Effect of Hydrostatic Pressure on Corrosion Behavior of Ultra Pure Al Coupled with Ultra Pure Fe
MA Rongyao1,2,MU Xin1,LIU Bo3,WANG Changgang1,WEI Xin1,ZHAO Lin1,DONG Junhua1(),KE Wei1
1. Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China 2. University of Chinese Academy of Sciences, Beijing 100049, China 3. School of Metallurgy, Northeastern University, Shenyang 110819, China
Cite this article:
MA Rongyao, MU Xin, LIU Bo, WANG Changgang, WEI Xin, ZHAO Lin, DONG Junhua, KE Wei. Effect of Hydrostatic Pressure on Corrosion Behavior of Ultra Pure Al Coupled with Ultra Pure Fe. Acta Metall Sin, 2019, 55(12): 1593-1605.
Hydrostatic pressure was one of the critical factors affecting deep-sea corrosion. Theoretical research showed that increasing hydrostatic pressure could improve the activity of metal materials, increase the difference in activity between coupled metal materials, and aggravate the galvanic corrosion. At present, there were many researches on the corrosion behavior of metallic materials under hydrostatic pressure, but there were few researches on the influence of hydrostatic pressure on the corrosion behavior of metal materials. Due to the requirements of structure and performance in the marine environment, equipment components with different electrochemical properties must be connected. In such a harsh environment, galvanic corrosion would obviously accelerate. Therefore, it was very necessary to study the galvanic corrosion behavior of metallic materials under the condition of the deep sea. Fe-based alloys and Al-based alloys have been widely used in the marine environment, and there have been many studies on corrosion of Fe-based alloys and Al-based alloys in the deep-sea environment. As a result of single composition and structure, taking ultra-pure Al and ultra-pure Fe as the research object, the influence of phase, inclusion and other factors on corrosion behavior under hydrostatic pressure could be avoided, which was helpful to clarify the influence of hydrostatic pressure on corrosion behavior of ultrapure Al coupled with ultra-pure Fe. The influence of hydrostatic pressure on the corrosion behavior of ultrapure Al coupled with ultrapure Fe was studied in 3.5%NaCl using electrodynamic polarization and electrochemical noise. The discrete wavelet transform was utilized to remove the direct current drift of noise signal, and then the stochastic analysis based on the shot noise theory was carried out. The Hilbert-Huang transform was utilized to analyze the time-frequency characteristics of the noise signal. The surface morphology of corrosion samples was observed by SEM. The pressure distribution was simulated by finite element method. The results showed that the ultrapure Al was self-passivation in 3.5%NaCl solution under different hydrostatic pressures, pitting corrosion occurred after coupling with ultrapure Fe. With the increase of hydrostatic pressure, the galvanic potential of coupled ultrapure Al and ultrapure Fe decreased gradually, and the galvanic current increased gradually. The increase of hydrostatic pressure accelerated the pitting generation rate of ultrapure Al in galvanic corrosion, but inhibited the growth probability of pitting corrosion and reduced the tendency of local corrosion. When hydrostatic pressure was atmospheric, pitting corrosion could expand along the horizontal and vertical directions. In the presence of hydrostatic pressure, pitting corrosion was easier to expand along the horizontal direction.
