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BK 21 Project Supported by POSCO

BK 21 Project Supported by POSCO. Effects of Alloying Elements on the Passivity of Stainless Steels. Kyung-Jin Park, Ihsan ul Haq Toor and Hyuk-Sang Kwon. Mid-term Report/ 2007. 07. 27 Corrosion and energy storage materials lab. Dept. of Materials Science & Engineering. Midterm.

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BK 21 Project Supported by POSCO

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  1. BK 21 Project Supported by POSCO Effects of Alloying Elements on the Passivity of Stainless Steels Kyung-Jin Park, Ihsan ul Haq Toor and Hyuk-Sang Kwon Mid-term Report/ 2007. 07. 27 Corrosion and energy storage materials lab. Dept. of Materials Science & Engineering

  2. Midterm BK 21 Project Supported by POSCO Resistance to corrosion Composition and Semiconducting properties of passive film Preparation of specimens • 400 series commercial • STS (POSCO) • Vacuum arc melting • Fe-21Cr • Fe-21Cr-(0.5, 1)Mo • Fe-21Cr-(0.5, 1)Cu • Fe-21Cr-(0.5, 1)Si • Fe-18Cr-1Mo • Fe-21Cr-0.5Mo-0.5Cu • Fe-21Cr-0.2Mn • Fe-21Cr-0.2Ti • Fe-21Cr-0.4Nb • Fe-21Cr-1W • Mott-Schottky analysis • XPS analysis Film breakdown (pit initiation) Effects of Mn, N on the SCC susceptibility Film growth (repassivation)

  3. pitting Epit Passivity • Passivity is the phenomenon that infers kinetic stability on reactive metals (Fe, Ni, Cr, Ti, Zn, Al, Mg, W, Mo, Zr, etc.) and alloys in contact with oxidizing aqueous environments. Oxygen evolution or oxidative dissolution of metal (ex. Cr3+→ Cr6+) Noble transpassive Passive film Applied Potential passive Oxide formation Epp active Ecorr Uniform corrosion ~10-6 A/cm2 : ~10 mm/year Active ip ic Log(Current Density)

  4. 1 2 3  [H+]  [Cl-]  T Icorr 1 Icorr 2 Icorr 3 log i Effects of environments on the passivity of alloys Severe environmental conditions of high acidity & temperature decrease the passive potential range and increase corrosion rates at all potentials.

  5. Effects of alloying elements on the passivity of alloys Noble Mn Epit Cu Cr, Mo, N, W, Si, V, Ni Applied Potential Cr, Ni, W, N Epp Ni, Cu Ecorr Cr, Ni, V, Mo, N, Cu Active ip ic Log(Current Density)

  6. Ecorr A Potential EFB B E vs. time Time Film breakdown and repassivation MxOy Passive Surface (-) Potential (+) A Ecorr Repassivation (Film growth) Pit initiation (Film breakdown) Metastable pitting Mn+ Stable pit B EFB Stable pit growth ip Irreversible damage iFB log (current density) iFB B Current density A ip i vs. time Time

  7. Importance of repassivation kinetics in SCC SCC of stainless steels in Cl- solution occurs via repetitive processes of film breakdown/ dissolution / repassivation (film rupture & slip dissolution model). Repassivation rate = For an alloy/environment system where a smooth specimen of alloy is subjected to a constant loading condition, the inherent repassivation rate of alloy will determine SCC susceptibility. A close relationship between SCC susceptibility and repassivation rate for stainless steels.

  8. Analysis of repassivation current transient based on high field ion conduction model CE WE 5 RE Air gas inlet 2 N 3 V : potential drop across film. h(t) : thickness of film.B : constant associated with ion movement. POTENTIO - - GALVANOSTAT TEMPERATURE IBM PC/AT CONTROLLER 8 PRINTER 9 4 r : density of the film. M : molecular weight of the film. z : valence state. 6. Scratcher 1. Magnetic Plate 7 6 2. Stirrer 7. Thermometer 2 3. Condensor 8. Luggin Probe Detail 6 9. Diamond Tip 4. Holder 1 5. Solenoid Valve The cBVis the slope in the linear part of log i vs. 1/q(t) plot.

