Electrochemical corrosion behavior of 2205 and 316L stainless steel flanges in hydrofluoric acid
The electrochemical behaviors of 2205 duplex stainless steel flange and 316L stainless steel flange in 5% (volume fraction) HF solution were investigated by potentiodynamic polarization and electrochemical impedance spectroscopy. The semiconductor properties of the passive films on the surfaces of the two stainless steels were analyzed by Mott schokkty curves. The results show that both kinds of stainless steel can be passivated in hydrofluoric acid solution, and 2205 duplex stainless steel has wider passivation range and lower dimensional passivation current density. The results show that the surface passive film of 2205 duplex stainless steel has higher passive film resistance and charge transfer resistance, and its hydrofluoric acid corrosion resistance is better than that of 316L stainless steel, which is mainly related to the high content of Mo and Cr in 2205 duplex stainless steel, less surface passive film defects and easy repair of passive film.
Hydrofluoric acid is a kind of reducing acid, which can be used as catalyst for alkylation oil production, and also as cleaning agent for cleaning silicate scale and iron oxide scale. Hydrofluoric acid has strong corrosiveness. At present, it mainly reduces the corrosion of hydrofluoric acid by selecting materials reasonably, using organic coating and adding corrosion inhibitor, so as to prolong the service life of equipment or equipment.
Monel alloy steel is the best material with the best corrosion resistance to hydrofluoric acid at present, but it is not realistic to use Monel alloy in large petrochemical plants or whole equipment. In many cases, some parts involving a small amount of hydrofluoric acid mostly adopt carbon steel, but this increases the risk of equipment leakage and the maintenance times, and shortens the safe operation cycle of the device. Therefore, it is necessary to study the corrosion resistance of other materials. 316L stainless steel and 2205 duplex stainless steel are commonly used in petrochemical plants. Both of them have good corrosion resistance and are widely used in the field of application. At present, there are many researches on the two solutions, such as Cl-containing solution, brominated acetic acid and sulfuric acid, but few studies on the corrosion resistance of hydrofluoric acid. Therefore, the electrochemical method is used to compare the corrosion resistance of hydrofluoric acid, and it is hoped that it can be used for reference in engineering application.
See Table 1 for chemical composition of 2205 flange and 316L flange. The corrosion medium is 5% (volume fraction) HF, and the solution is prepared by hydrofluoric acid chemical purity and deionized water.
Table 1 Chemical compositions of 2205 flange and 316L flange (mass fraction / %)
2205 flange and 316L flange are working electrodes, and the test area of both samples is 1 cm2. After the sample is polished to 1000 # sand paper, wipe the surface with ethanol, put it into a dryer for drying and wait for testing.
The electrochemical test was completed by the parstat 2273 electrochemical workstation. The three electrode system was used in the test, platinum plate was the auxiliary electrode, and the saturated mercury electrode (SCE) was the reference electrode (the potential without special explanation in this paper is relative to SCE). The working electrode was polarized at -1.3 V for 3 minutes to remove the oxide film formed on the surface of the sample in air.
The polarization curves of 2205 flange and 316L flange in 5% HF solution were measured by dynamic potential polarization method. The scanning potential range starts from the open circuit potential-0.25 V, and the potential reaches the passivation potential and ends with scanning rate of 0.5 MV / s. The electrochemical impedance test was carried out at the Ecorr potential, the amplitude of the disturbance voltage was 10 MV, and the frequency range was 105-10-2 Hz. The impedance data were fitted by zsimpwin3.10 software. The temperature of the solution is controlled at (25 ± 2) ℃. The corrosion morphology of polarization was observed by Leica q500mw metallographic microscope.
The passivation film was formed on the surface of 2205 flange and 316L flange after 0.4 V potentiostatic polarization for 2 hours. Then Mott Schottky curve test was carried out on the passivation film. The test frequency was 1000 Hz, the excitation signal was 5 mV, and the potential scanning interval was -0.20~+1.1 v.
