Effect of Aging Time on Hydrofluoric Acid Corrosion Resistance of 2205 Duplex Stainless Steel
The precipitate will affect the corrosion performance of 2205 duplex stainless steel in hydrofluoric acid solution. In order to understand this effect, the microstructure of the solution-treated 2205 duplex stainless steel after aging at 800 ℃ for 15, 60 and 120 min was observed through a metallographic microscope, and X-ray diffraction technology was used to analyze the material under different aging times. Phase structure, with the aid of electrochemical polarization and electrochemical impedance methods, the influence of aging time on the hydrofluoric acid corrosion resistance of 2205 duplex stainless steel was tested. The results show that when the aging time is 15 minutes, the σ phase has been precipitated in the duplex stainless steel, and the volume fraction of the σ phase increases with the aging time. With the extension of the aging time, the self-corrosion current density and the passive current density of 2205 duplex stainless steel in the hydrofluoric acid solution gradually increase, and the hydrofluoric acid corrosion resistance decreases, which is mainly due to the precipitated σ phase As a result, the carrier density in the passivation film gradually increases, which accelerates the transmission speed of electrons and makes electrochemical reactions easier to occur.
Duplex stainless steel (DSS) is composed of ferrite phase and austenite phase. The ferrite phase mainly improves the stress corrosion resistance and crevice corrosion resistance of stainless steel, while the austenite phase mainly improves the toughness and toughness of stainless steel. Plasticity and weldability , therefore, duplex stainless steel has both the advantages of austenite and ferrite, and is widely used in many fields such as petroleum, chemical industry, shipbuilding, and ocean . But when the duplex stainless steel is at 300~1000 ℃, it is easy to produce carbides (M23C6), nitrides (Cr2N), σ phase, χ and other intermetallic compounds , so that its structure, mechanical properties and corrosion properties are all Will have an impact. Wang Yuexiang et al.  showed that the precipitation temperature of σ phase in 2205 duplex stainless steel is 850 ℃, and the as-cast or hot-rolled state does not affect the precipitation temperature of σ phase. Ananya et al.  showed that UNS S32205 stainless steel after solution treatment will precipitate σ and χ phases when aging at 800 ℃. These two phases have no effect on the stress corrosion performance, but after solid solution at 1150 ℃ and aging at 475 ℃ After treatment, UNS S32205 stainless steel experienced severe intergranular corrosion.
Hydrofluoric acid is an important catalyst that petrochemical companies need to use when producing alkylated oil. This substance is very corrosive. How to extend the service life of equipment has become an important topic for companies. The previous research of this research group has shown that 2205 duplex stainless steel has good hydrofluoric acid corrosion resistance and can replace Q235 steel commonly used in equipment. However, if there are precipitates in 2205 duplex stainless steel, how the precipitates affect its anti-hydrofluoric acid corrosion effect and the reasons for the changes have not been seen in the corresponding literature or related reports. Since 800 ℃ is in the temperature range where 2205 duplex stainless steel is easy to produce precipitates [4, 5], this article has aged 2205 duplex stainless steel at 800 ℃, and investigated the resistance of 2205 duplex stainless steel after aging for different times. Corrosion performance, hope to provide data support for enterprises to apply this material.
The experimental material is 2205 duplex stainless steel (2205 DSS Pipe Fittings), its chemical composition (mass fraction, %): C 0.025, Si 0.60, Mn 1.16, S 0.001, P 0.026, Ni 5.80, Cr 22.50, Mo 3.00, Fe balance. The corrosive medium is 5% HF (volume fraction), and the solution is prepared with chemically pure hydrofluoric acid and deionized water.
2205 DSS Pipe Fittings was solid-dissolved at 1050 ℃ for 60 minutes, and then the solid-dissolved samples were aged at 800 ℃ for 15, 60 and 120 minutes, and air cooled. Inlay the aged sample into a metallographic sample. After being polished and polished, it is corroded with an etching solution composed of 30 g KOH+30 g K3F (CN)6+100 mL deionized water for 1~3 min (85 ℃). Then use Leica metallurgical microscope to observe the morphology of each sample after solid solution and aging. X-7000 X-ray diffractometer (XRD) was used to analyze the precipitated phase composition of each sample after solid solution and aging.
The electrochemical test is completed by 2273 electrochemical workstation. A three-electrode system was used in the test. The 2205 DSS Pipe Fittings samples aged at different times were the working electrodes, and the test area of the samples was 1 cm2. The bulk graphite is the auxiliary electrode, and the saturated calomel electrode (SCE) is the reference electrode. During the test, the working electrode was pre-polarized at -1.3 V for 3 minutes to remove the oxide film formed on the surface of the sample in the air.
