1. Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China 2. University of Chinese Academy of Sciences, Beijing 100049, China
Cite this article:
Rongyao MA,Changgang WANG,Xin MU,Xin WEI,Lin ZHAO,Junhua DONG,Wei KE. Influence of Hydrostatic Pressure on Corrosion Behavior of Ultrapure Fe. Acta Metall Sin, 2019, 55(7): 859-874.
Hydrostatic pressure is part of the crucial factors affecting deep sea corrosion. At present, there have been a lot of studies on the pitting behavior of metallic materials under hydrostatic pressure, but most of them take passive metallic materials as the research object, and the influence rule of hydrostatic pressure on the pitting behavior of metallic materials also presents diversity. People not only have no clear understanding of its mechanism, but also have some disputes. The generation and growth of pitting corrosion are dependent on the structure of materials, chemical composition and service environment. Inclusion, passivation ability and surface roughness can all affect the pitting behavior of metal materials. Due to the single composition and simple structure of ultrapure Fe, the influence of phase, inclusion and other factors on corrosion behavior under hydrostatic pressure can be avoided, which is more conducive to elucidate the mechanism of hydrostatic pressure on metal corrosion behavior. In addition, the influence of hydrostatic pressure on the corrosion behavior of ultrapure Fe is rarely reported. So, the effect of hydrostatic pressure on the corrosion behavior of ultrapure Fe exposed to 3.5%NaCl aqueous solution is investigated by potentiodynamic polarization curves and electrochemical noise method. The noise signals are analyzed by shot noise theory, stochastic analysis and Hilbert-Huang transform. Besides, the surface morphology of the corrosion sample is observed by SEM. The results of weight loss test and potentiodynamic polarization study show that increasing hydrostatic pressure accelerated the corrosion rate of ultrapure Fe exposed to 3.5%NaCl. The results of electrochemical noise study show that increasing hydrostatic pressure promotes the development of pitting corrosion and increases the tendency of local corrosion throughout the immersion. At the beginning of soaking, local corrosion (such as pitting nucleation, metastable pitting and stable pitting) mainly occurred in ultrapure Fe, increasing of hydrostatic pressure inhibits the pitting nucleation process, but promotes the development of metastable pitting and steady pitting, and increases the growth probability of pitting. With the immersion time prolonging, the uniform corrosion gradually changed into the principal corrosion type, increasing hydrostatic pressure still promotes the development of metastable pitting and stable pitting and improves the growth probability of pitting corrosion, but relatively inhibits the uniform corrosion process.
Fund: National Key Research and Development Program of China(No.2017YFB0702302);National Natural Science Foundation of China(Nos.51671200);National Natural Science Foundation of China(51501204);National Natural Science Foundation of China(51801219)
Table 1 Time and frequency scale of each crystal after wavelet decomposition in the case of 0.1 s sampling interval time
Fig.1 Schematic of simulated deep sea corrosion electrochemical test system (1—high purity nitrogen bottle, 2—autoclave, 3—electrode, 4—thermocouple, 5—pressure transmitter, 6—circulating water jacket, 7—low temperature constant temperature circulating water tank, 8—console, 9—electrochemical workstation)