Open circuit potential curve
Figure 1 shows the open circuit potential curve of 2205 flange and 316L flange in 5% HF solution. It can be seen that with the corrosion time, both stainless steel open circuit potential moves in the positive direction, mainly because after the sample is immersed in the solution, the passivation film begins to form on the fresh metal surface. When the time reaches 2200 s, the potential fluctuation range is very small, and the value tends to be stable, which is stable at +0.03 V and-0.13 V respectively. The former is higher than the latter, which shows that the passivation film on the surface of 2205 flange has a more stable trend.
Fig.1 open circuit potentials of 2205 flange and 316L flange in 5% HF solution
Dynamic potential polarization curve
Figure 2 shows the dynamic potential polarization curve of 2205 flange and 316L flange in 5% HF solution. It can be seen that both cathode parts of polarization curve show hydrogen evolution reaction, and anode part has obvious passivation behavior, which indicates that both materials can be passivated in this medium, but the anode polarization curve of 316L flange has activation passivation transition zone, while 2205 flange does not exist in this region, and it enters passivation state directly, and the dimension blunt current density of 2205 flange is smaller, It is shown that the passivation of the material is more easily than that of 316L flange.
Fig.2 potentiodynamic polarization curves of 2205 flange and 316L flange in 5% HF solution
The electrochemical parameters of the two polarization curves are numerically fitted, and the results are shown in Table 2. It can be seen that the self corrosion potential of 2205 flange in this solution is significantly higher than that of 316L flange. Therefore, from the thermodynamic point of view, 2205 flange shows better resistance to hydrofluoric acid corrosion; from the dynamic point of view, the self-corrosion current density of 2205 flange in this solution is obviously lower than that of 316L flange, and the corrosion resistance of HFR is stronger. It can be seen that the passivation range of 2205 flange is much wider than that of 316L flange, and the over passivation potential of 2205 flange is about 1.2502 × 10-5 a/cm2, and the current density of 316L flange is about 2.3823 × 10-5 a/cm2. These parameters show that 2205 flange has better resistance to hydrofluoric acid corrosion than 316L flange.
Table 2 fixed results of potentiodynamic polarization curves of 2205 flange and 316L flange
|Steel||Ecorr||Icorr||Passive current densityμAcm-2||Pitting potentialmVSCE||Passive potential rangemVSCE|
Figure 3 shows the macro morphology of 2205 flange and 316L flange after polarization in 5% HF solution. It can be seen that pitting has occurred on both stainless steel surfaces, but the number of pits on the surface of 2205 flange is much less than that of 316L flange, which is consistent with the law reflected by polarization curve.
Fig.3 metal images of 2205 flange (a) and 316L flange (b) in 5% HF solution after polarization
Electrochemical impedance curve
FIG. 4A and B are the electrochemical impedance spectra of 2205 flange and 316L flange in 5% HF solution. It can be seen that Nyquist curves of both are characteristic of resistance arc, but the real and imaginary part values of Nyquist curve of 2205 flange are much larger than that of 316L flange.
Fig.4 Nyquist (a, b) and Bode (C, d) plots of passive films formed on 2205 flange and 316L flange in 5% HF solution
Figure 4C shows bode pattern of 2205 flange and 316L flange in 5% HF solution. It can be seen that the maximum phase angle of 2205 flange is about 82 °, and the range of maximum phase angle is wide, while the maximum phase angle of 316L flange is about 70 °, the range of maximum phase angle is narrow, and the phase angle in low frequency section is significantly lower than that of 2205 flange. In addition, it can be seen from Fig. 4D that the impedance mode value of 2205 flange is far greater than that of 316L flange. Therefore, the curves of electrochemical impedance can reflect that the resistance of 2205 flange to hydrofluoric acid is better than 316L flange.
Fig.5 Equivalent circuit for the electrochemicalimpedance plots
The electrochemical impedance curves of 2205 flange and 316L flange are fitted by using equivalent circuit diagrams 5A and B respectively. RS is solution resistance, QF is corrosion product membrane capacitance, RF is corrosion product film resistance, QD is double layer capacitance on electrode surface, RT is charge transfer resistance, l is inductance, RL is resistance related to inductance. The fitting results are shown in Table 3. It can be seen that the film resistance of 2205 flange is about 2.5 times of that of 316L flange and 49.5 times of 316L flange, which shows that the corrosion resistance of 2205 flange is better than that of 316L flange.
