INVESTIGATION ON PITTING CORROSION BEHAVIOR OF ULTRAFINE-GRAINED 304L STAINLESS STEEL IN Cl- CONTAINING SOLUTION
Nan PIAO1,2,Ji CHEN1(),Chengjiang YIN1,3,Cheng SUN4,Xinghang ZHANG4,Zhanwen WU5
1 Center of Corrosion and Protection Technology in Petro-Chemical Industry, Department of Mechanical Engineering, Liaoning Shihua University, Fushun 113001 2 Institute of Nuclear and New Energy Technology, Tsinghua University, Beijing 100084 3 Department of Mechanical Engineering, Northeast Petroleum University, Daqing 163318 4 Department of Mechanical Engineering, Texas A&M University, College station, TX 77843-3123, USA 5 CNOOC Energy Technology and Services-Pipe Engineering Co., Tianjin 300452
Cite this article:
Nan PIAO,Ji CHEN,Chengjiang YIN,Cheng SUN,Xinghang ZHANG,Zhanwen WU. INVESTIGATION ON PITTING CORROSION BEHAVIOR OF ULTRAFINE-GRAINED 304L STAINLESS STEEL IN Cl- CONTAINING SOLUTION. Acta Metall Sin, 2015, 51(9): 1077-1084.
The electrochemical behavior and pitting corrosion in a Cl- containing solution (0.05 mol/L H2SO4+0.05 mol/L NaCl) of the ultrafine-grained 304L stainless steel (304L SS) with average grain size of (130±30) nm prepared by equal channel angular pressing (ECAP) technique were examined using potentiodynamic polarization curves, cycle polarization curves, electrochemical impedance spectroscopy (EIS), Mott-Schottky (M-S) curve measurements together with SEM observation of surface morphology. As compared to the coarse-grained counterpart, the ultrafine-grained sample exhibited a higher corrosion current density icorr of 81.74 Acm2 and a lower corrosion potential Ecorr (vs SCE) of -466 mV, and having a higher passivation current density ip of 32.38 mAcm2 and a narrower passive region (-315~450 mV) together with a breakdown potential Eb decrease of 100 mV and a protection potential Ebp decrease of 190 mV. On one hand, the grain refinement induced by severe plastic deformation deteriorates the compactness of the passive films and is helpful for the Cl- absorption, resulting in a 1.6 times increase of the carrier density and one order of magnitude increase of the diffusion coefficient in the passive films. On the other hand, the significant increase of grain boundaries provides more possibility for Cl- diffusion along grain boundaries, and thus promotes the pitting nucleation and growth.
Fig.1 Potentiodynamic polarization curves (a) and cyclic polarization curves (b) of UFG-304L stainless steel (SS) and CG-304L SS in 0.05 mol/L H2SO4 + 0.05 mol/L NaCl solution ( iUFGp and iCGp—passive current densities of UFG-304L SS and CG-304L SS, EUFGp and ECGp—passivation potentials of UFG-304L SS and CG-304L SS, EUFGb and ECGb—breakdown potentials of UFG-304L SS and CG-304L SS, EUFGbp and ECGbp—protection potentials of UFG-304L SS and CG-304L SS)
Sample
Ecorr / mV
icorr / (mAcm-2)
Ep / mV
ip / (mAcm-2)
Eb / mV
Ebp / mV
UFG-304L
-466
81.74
-315
32.38
552
198
CG-304L
-401
19.45
-305
20.12
655
385
Table 1 Fitting results of the potentiodynamic polarization curves and cyclic polarization curves of UFG-304L SS and CG-304L SS in 0.05 mol/L H2SO4 + 0.05 mol/L NaCl solution
Fig.2 SEM images of surface morphology for UFG-304L SS (a) and CG-304L SS (b) after cyclic polarization in 0.05 mol/L H2SO4 + 0.05 mol/L NaCl
Fig.3 M-S curves of passive films formed on UFG-304L SS (a) and CG-304L SS (b) at different Ef in 0.05 mol/L H2SO4 + 0.05 mol/L NaCl solution (C—space charges capacitance of passive film, E—applied potential, Ef—formation potential of passive film, k—slope)
Fig.4 Relationships between Nd and Ef for the passive films of UFG-304L SS and CG-304L SS formed in 0.05 mol/L H2SO4 + 0.05 mol/L NaCl solution for 30 min (Nd—donor concentration)
Sample
0.1 V
0.2 V
0.3 V
0.4 V
UFG-304L
7.52
6.04
5.32
4.55
CG-304L
6.57
5.26
4.59
4.11
Table 2 Carrier densities (Nd) of passive films formed on UFG-304L SS and CG-304L SS at different Ef in 0.05 mol/L H2SO4 + 0.05 mol/L NaCl solution
Fig.5 Relationships between Lss and Ef for the passive films of UFG-304L SS and CG-304L SS formed in 0.05 mol/L H2SO4 + 0.05 mol/L NaCl solution for 30 min (Lss—thickness of passive film)
Fig.6 Nyquist plots of passive films on the UFG-304L SS and CG-304L SS formed in 0.05 mol/L H2SO4 + 0.05 mol/L NaCl solution at Ef =0.1 V (a), 0.2 V (b), 0.3 V (c) and 0.4 V (d) (Inset in Fig.6a shows the equivalent circuit. Rs—solution resistance, Rf—passive film resistance, Qf—passive film capacitance)
Fig.7 Schematics of pitting corrosion for UFG-304L SS (a) and CG-304L SS (b) in 0.05 mol/L H2SO4+0.05 mol/L NaCl (DB—diffusion coefficient of Cl- along grain-boundary, DL—diffusion coefficient of Cl- in lattice)
Sample
Ef / V
Rs / (Ωcm-2)
Qf/ (10-4 Fcm-2)
n
Rf / (104 Ωcm-2)
UFG-304L
0.1
9.58
1.96
0.80
2.19
0.2
8.56
2.06
0.82
2.99
0.3
7.54
2.31
0.86
3.49
0.4
10.78
1.99
0.90
4.03
CG-304L
0.1
11.35
1.05
0.87
3.88
0.2
10.34
1.26
0.87
4.06
0.3
9.86
1.57
0.90
4.52
0.4
10.84
1.70
0.92
5.03
Table 3 Fitting impendance parameters of Nyquist plots of the passive films on the UFG-304L SS and CG-304L SS formed in 0.05 mol/L H2SO4 + 0.05 mol/L NaCl solution at different Ef
Fig.7 Schematics of pitting corrosion for UFG-304L SS (a) and CG-304L SS (b) in 0.05 mol/L H2SO4+0.05 mol/L NaCl (DB—diffusion coefficient of Cl- along grain-boundary, DL—diffusion coefficient of Cl- in lattice)
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