The polarization curve of 2205 DSS Pipe Fittings in 5%HF solution was tested by using the potentiodynamic polarization method. The scanning potential range starts from -0.20 V relative to the open circuit potential and ends when the potential reaches the overpassivation potential. The scan rate is 0.5 mV/s. The electrochemical impedance was tested under Ecor potential, the AC disturbance voltage amplitude was 10 mV, the measurement frequency range was 105~10-2 Hz, and ZSimpWin3.21 software was used to fit the impedance data. The Mott-Schottky curve test was performed on 2205 DSS Pipe Fittings under different aging times, the test frequency was 1000 Hz, and the excitation signal was 5 mV.
Results and discussion
The influence of aging time on tissue morphology
Figure 1 shows the tissue images of 2205 DSS Pipe Fittings at different times. It can be seen that the 2205 DSS Pipe Fittings in the solid solution state is composed of a white austenite phase and a gray ferrite phase, and the two structures are more evenly distributed. When the ageing treatment reaches 15 min, precipitates begin to occur at the ferrite grain boundary and the ferrite/austenite phase boundary; when the ageing time reaches 60 min, the ferrite grain boundary is significantly coarsened, resulting in precipitation There are also more phases, and the precipitated particles are also significantly larger.At the same time, the content of the ferrite phase begins to decrease; when the ageing time reaches 120 min, the content of the precipitated phase continues to increase, and the precipitate becomes coarser. The volume content is also reduced more significantly.
Fig.1 Microstructure images of 2205 DSS Pipe Fittings after aging at 800 ℃ for 0 min (a), 15 min (b), 60 min (c) and 120 min (d)
The effect of aging time on phase
The XRD test was performed on the 2205 DSS Pipe Fittings samples under different aging times, and the results are shown in Figure 2. When the aging time is 0 min, 2205 DSS Pipe Fittings has only γ and α phases; samples after aging for different times begin to show σ phase diffraction peaks, and as the aging time increases, the intensity of the σ phase diffraction peaks gradually becomes obvious, indicating The σ phase content is also gradually increasing , which is consistent with the results observed by the metallographic microscope. Due to the high content of Cr and Mo in the ferrite phase and the grain boundaries, these two elements are also the forming elements of the σ phase, thus promoting the nucleation of the σ phase . In addition, atoms diffuse more easily in the ferrite body-centered cubic lattice than in the austenite face-centered cubic lattice, therefore, the σ phase is preferentially formed at the ferrite phase and the γ/α junction, and With the extension of the aging time, this promotion effect will increase.
Fig.2 XRD spectra of 2205 DSS Pipe Fittings after aging at 800 ℃ for different time
Effect of Aging Time on the Polarization Curve of Potential
Figure 3 shows the potentiodynamic polarization curves of 2205 DSS Pipe Fittings in hydrofluoric acid solution under different aging time. It can be seen that compared with the solid solution state, the anodic polarization curve of the 2205DSS after the aging treatment still has an obvious passivation area, indicating that the aging sample can still be passivated and still has a good corrosion protection effect; However, compared with the solid solution state, the anodic polarization curve has shifted to the right, and the longer the aging time, the greater the shift to the right, indicating that the aging treatment reduces the hydrofluoric acid corrosion resistance of 2205 DSS Pipe Fittings.
Fig.3 Potentiodynamic polarization curves of 2205 DSS Pipe Fittings after aging in HF solution for different time
Table 1 shows the fitting results of the polarization curve in Figure 3. It can be seen that the self-corrosion potential of 2205 DSS Pipe Fittings in the solid solution state in 5% HF solution is relatively positive, while the self-corrosion potential of 2205 DSS Pipe Fittings after aging treatment becomes significantly negative, indicating that the aging treatment increases the corrosion of 2205 duplex stainless steel. trend. The fitting results of the self-corrosion current density show that the aging treatment increases the corrosion rate of 2205 DSS Pipe Fittings, and the longer the aging time, the greater the self-corrosion current density. In addition, the passive current density of 2205 DSS Pipe Fittings after aging treatment also gradually increased, indicating that the dissolution rate of the passive film formed by 2205 DSS Pipe Fittings in the solution gradually increased, and the corrosion resistance gradually decreased.
Table.1 Fitted results of polarization curves of 2205 DSS Pipe Fittings for different aging time in HF solution
The influence of aging time on electrochemical impedance spectroscopy
The electrochemical impedance spectra of 2205 DSS Pipe Fittings under different aging time are shown in Figure 4. It can be seen that the 2205 DSS Pipe Fittings in each state is characterized by capacitive reactance arc, and when the aging time is 15 minutes, the diameter of the capacitive reactance arc decreases significantly, and as the aging time increases, the diameter of the capacitive reactance arc gradually decreases. , The maximum phase angle also gradually decreases, which shows that the corrosion resistance of hydrofluoric acid is getting worse.
