Seong-Jong Kim

Improvement of hydrogen embrittlement and stress corrosion cracking by annealing for Al-4.4Mg-0.6Mn alloy

Abstract With no annealing treatment, cathodic polarization trends in 5083F Al alloy revealed concentration polarization and activation polarization. However, the annealed specimens have lower current densities at corrosion protection potential compared to the non-annealed specimen. The results of SSRTs conducted in seawater at the applied potential range of −1.8 V to −0.5 V indicated that the maximum tensile strength, elongation, and time-to-fracture had high values at applied potentials of −0.7 to −1.4 V. The maximum tensile strength, elongation, and time-to-fracture decreased when the potential values were beyond this range in either anodic or cathodic direction. In general, the increased shear lip caused by annealing treatment indicates elongation. Time-to-fracture would likely increase with elongation. Potentials between −0.5 V to −0.6 V were found to be in the region of stress corrosion cracking. The corrosion protection zone was determined to be −0.7 V to −1.4 V because these potential ranges produced good mechanical properties. Potential less than −1.4 V produced a fractured surface with a mixture of dimples (ductile fractures) and a quasi-cleavage pattern resulting from the effects of hydrogen gas.

Effects of silicate ion concentration on the formation of ceramic oxide layers produced by plasma electrolytic oxidation on Al alloy

Plasma electrolytic oxidation (PEO) coatings were fabricated on 5083 Al alloy in KOH electrolyte solution with adding various concentrations of Na2SiO3. Changes in voltage?time response and micro-discharge evolution were analyzed, and the surface and cross-section of the resulting coating layer were further characterized by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and X-ray diffraction (XRD). The results showed that discharge characteristics were evidently changed with different Na2SiO3 concentrations, particularly higher Na2SiO3 concentrations leading to lower dielectric breakdown voltages. It was found that porous surface structure became prevalent with increasing Na2SiO3 concentration. The EDS analysis confirmed the incorporation of Si element in the PEO coatings. The result of XRD analysis revealed that metastable phases such as ?- and ?-alumina were produced as a result of PEO, while amorphous phases appeared with excessive Na2SiO3 concentrations (10 and 14 g/L). The coating thickness was significantly increased about 2?8 times with increasing Na2SiO3, almost depending on Na2SiO3 concentration.

Evaluation of Electrochemical Characteristic and Investigation on Optimum Condition in Friction Stir Welding for 6061-T6 Al Alloy

In friction stir welding for 6061-T6 with various traveling speed and rotation speed conditions, the best mechanical characteristics presented in traveling speed of 507 mm/min and rotation speed of 1100RPM. The maximum tensile strength and yield strength increased with the increasing of traveling speed. The result of the electrochemical characteristic evaluation in friction stir welding at optimum conditions for 6061-T6 Al alloy presented a good characteristics compare to base metal.

Influence of Na2SiO3 addition on surface microstructure and cavitation damage characteristics for plasma electrolytic oxidation of Al–Mg alloy

Recently, plasma electrolytic oxidation (PEO) has emerged as a promising surface modification technique to improve surface properties of Al alloys. In this study, PEO coating process for Al–Mg alloy was conducted with two different electrolyte solutions under the same electrical parameters: one was potassium hydroxide (KOH) aqueous solution, and the other involved potassium hydroxide aqueous solution with sodium silicate (Na2SiO3). The surface morphology was observed with scanning electron microscope (SEM) and elemental compositions were identified with energy dispersive spectroscopy (EDS) analysis. The chemical structures of PEO coatings were identified by X-ray diffraction analysis. Cavitation experiment was performed using ultrasonic vibratory cavitation erosion testing apparatus. Cavitation damage of PEO coatings was characterized using SEM and three-dimensional (3D) microscope. The result indicated that the surface of Al–Mg alloy were successfully modified having complete different surface morphologies by changing electrolyte composition. It was found that the surface morphology had a great influence on the cavitation damage behavior of PEO coating.

Electrochemical Behavior of CoNiCrAlY/ZrO2–Y2O3 Coated Layers with Atmospheric Pressure Plasma Technology in Seawater

Application of surface treatment has become common for protecting machine parts from oxidation, abrasion and corrosion induced by external environment. In particular, thermal spraying techniques are widely employed to improve wear, corrosion and thermal resistance. And compared to other methods they are simple and cost effective. However, the presence of porosity in the thermal spray coating can be highly detrimental because it provides access to penetration of corrosive matters, lowering corrosion resistance. Therefore, this research evaluate the electrochemical behavior under marine environment for aluminum–bronze alloy coated with MCrAlY and yttria-stabillized zirconia (YSZ) by atmospheric pressure plasma (APP) coating technology. Further application of carbon-based sealer removed voids and defects in the coating. The result reveled that, in case the voids and defects are completely removed, excellent corrosion resistance can be archived by application of good coating material along with formation of compact sealing layer.