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)
Fig.1 Equivalent circuit of the cell (ia and ic—current noise sources of anode and cathode, respectively; Za and Zc—noise impedances of anode and cathode, respectively; ZRE—noise impedance of reference electrode; Rs—solution resistance; and —volatilities of potential and current, respectively; V—potentiometer, ZRA—zero resistance ammeter)
Fig.2 Schematic of simulated deep sea corrosion electrochemical test system (1—high purity nitrogen bottle, 2—water tank, 3—booster pump, 4—low temperature constant temperature circulating water tank, 5—autoclave, 6—circulating water jacket, 7~9—electrodes, 10—thermocouple, 11—pressure transmitter, 12—console, 13—electrochemical workstation)
Fig.3 Electrodynamic polarization curves of ultrapure Al and ultrapure Fe in 3.5%NaCl at hydrostatic pressures of 0.1 and 10 MPa (iFe,a and iFe,c—anode and cathode current densities of Fe dissolution reaction at -740 mV respectively, i'Fe,a and i'Fe,c—anode and cathode current densities of Fe dissolution reaction at -750 mV respectively)
Fig.4 Galvanic potential noise (EPN) spectra of coupled pairs of ultrapure Al and ultrapure Fe in 3.5%NaCl before (a) and after (b) a direct current (DC) drift remove by using discrete wavelet transform (DWT) at different hydrostatic pressures
Fig.5 Galvanic current noise (ECN) spectra of coupled pairs of ultrapure Al and ultrapure Fe in 3.5%NaCl before (a) and after (b) a DC drift remove by using DWT at different hydrostatic pressures
Fig.6 SEM image of ultrapure Al coupled with ultrapure Fe in 3.5% aqueous NaCl solution for 18000 s at different hydrostatic pressures(a1~a3) 0.1 MPa (b1~b3) 10 MPa (c1~c3) 20 MPa
Fig.7 Plot of noise resistance (Rn) versus time (a) and cumulative probability plot of Rn(b) for coupled pairs of ultrapure Al and ultrapure Fe in 3.5%NaCl at different hydrostatic pressures
Fig.8 Plot of the frequency of events (fn) versus time (a) and plot of the average charge in each event (q) versus time (b) for coupled pairs of ultrapure Al and ultrapure Fe in 3.5%NaCl at different hydrostatic pressures
Fig.9 Weibull probability plots for fn of coupled pairs of ultrapure Al and ultrapure Fe in 3.5%NaCl at different hydrostatic pressures
Pressure
MPa
Uniform corrosion
Pitting corrosion
m
n
m
n
0.1
0.5245
12.5346
0.8165
12.4497
10
2.4589
0.6219
1.2681
2.2702
20
2.8706
0.0775
1.4878
0.7527
Table 1 The shape parameter (m) and the scale parameter (n) determined from the linear slope of ln{ln[l/(l-F(1/fn))]} vs ln(1/fn) plot for coupled pairs of ultrapure Al and ultrapure Fe in 3.5%NaCl at different hydrostatic pressures
Fig.10 Plots of the uniform corrosion generation rate (a) and pitting corrosion generation rate (b) against exposure time for coupled pairs of ultrapure Al and ultrapure Fe in 3.5%NaCl at different hydrostatic pressures
Fig.11 Gumbel distribution plots for q of coupled pairs of ultrapure Al and ultrapure Fe in 3.5%NaCl at different hydrostatic pressures (Y—reduced variant)
Pressure
MPa
Metastable pitting
Stable pitting
α
μ / C
α
μ / C
0.1
3.82×10-5
4.29×10-5
6.83×10-5
3.63×10-5
10
6.09×10-6
9.77×10-6
1.31×10-5
1.91×10-6
20
2.38×10-6
4.83×10-6
4.42×10-6
4.43×10-6
Table 2 Typical Gumbel distribution parameters of the scale parameter (α) and the shape parameter (μ) for coupled pairs of ultrapure Al and ultrapure Fe in 3.5%NaCl at different hydrostatic pressures
Fig.12 Corrosion growth probabilities (Pc) versus q of coupled pairs of ultrapure Al and ultrapure Fe in 3.5%NaCl at different hydrostatic pressures
Fig.13 Noise signals and Hilbert spectra of coupled pairs of ultrapure Al and ultrapure Fe in 3.5%NaCl at different hydrostatic pressures for a duration of 1024 s (The normalized amplitude of the original noise signal is plotted on the back side of the graph)Color online(a) 0.1 MPa, EPN (b) 10 MPa, EPN (c) 20 MPa, EPN(d) 0.1 MPa, ECN (e) 10 MPa, ECN (f) 20 MPa, ECN
Fig.14 Hilbert marginal spectra of the signal of coupled pairs of ultrapure Al and ultrapure Fe in 3.5%NaCl at different hydrostatic pressures(a) EPN (b) ECN
Fig.15 Schematic of influence of hydrostatic pressure on corrosion morphology obtained by finite element simulation (Arrows show the easy expand directions)Color online(a1, a2) 0.1 MPa (b1, b2) 10 MPa
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