AC8A 알루미늄합금 주조재의 열처리에 의한 특성 평가

Aluminum is a active metal and it is well known that its oxide film exhibits a thin and protective barrier which is comparatively stable in both air and neutral aqueous solution. Thus, Aluminum alloys have been widely applied in architectural trim, cold & hot-water storage vessels and piping etc., furthermore, the aluminum alloy of AC8A have been widely used in mold casting material of engine piston because of its properties of temperature and wear resistance. In recent years, the oil price is getting higher and higher, thus the using of oil of low quality have been significantly increased at engines of ship and vehicle. Therefore it is considered that evaluation of corrosion resistance as well as wear resistance of AC8A material is also important to improve its property and prolong its lifetime. In this study, the effect of solution and tempering heat treatment to corrosion and wear resistance is investigated with electrochemical method and measurement of hardness. The hardness decreased with solution heat treatment compared to mold casting condition, however, its value increased with tempering heat treatment and exhibited the highest value of hardness with tempering heat treatment(temperature at 190OC for 8 hrs). Furthermore, corrosion resistance was increased and decreased with decreasing and increasing of hardness respectively. As a result, it is suggested that the optimum heat treatment to improve both corrosion and wear resistance is tempering heat treatment(temperature at 190OC for 2 hrs).목 차ⅰ List of Figuresⅲ List of Tableⅴ 1. 서 론2 2. 이론적 배경4 2.1. 부식의 이론4 2.2. 부식의 전기화학적 원리6 2.3. 양극과 음극의 정의 9 2.4. 부식의 발생 9 2.5. 기전력 계열과 갈바닉 계열11 2.6. 부식전지의 종류13 2.7. 부식의 종류16 2.8. 전위(potential)20 2.9. 전위-pH도24 2.10. 분극 (polarization)26 2.11. 임피던스(impedance)30 2.12. 알루미늄과 그 합금의 특성36 2.13. 주조용(鑄造用) Al 합금(合金)44 2.14. 알루미늄의 부식특성 45 2.15. 담수(淡水) 또는 해수(海水)에서의 부식46 2.16. 알루미늄의 부식에 영향을 미치는 인자47 2.17. 알루미늄의 응력부식균열(應力腐蝕龜裂)51 3. 연구결과 및 고찰52 3.1. 주조용Al AC8A합금의 내식성에 관한 전기화학적 고찰52 3.1.1. 연구목적 및 배경52 3.1.2. 실험방법52 3.1.3. 실험결과 및 고찰54 3.1.4. 결론64 참고문헌65 Fig. 2.1 Schematic diagram for corrosion reactions of metal in electrolyte. 5 Fig. 2.2 Anodic and cathodic reactions by potential difference of two metals in seawater. 6 Fig. 2.3 Schematic diagram of electrochemical corrosion on metal in electrolyte (HCl solution). 8 Fig. 2.4 Schematic diagram of five elements for corrosion occurrence. 11 Fig. 2.5 Schematic diagram of salt concentration cell. 14 Fig. 2.6 Schematic diagram of oxygen concentration cell. 15 Fig. 2.7 Schematic diagram of oxygen concentration cell performed by rust. 15 Fig. 2.8 Schematic diagram of oxygen concentration cell performed by water surface. 16 Fig. 2.9 Schematic diagram for corrosion occurrence in bottom place of rust. 18 Fig. 2.10 Schematic diagram of pitting corrosion occurrence. 19 Fig. 2.11 Schematic diagram of crevice corrosion. 20 Fig. 2.12 E‐pH diagram of Fe‐H2O (298°K, ion activity: 10‐6 (mol/ℓ)). 25 Fig. 2.13 Polarization curve for Stern Geary's equation. 28 Fig. 2.14 Polarization curve for Tafel’s extrapolation method. 30 Fig. 2.15 Vector sum of resistance and capicitor. 32 Fig. 2.16 Representation of the impedance, Z, of a cell on a vector. Z' and Z" are respectively the real and imaginary components of the complex impedance. 33 Fig. 2.17 Representation of the sinusoidal voltage and current, at a given frequency, assodiated with a cell. 34 Fig. 2.18 Complex impedance plots for a combination of a resistor, R(a), and capacitor, C(b), (c) in series and (d) in parallel. 35 Fig. 2.19 State diagram of Al-Cu. 39 Fig. 2.20 State diagram of Al-Mg. 42 Fig. 2.21 State diagram of Al-Mn. 43 Fig. 2.22 State diagram of Al-Si. 44 Fig. 3.1 Variation of corrosion potential of welding metals in seawater solution. 54 Fig. 3.2 Variation of corrosion potential of welding metals in seawater solution in case of annealing heating treatment(H) (625℃, 2h)55 Fig. 3.3 Variation of corrosion potential of welding metals in case of heating treatment(H) or not(A) in seawater solution 56 Fig. 3.4 Variation of cathodic and anodic polarization curves with (510oC : 4hrs) and without heat treatment 57 Fig. 3.5 Comparison of cathodic and anodic polarization curves with solution(510oC:4hrs) and tempering(190oC:8hrs) heat treatment 58 Fig. 3.6 Variation of cathodic and anodic polarization curves as a function of tempering time(hrs) 59 Fig. 3.7 Variation of cathodic and anodic polarization curves with and without heat treatment59 Fig. 3.8 Comparison of AC impedance with and without heat treatment60 Fig. 3.9 Variation cyclic voltammogram with and without heat treatment61 Fig. 3.10 Comparison of AC impedance with and without heat treatment 62 Fig. 3.11 Morphologies of corroded surfaces aftre drawing polarization curves with and without heat treatment(x150) 63 Table 2.1 The series of electro motive force for various metals. 12 Table 2.2 The galvanic series of various metals in sea water. 13 Table 2.3 Corrosion potentials of metals in seawater. 18 Table 2.4 Standard potentials for various metal‐ion, gas or redox electrodes vs SHE at 25℃. 23 Table 2.5 Physical properties of Al 36 Table 2.6 Mechanical properties of Machining Al 36 Table 3.1 Kinds of AC8A alloy composition. 53 Table 3.2 The relationship between corrosion current density and vickers hardness6