열처리에 의한 알루미늄 합금의 내식성 개선

Abstract

Aluminum is a active metal that owes its resistance to a thin, protective barrier on the oxide surface layer, which is stable in the air and neutral aqueous solution. Thus Al alloys are widely used in architectural trim, cold & hot-water storage vessels and piping. However Al and most of its alloys may be corroded with some forms such as pitting corrosion, intergranular corrosion and galvanic corrosion in case of exposure to various industrial and marine atmosphere. Therefore, some kinds of metal such as Mn, Mg, Si or Cu are added to pure Aluminium and were carried out heat treatments for improving mechanical property and corrosion resistance.

In this study, relative evaluations of corrosion resistance for three types of Al alloy (ALDC 3, ALDC 5 and ALDC 12) were carried out with electrochemical methods such as variation of corrosion potential, anodic/cathodic polarization curves, cyclic voltammogram, AC impedance, SEM photograph and Vickers hardness measurement etc. And the effect of corrosion resistance improvement by heat treatments for ALDC 12 was investigated by the electrochemical methods.

Chapter 3.1 shows the difference of electrochemical property between these three kinds of Al alloy mentioned above. It was known that corrosion potential is shifted to positive or negative direction only by alloying components regardless of corrosion resistance. Moreover the data of electrochemical properties for the corrosion resistance evaluation obtained from cathodic polarization curve, cyclic voltammogram and AC, DC impedance showed a good correspondence each other. However variation of corrosion potential, passivity current density of anodic polarization curve and corrosion current density by Tafel extrapolation and Stern-Geary method did not correspond with considerably not only each other but also the data of electrochemical properties discussed above. Therefore, it is suggested that an optimum electrochemical evaluation for corrosion resistance of Al alloys should be calculated by the diffusion limiting current density of cathodic polarization curve, impedance of AC or DC and polarization resistance of cyclic voltammogram. And from those results, ALDC 5 showed the best corrosion resistance than other Al alloys. On the contrary, its resistance of ALDC 12 was comparatively lower than those, indicating intergranular corrosion.

Chapter 3.2 shows the effect of annealing heat treatments (annealing temperature at 100oC, 200oC, 300oC, 400oC, 500 for 1h respectively) to corrosion resistance of ALDC 12. The value of Vickers hardness was decreased through all range of annealed temperatures compared to non heat treatment. However corrosion resistance was relatively improved with annealing heat treatment. Especially, annealed at 500oC, the improving effect of corrosion resistance was the highest than other annealing temperatures without indicating intergranular corrosion. Therefore, it is suggested that optimum annealing temperature for corrosion resistance should be at 500oC. But recommendable temperature for increasing mechanical properties and corrosion resistance should be at 200oC.

Chapter 3.3 shows the effect of solution heat treatment (for 1hr at 500oC) and aging treatment (artificial aging: for 5hrs at 160oC , and natural aging: for 36hrs at room temperature) regarding corrosion resistance of ALDC 12. In case of water quenching after solution heat treatment, their Vickers hardness is relatively decreased compared to normalizing after solution heat treatment. Especially the value of Vickers hardness by natural aging after water quenching shows the lowest value. However, artificial aging after normalizing increased considerably the Vickers hardness than other heat treatments. The best optimum heat treatment for improving corrosion resistance is a natural aging after water quenching, but intergranular corrosion was observed. Intergranular corrosion could be restrained slightly by normalizing than water quenching after solution heat treatment.

Eventually it is known that ALDC 12 can be improved the corrosion resistance and the mechanical properties by heat treatments such as annealing, solution heat treatment and aging at the economical points of view.