Aluminum alloys have favorable properties such as light weight high strength high ductility and good machinability Although the oxide film formed on aluminum alloys due to the oxygen in the air enhances the corrosion resistance it is not enough to meet the needs in industry In this dissertation we investigated anodic aluminum oxide (AAO) and micro–arc oxide (MAO) films on 6061 aluminum alloy (AA6061) to improve the corrosion resistance Our study of AAO films can be divided into three sections In the first section the effects of an applied current density (0 3–3 A/dm2) and sulfuric acid concentration (1–5 M) over a wide range on the AA6061 alloy are discussed Applying high current density resulted in a higher growth rate of the oxide; however this also made the oxide layer more porous and less protective Increasing the sulfuric acid concentration also enhanced the reaction rate The vigorous reaction that took place in the electrolyte with the high sulfuric acid concentrations of 3–5 M led to the deposits cracking which deteriorated the corrosion resistance In the second section the effects of electrolyte concentration current density and anodization-time parameters on the evolution of the corrosion resistance for the AA6061 alloy were investigated using the Taguchi method Each anodization parameter had three levels using an experimental set of L9 orthogonal arrays Variations in the bias voltage were recorded during the AAO process and an energy-dispersive spectrometer was used to analyze the variations in the composition of the oxidized films Variations in surface morphology and cross-section of the AAO were examined using a scanning electron microscope Finally the potentiostatic polarization method was used to characterize the relative corrosion resistance of the oxidized films In the third section the crack formation and corrosion behavior of the AA6061 oxide films were investigated A number of cracks formed on the surface of the anodized AA6061 oxide film at the high electrolyte concentration of 5 M and low current density of 0 3 A/dm2 Moreover increasing the current density suppressed crack formation at the high concentration of 5 M; alternatively decreasing the electrolyte concentration was also helpful for eliminating cracks at a constant current density of 1 A/dm2 Moreover the more the cracks the higher the corrosion current was Suppressing crack formation was crucial for promoting the corrosion resistance of the AA6061 film with less corrosion current The best corrosion resistance of the anodized films with Icorr = 8 516 × 10–11 A/cm2 was attained with 1 M sulfuric acid a current density of 1 A/dm2 and an anodization time of 20 min Our study of MAO films can be divided into three sections In the first section the specimens were oxidized at the various direct current densities of 1–3 A/dm2 in a 1 g/L NaOH and 4 g/L Na2SiO3 electrolyte solution for a reaction time of 60 min to characterize the micro–arc oxidation behavior The thickness of the oxidized films was roughly linearly proportional to the applied current density The vigorous reaction that occurred at the higher current densities of 2–3 A/dm2 led to more thermal stress in the deposits which then cracked thereby deteriorating the corrosion resistance In the second section the current density and reaction time were fixed at 1 A/dm2 and 60 min The electrolyte comprised 1 g/L of sodium hydroxide with the different concentrations of 4 8 and 12 g/L sodium silicate solution added With the NaOH solution at the concentration of 1 g/L the increases in sodium silicate (4–12 g/L) concentration led to the initial arc voltage decreasing (380–320 V) With the sodium hydroxide solution at the concentration of 1g/L the thickness of the MAO films was not significantly changed when the sodium silicate concentration was varied between 4–8 g/L But when the sodium silicate concentration was enhanced to 12 g/L the MAO films grew thicker In the third section the specimens were oxidized at the various reaction times of 30–90 min in a concentration of 1 g/L sodium hydroxide and 4 g/L sodium silicate electrolytes solution at a fixed current density of 1 A/dm2 to characterize the micro–arc oxidation behavior The thickness of the oxidized films was roughly linearly proportional to the applied reaction time The reaction time of 90 min led to the accumulation of more thermal stress in the deposits which then cracked thereby deteriorating the corrosion resistance The best corrosion resistance of the MAO film with Icorr=2 945×10–10 A/cm2 was attained at a current density of 1 A/dm2 under a concentration of 1 g/L NaOH and 4 g/L Na2SiO3 electrolyte solution and with a reaction time of 60 min
A Study on Oxidation and Corrosion Resistance Behavior of 6061 Aluminum Alloy
逸駿, 鍾. (Author). 2017 8月 9
學生論文: Doctoral Thesis