Fig.4 Nyquist plots (a) and Bode plots (b) of 2205 DSS Pipe Fittings after aging at 800 ℃ for different time
The equivalent circuit diagram of Figure 5 is used to fit the electrochemical impedance curve, where Rs is the solution resistance, Qf is the corrosion product film capacitance, Rf is the corrosion product film resistance, Qd is the electric double layer capacitance of the electrode surface, and Rt is the charge transfer. resistance. Figure 6 is a graph showing changes in charge transfer resistance and passivation film resistance under different aging times. It can be seen that with the extension of the aging time, the resistance of the passivation film and the charge transfer resistance show a downward trend, and the corrosion resistance gradually deteriorates.
Fig.5 Equivalent circuit for the electrochemical impedance plots
Fig.6 Rct and Rf curves of 2205 DSS Pipe Fittings after aging at 800 ℃ for different time
Mott-Schottky curve test
Generally speaking, the semiconductor performance of the passivation film affects the corrosion resistance of the material. Studies have shown that the passive film on the surface of stainless steel is composed of Fe oxide and Cr oxide . The semiconductor performance of the passivation film can be characterized by the Mott-Schottky equation . When the passivation film is an n-type semiconductor, the relationship between C and E is as in formula (1); when the passivation film is a p-type semiconductor, the relationship between C and E is as in formula (2).
In the formula, ε0 is the vacuum permittivity (8.85419×10-12 F/m), and ε is the dielectric constant of the passivation film at room temperature. This paper takes 15.6, ND is the donor density, NA is the acceptor density, and Efb is the flat belt Potential, E is the electrode potential, K is Boltzmann’s constant (1.38×10-23 J/K), T is the absolute temperature (K), e is the electron power (1.6×10-19 C), at room temperature KT/ q is about 25 mV, which can be ignored. According to the straight line formed by C -2~E in Mott-Schottky, the donor density ND and acceptor density NA in the passive film on the stainless steel surface can be obtained.
The Mott-Schokkty curve test was performed on the 2205 DSS Pipe Fittings samples under different aging times, and the results are shown in Figure 7. It can be seen that when the potential is 0.20~0.80 V, the slope of the Mott-Schokkty curve is positive, indicating that the passivation film formed is an n-type semiconductor. When the potential is 0.80~1.05 V, the slope of the Mott-Schokkty curve is negative, indicating that the passivation film formed is a p-type semiconductor. Therefore, the 2205 DSS Pipe Fittings surface passivation film in the solid solution state and the aging treatment has a p-n heterojunction double-layer structure .
Fig.7 Mott-Schottky plots of 2205 DSS Pipe Fittings after aging at 800 ℃ for different time
Table 2 shows the carrier density ND and NA of the passivation film formed on the surface of 2205 DSS Pipe Fittings under different aging times. It can be seen that in 5% HF solution, with the extension of aging time, the σ phase in 2205 DSS Pipe Fittings gradually increases , The carrier density ND and NA of the passivation film are both increasing, and the increase of the carrier density accelerates the transmission of electrons and promotes the electrochemical reaction at the passivation film/solution interface , So that the corrosion reaction occurs more quickly, thereby increasing the corrosion rate of 2205DSS in hydrofluoric acid solution. In addition, the increase of σ phase may also promote the change of the composition and structure of the passive film, which may also affect the corrosion resistance of duplex stainless steel, and the research on this aspect needs further research.
Table.2 Fitted results of donor density and acceptor density of 2205 DSS Pipe Fittings for different aging time
After 2205 duplex stainless steel is aged at 800 ℃, as the aging time increases from 15 min to 120 min, the self-corrosion current density and the passive current density of 2205 DSS Pipe Fittings in the hydrofluoric acid solution gradually increase, and the corrosion resistance is due to aging. And reduce. This may be because with the extension of the aging time, the σ phase precipitated by the 2205 DSS Pipe Fittings continues to increase, resulting in the gradual increase of the carrier density in the passivation film, the faster electron transmission, and the electrochemical reaction at the passivation film/solution interface As the speed increases, the corrosion rate increases.
Author: AN Pengliang, SONG Fei, LIANG Ping, QIN Hua, ZHAO Yan, PANG Xuguang
The authors have declared that no competing interests exist.
Source: China Duplex Stainless Steel Pipe Fittings Manufacturer – Yaang Pipe Industry Co., Limited (www.ugsteelmill.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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