  9. cBV(A) cBV(B) cBV as a measure of repassivation rate • Assume that aA= aB, and ip,A > ip,B. for two alloys A & B • For the same degree of repassivation, ir - Repassivation time : tA > tB - Repassivation rate (Vr) : Vr,B > Vr,A - From log i(t) vs. 1/q(t) plot, (cBV)A > (cBV)B decay gradient ↓ cBV ↑ repassivation rate

  10. Prediction of SCC susceptibility by cBV Type 304, 50 ℃ , 4 M NaCl • 1st stage : •  cBV with  No SCC • 2nd stage : • cBV converges to a limiting value SCC • 3rd stage : • Appearance of inflection point in log i vs 1/q(t). Severe dissolution [H. S. Kwon et. al., Corrosion56 (2000), p. 32]

  11. Objective To investigate the effects of alloying elements (Mo, Cu, Si, Ti, Nb, W, Mn) on the passivity of STS

  12. Materials and experiments (POSCO commercial STS) 400 series commercial STS (POSCO) Corrosion behavior

  13. Materials and experiments (Artificial specimens) Specimens (prepared by vacuum arc melting) Fe-21Cr Fe-21Cr-(0.5, 1)Mo Fe-21Cr-(0.5, 1)Cu Fe-21Cr-(0.5, 1)Si Fe-18Cr-1Mo Fe-21Cr-0.5Mo-0.5Cu Fe-21Cr-0.2Mn Fe-21Cr-0.2Ti Fe-21Cr-0.4Nb Fe-21Cr-1W Resistance to corrosion Pit initiation (film breakdown) Fe-25Cr-(0, 2, 4)Mo Fe-20Cr-(0. 1, 2)Cu Fe-20Cr-(0, 2, 4)Si Repassivation (film growth) Hot rolling (1200 oC, 2h, 40%) Cold rolling (40%) Annealing (1050 oC, 1h) 100 % ferritic STS!!

  14. Resistance to pitting corrosion (PD test)- 400 series commercial STS (POSCO) - (PRE = Cr% +3.3 Mo% + 30 %N -1.0 Mn%) ↑PRE → ↑Epit

  15. Resistance to general corrosion (PD test)- 400 series commercial STS (POSCO) - (PRE = Cr% +3.3 Mo% + 30 %N -1.0 Mn%) ↓ Cr and Mo → ↑ ic

  16. Effects of Mo on the resistance to corrosion - Fe-25Cr-(0,2,4)Mo - ↑Mo → ↑ Ecorr, Epit → ↑ resistance to pitting corrosion ↑Mo → ↓ ic, Epp, ↑Ecorr → ↑resistance to general corrosion

  17. Effects of Mo on the pit initiation process - Fe-25Cr-(0,2,4)Mo - Experiment procedure 1. Cathodic cleaning : - 1 V/3 min. 2. Initial delay : 600 sec. 3. Potentiodynamic scan (scan rate : 1mV/sec) to Eapp 4. Potentiostatic test, Eapp = -0.05 V Mo addition → ↑ pitting induction time → ↓ pitinitiation

  18. Effects of Mo on the repassivation kinetics - Fe-25Cr-(0,2,4)Mo - 50 oC 1 M MgCl2, Eapp= -0.3 VSCE 0Mo 2Mo 4Mo ↑Mo → ↓ cBV → ↑repassivation rate

  19. Effects of Cr on the repassivation kinetics - Fe-(18,20,25,29)Cr - 50 oC 1 M MgCl2, Eapp= -0.3 VSCE Fe-18Cr Fe-20Cr Fe-25Cr Fe-29Cr ↑Cr → ↓ cBV → ↑repassivation rate

  20. Effects of Cr vs. Mo on the repassivationkinetics 50 oC 1 M MgCl2, Eapp= -0.3 VSCE Cr Mo cBV(X10-3)=3.4-0.65XMo cBV(X10-3)=9.1-0.23XCr Cr content (wt%) Mo content (wt%) • ↑ Cr and Mo → ↓ cBV → ↑ repassivation rate • Mo has stronger effect on decreasing cBV than Cr. (about 3 times)

