 kW and 300 mm/min thereby exhibiting a maximum hardness of 447 HV.The corrosion rate for the entire laser treated samples showed a decreasing trend with a minimum value of 2.07172 x10^-2 mm/year, compared to base alloy. This paper demonstrates that laser surface melting is a viable method to improve the wear and corrosion properties of Hastelloy C-276.

Abstract: Boronizing is a surface thermochemical treatment in which boron atoms are made to diffuse into a metallic surface at high temperatures. A nano-crystalline surface with larger defect density assists in enhancing the diffusion rate even at low temperatures. In the present work Ni-Cr-Mo steel is subjected to a surface mechanical attrition treatment (SMAT) to activate the surface with nanocrystalline structures and crystal defects. Subsequently the samples were boronized at low temperature regime (400°C - 600°C) for 5 hours using a pack boronizing technique. The microstructure, chemical analysis and hardness of borided layers were investigated using optical microscope, SEM – EDX and Microvicker’s Hardness Tester. The SMAT treated samples showed severe plastic deformation of the surface, nano-structured grains (10-30 nm) and larger defect density illustrating mechanically activated surface for diffusion. The boronizing had clearly demonstrated the diffusion of boron even at 400°C. The thickness of diffused layer was found to be about 20 µm at 400°C and 50 µm at 600°C for SMAT samples while the untreated samples showed practically no diffusion at 400°C and 12 µm at 600°C. The SEM-EDX results had confirmed the presence of boron at the diffused layer; however the hardness was found to be low. A maximum of 650 HV_0.3 was achieved by low temperature boronizing of SMAT treated samples.