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Title Page

Contents

ABSTRACT 9

1. INTRODUCTION 10

2. LITERATURE REVIEW 12

2.1. Corrosion of Steel Structures 12

2.1.1. Fundamental Theory of Corrosion 12

2.1.2. Uniform Corrosion 17

2.1.3. Galvanic Corrosion 17

2.1.4. Pitting Corrosion 20

2.2.Volta potential of surface 23

2.2.1. Definition of Electron Work Function (EWF) 23

2.2.2. Relation between the Volta and the Electrode potential 24

2.3. Electrochemical Tests for the Study of Corrosion 26

2.3.1. Electrochemical Impedance Spectroscopy (EIS) 26

2.3.2. Polarization method 33

2.4. Surface analysis using Kelvin probe force microscopy (KFPM) 37

3. EXPERIMENTAL PROCEDURES 41

3.1. Materials and Preparation 41

3.2. Electrochemical Measurements 43

3.3. Surface Analyses 45

4. RESULT AND DISCUSSION 46

4.1. Corrosion behaviors of carbon steel 46

4.2. Corrosion behaviors of stainless steel 62

4.3. Comparative effect of roughness on uniform and pitting corrosion 87

5. CONCLUSION 89

6. REFERENCES 90

논문요약 93

List of Tables

Table 2-1. Galvanic series of some commercial metals and alloys in seawater 19

Table 3-1. Chemical composition of the specimens 42

Table 3-2. Composition of the synthetic groundwater 44

Table 3-3. Composition of the synthetic acid rain 44

Table 4-1. Electrochemical parameters of the potentiodynamic polarization measurements of carbon steel in synthetic groundwater at 25℃ 48

Table 4-2. EIS measurements as a function of roughness in synthetic groundwater 52

Table 4-3. EIS measurements for stainless steel as a function of roughness in synthetic acid rain 68

Table 4-4. EIS measurements for stainless steel as a function of roughness after potentiostatic test 75

Table 4-5. The number of pitting as a function of surface roughness after electrochemical tests 84

List of Figures

Figure 2-1. Potential-pH equilibrium diagram for the system iron-water at 25℃ 15

Figure 2-2. Calculated polarization curve of steel with the potential as a function of the applied current density 16

Figure 2-3. Autocatalytic process occurring in a corrosion pit. The metal, M, is being pitted by an aerated NaCl solution 22

Figure 2-4. The results of electrochemical impedance spectroscopy; (a) and (c) : Nyquist plot, (b) and (d) : Bode plot 27

Figure 2-5. Depression parameters about R-C circuit in parallel: (a) Nyquist plot, (b) Bode plot 29

Figure 2-6. Geometrical extrapolation technique: (a) Geometrical extrapolation technique about depression circle, (b) The schematic of finding center point of semi-circle using geometrical extrapolation technique, (c) The calculation of depression angle using geometrical extrapolation technique 32

Figure 2-7. The Tafel extrapolation method 35

Figure 2-8. Experimentally measured Tafel polarization plot 36

Figure 2-9. Schematic setup of the scanning Kelvin probe 40

Figure 4-1. Potentiodynamic polarization curves of carbon steels as a function of surface roughness in synthetic groundwater at 25°C 47

Figure 4-2. Nyquist plots for carbon steel in synthetic groundwater at 25°C 50

Figure 4-3. Equivalent circuit for describing the impedance behavior of rust/steel system 51

Figure 4-4. Variation of the corrosion rate as a function of average roughness of carbon steel 54

Figure 4-5. Topography and Volta potential mapping image measured by KPFM with different average roughness: (a) Ra 108.0 nm, (b) Ra 39.3 nm, (c) Ra 16.7 nm 56

Figure 4-6. Surface morphologies and Volta potential profiles of inset line in Figure 4-5: (a) Ra 108.0 nm, (b) Ra 39.3 nm, (c) Ra 16.7 nm, and (d) difference of Volta potential between peak and valley with average roughness 58

Figure 4-7. Surface morphologies after 40 min immersion: (a) SEM image (500x), (b) optical profile image of region A in (a) 60

Figure 4-8. Potentiodynamic polarization curves of stainless steels as a function of surface roughness in synthetic acid rain solution at 80°C 63

Figure 4-9. Pitting potential of stainless steels as a function of surface roughness in synthetic acid rain solution at 80°C 64

Figure 4-10. Nyquist plots for stainless steel in synthetic acid rain at 80°C 66

Figure 4-11. Equivalent circuit to fit the EIS data for the one time constant 67

Figure 4-12. Current variation of stainless steel with time at 200 mVSCE 70

Figure 4-13. Nyquist plots of the stainless steel after potentiostatic test 72

Figure 4-14. Equivalent circuit to fit the EIS data for the two-time constant 73

Figure 4-15. Depressed angle from EIS measurements as a function of surface roughness 74

Figure 4-16. Topography and Volta potential mapping image measured by KPFM with different average roughness: (a) Ra 154.1 nm, (b) Ra 45.9 nm, (c) Ra 35.8 nm, (d) Ra 16.3 nm 77

Figure 4-17. Surface morphologies and Volta potential profiles of inset line in Figure 4-16: (a) Ra 154.1 nm, (b) Ra 45.9 nm, (c) Ra 35.8 nm and (d) Ra 16.3 nm 79

Figure 4-18. Surface morphologies after electrochemical tests: (a) Ra 154.1 nm, (b) Ra 45.9 nm, (c) Ra 35.8 nm and (d) Ra 16.3 nm 82

Figure 4-19. Schematic illustration of residual stress distributed on the (a) rough and (b) smooth surface 86

초록보기

본 연구에서는 표면조도가 각각 균일부식과 공식에 미치는 영향을 비교하기 위하여 탄소강과 스테인리스강 시편을 사용하여 전기화학적 시험과 표면분석을 진행하였다. 전기화학적 시험을 통해 표면조도가 증가할수록 균일부식의 내식성과 공식에 대한 저항성이 증가하는 것을 확인하였다. 그러나 표면 분석을 통해 표면조도 가 균일부식과 국부부식에 미치는 영향이 다름을 확인하였다. 균일부식의 경우, 표 면조도가 증가할수록 골과 봉우리 사이의 Volta 전위의 차이가 증가하여 미세 갈바닉 부식이 가속화되고, 시간이 경과함에 따라 시편 전체 면적에 걸쳐 균일하게 분포하는 미세 갈바닉 전지의 활성화로 인해 균일부식으로 전환되는 거동을 보였다. 반면 공식의 경우, 표면조도가 증가할수록 공식의 개수와 크기가 증가하며, 공식전 위가 낮아지는 거동을 보였다. 이는 표면의 조도가 증가할수록 부동태 피막의 봉우리에 응력이 집중되는 현상이 증가하고, 응력이 집중될수록 부동태 피막에 결함이 생기는 확률이 높아지기 때문이다. 부동태 피막에 생긴 결함에서 기지금속이 노출 되어 준안정 공식이 발생하여 공식의 발생 가능성을 증가시킨다.

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