  21. Effects of Cu on the resistance to corrosion - Fe-20Cr-(0,2,4)Cu - ↑Cu → ↓Epit ,↑Ecorr → ↓ resistance to pitting corrosion ↑Cu → ↓ ic, Epp, ↑Ecorr → ↑resistance to general corrosion

  22. Effects of Cu on the pit initiation process - Fe-20Cr-(0,2,4)Cu - ↑Cu → ↑# of metastasble pitting → ↑pit initiation

  23. Effects of Cu on the repassivation kinetics - Fe-20Cr-(0,2,4)Cu - 25 oC 0.1M NaCl, Eapp= -0.1 VSCE 4Cu 2Cu 0Cu ↑Cu ↑Cu → ↑ cBV → ↓ repassivation rate

  24. Effects of Si on the resistance to corrosion - Fe-20Cr-(0,1,2)Si - ↑Si → ↑Epit → ↑ resistance to pitting corrosion ↑Si → ↑ic, Epp → ↓ resistance to general corrosion

  25. Effects of Si on the pit initiation process - Fe-20Cr-(0,1,2)Si - Si addition → ↑ metastable pitting → ↑ pit initiation

  26. Effects of Si on the repassivation kinetics - Fe-20Cr-(0,1,2)Si - 25 oC 0.1M NaCl, Eapp= 0.2 VSCE 0Si 1Si 2Si ↑Si ↑Si → ↓ cBV → ↑ repassivation rate

  27. Effects of Mo on the resistance to corrosion - Fe-21Cr-(0, 0.5, 1)Mo - ↑Mo → ↑ Epit → ↑ resistance to pitting corrosion ↑Mo → ↓ ic, Epp → ↑ resistance to general corrosion

  28. Effects of Cu on the resistance to corrosion - Fe-21Cr-(0, 0.5, 1)Cu - ↑Cu → ↓Epit → ↓ resistance to pitting corrosion ↑Cu → ↓ ic, Epp → ↑resistance to general corrosion

  29. Effects of Si on the resistance to corrosion - Fe-21Cr-(0, 0.5, 1)Si - ↑Si → almost same Epit ↑Si → ↑ic, Epp → ↓resistance to general corrosion

  30. Effects of Cr substitution by Mo on the corrosion resistance PRE (Fe-21Cr) = 21 / PRE(Fe-18Cr-1Mo) = 21.3 Cr Substitution by Mo →↓ ic, Epp → ↑resistance to general corrosion Cr Substitution by Mo → almost sameEpit → ↑Ecorr

  31. Effects of Mo+Cu on the resistance to corrosion Addition of 0.5%Mo and 0.5%Cu →↓ ic, Epp → ↑ resistance to general corrosion Addition of 0.5%Mo and 0.5%Cu → almost sameEpit and ↑ Ecorr

  32. Effects of Mn on the resistance to corrosion ↑Mn → ↓Epit → ↓ resistance to pitting corrosion ↑Mn → ↑ic, Epp → ↓ resistance to general corrosion

  33. Effects of W on the resistance to corrosion ↑W → ↑Epit → ↑ resistance to pitting corrosion ↑W → ↓ ic, Epp → ↑resistance to general corrosion

  34. Effects of Nb on the resistance to corrosion ↑Nb → ↑almost same Epit ↑Nb → ↑icc, Epp → ↓ resistance to general corrosion

  35. Effects of Ti on the resistance to corrosion ↑Ti → ↓Epit → ↓ resistance to pitting corrosion ↑Ti → almost same ic, Epp

  36. Conclusions • Effects of alloying elements on the passivity of STS

  37. Midterm Final Future Work Resistance to corrosion Composition and Semiconducting properties of passive film Preparation of specimens • 400 series commercial • STS (POSCO) • Vacuum arc melting • Fe-21Cr • Fe-21Cr-(0.5, 1)Mo • Fe-21Cr-(0.5, 1)Cu • Fe-21Cr-(0.5, 1)Si • Fe-18Cr-1Mo • Fe-21Cr-0.5Mo-0.5Cu • Fe-21Cr-0.2Mn • Fe-21Cr-0.2Ti • Fe-21Cr-0.4Nb • Fe-21Cr-1W • Mott-Schottky analysis • XPS analysis Film breakdown (pit initiation) Effects of Mn, N on the SCC susceptibility Film growth (repassivation)

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