Fig.2 The polarization curves of ultrapure Fe exposed to 3.5%NaCl aqueous solution for 2.16×104 s at hydrostatic pressures of 0.1 and 10 MPa (Solid line—polarization curve, dotted line—fitting line)
Fig.3 OM image of ultrapure Fe
Fig.4 SEM images of ultrapure Fe exposed to 3.5%NaCl aqueous solution for 6.2×104 s at hydrostatic pressures of 0.1 MPa (a) and 10 MPa (b) (Insets show the original SEM images drawing after grinding by 2000# abrasive paper)
Fig.5 Electrochemical potential noise (EPN) spectra of ultrapure Fe exposed to 3.5%NaCl aqueous solution at hydrostatic pressures of 0.1 and 10 MPa
Fig.6 Electrochemical potential noise (ECN) spectra of ultrapure Fe exposed to 3.5%NaCl aqueous solution at hydrostatic pressures of 0.1 and 10 MPa
Fig.7 Plot of noise resistance (Rn) versus time (a) and cumulative probability plot of Rn (b) for ultrapure Fe exposed to 3.5%NaCl aqueous solution for 0~6.2×104 s at hydrostatic pressures of 0.1 and 10 MPa
Fig.8 Plot of the frequency of events (fn) (a) and the average charge in each event (q) (b) versus time for ultrapure Fe exposed to 3.5%NaCl aqueous solution for 0~6.2×104 s at hydrostatic pressures of 0.1 and 10 MPa
Fig.9 Plots for the q-fn of ultrapure Fe exposed to 3.5%NaCl aqueous solution at hydrostatic pressures of 0.1 and 10 MPa
Fig.10 Weibull probability plots for fn of ultrapure Fe exposed to 3.5%NaCl aqueous solution at hydrostatic pressures of 0.1 and 10 MPa (F(1/fn)—cumulative probability for fn)
Fig.11 Plots of the uniform corrosion generation rate against exposure time for ultrapure Fe exposed to 3.5%NaCl aqueous solution at hydrostatic pressures of 0.1 and 10 MPa
Fig.12 Plots of the pitting corrosion generation rate against exposure time for ultrapure Fe exposed to 3.5%NaCl aqueous solution at hydrostatic pressures of 0.1 and 10 MPa
Corrosion style
Pressure
MPa
0~2.5×104 s
2.5×104~6.2×104 s
m
n
m
n
Pitting corrosion
0.1
0.4299
0.0202
0.5593
8.7726×10-4
10
0.4583
0.0323
0.7395
6.2403×10-4
Uniform corrosion
0.1
1.3411
7.2016×10-7
2.2436
2.9423×10-12
10
1.7697
1.1158×10-8
2.7843
5.1289×10-14
Table 2 The values of the shape parameter (m) and the scale parameter (n) determined from the linear slope of ln{ln[l/(l-F(1/fn)]}-ln(1/fn) plot for the ultrapure Fe exposed to 3.5%NaCl aqueous solution at hydrostatic pressures of 0.1 and 10 MPa
Fig.13 Gumbel distribution plots for q of ultrapure Fe exposed to 3.5%NaCl aqueous solution at hydrostatic pressures of 0.1 and 10 MPa (Y—reduced variant)
Fig.14 Plots for pitting corrosion growth probabilities (Pc) vs q of ultrapure Fe exposed to 3.5%NaCl solution at hydrostatic pressures of 0.1 and 10 MPa
Corrosion style
Pressure
MPa
0~2.5×104 s
2.5×104~6.2×104 s
α
μ / C
α
μ / C
Metastable pitting
0.1
5.10×10-9
-2.95×10-9
3.85×10-11
4.69×10-11
10
1.92×10-8
-1.05×10-9
7.41×10-10
7.41×10-11
Stable pitting
0.1
1.59×10-8
-3.24×10-8
-
-
10
4.00×10-8
-2.76×10-8
-
-
Table 3 Typical Gumbel distribution parameters of the scale parameter (α) and the shape parameter (μ) for the ultrapure Fe at hydrostatic pressures of 0.1 and 10 MPa
Fig.15 Four typical Hilbert spectra of the EPN signal of ultrapure Fe exposed to 3.5%NaCl aqueous solution at hydrostatic pressures of 0.1 MPa (a, c, e, g) and 10 MPa (b, d, f, h) for a duration of 204.8 s
Fig.16 Time-dependent variation of Hilbert marginal spectra of the EPN (a, b) and ECN (c, d) signal of ultrapure Fe exposed to 3.5%NaCl aqueous solution at hydrostatic pressures of 0.1 MPa (a, c) and 10 MPa (b, d) for a duration of 204.8 s
Fig.17 Hilbert marginal spectra of the EPN (a, b) and ECN (c, d) signal of ultrapure Fe exposed to 3.5%NaCl aqueoussolution at hydrostatic pressures of 0.1 and 10 MPa
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