Table 3 Fitted results of electrochemical impedance plots
|Steel||Rs / Ωcm2||Qf||nf||Rf / Ωcm2||Qt||nt||Rt / Ωcm2|
Mott schookty curve
In many cases, the semiconductor properties of passivation film are closely related to the Pitting Behavior of the materials. By testing Mott Schottky curve, the types of passivation film and the parameters of Nd and / or Na can be determined. The specific calculation formula can be referred to.
Figure 6 shows the Mott schookty curve of passivation film formed by 2205 flange and 316L flange under 0.4V potentiostatic polarization for 2 h. It can be seen that the slope of the fitting straight line of 316L flange and 2205 flange is positive in the range of -0.20~+0.80 V, indicating that the passivation film on both stainless steel surfaces is characterized by n-type semiconductor. With the increase of potential, i.e. in the range of +0.85~+1.1v, the fitting linear slope of 316L flange and 2205 flange is negative, indicating that both passivation films are p-type semiconductor characteristics, which is because the potential region is within the passivation range of passivation film, the oxide film dissolves and some high-priced oxides are broken down, However, Fe, Cr and Mo at the bottom of the film may be corroded to form low-cost oxides, which results in the change of oxide composition of passivation film, and the semiconductor characteristics of passivation film.
Fig.6 Mott Schottky plots of the passive films formed on 2205 flange and 316L flange
The fitting results of Nd and Na of passivation film on the surface of 316L flange and 2205 flange are shown in Table 4. It can be seen that the donor density and acceptor density in passivation film on the surface of 2205 flange are lower than that of 316L flange.
Table 4 fixed values of donor density and acceptor density of passive film for 2205 and 316L flange
|Potentialrange||Material||Slope109||ND / NA1021 cm-3|
Influence of material composition on passivation film
The results of polarization curve test show that passivation film can be formed in HF solution for 316L flange and 2205 flange. The literature shows that the passivation film of 316L flange in Cl-containing solution is mainly Cr2O3, FeO and NiO, and the passivation film composition in acetic acid solution is the same. The passivation film formed on the surface of 2205 flange in artificial seawater is in the form of Cr2O3, FeO, Fe3O4, MoO2, NiO, etc. the passivation film formed in 2 mol/l H2SO4 + 0.5 mol/l HCl solution is mainly in the form of Cr2O3, accounting for 94.6%. It can be seen that the main components of passivation film on 316L flange and 2205 flange will not change with the change of medium. It can be concluded that the passivation film formed by 316L flange in hydrofluoric acid medium should also be Cr2O3, FeO and NiO. However, 2205 flange contains more Cr and Mo than 316L flange, the former is the main forming element of passivation film, and the latter can enhance the stability of passivation film. Therefore, in addition to FeO and NiO, the passivation film of 2205 flange in HF medium contains MoO2 and more Cr2O3, The passivation film on the surface of 2205 flange is more compact and stable than that of 316L flange, which can be confirmed by the results of AC impedance curve fitting.
Influence of passivation film performance
The Mott schockky curve analysis results in Figure 6 have shown that, under the same film forming potential and film forming time, the applied and acceptor density of passivation film on the surface of 2205 flange are smaller than 316L flange, indicating that the number of defects on passivation film on the surface of 2205 flange is less, and these defects are often the main locations of corrosion reaction, Therefore, the corrosion of 2205 flange surface has less active point or active area. In addition, the passivation film also occurs self repairing process during the corrosion process. Compared with 316L flange, the repair process of passivation film of 2205 flange is easier and faster than that of 316L flange. Therefore, passivation film on 2205 flange surface shows lower pitting sensitivity.
The structure is also an important factor that affects the corrosion resistance of the material. 2205 flange is composed of ferrite phase and austenite phase, and austenite phase is dispersed in ferrite phase. TASI and other research results show that the corrosion resistance of passivation film on the surface of ferrite and austenite phase is different. Cheng and other studies also show that in 0.5 mol/l NaHCO3 + 0.5 mol/l NaCl solution, the thickness of passivation film on ferrite surface is a little thicker than that of austenite phase, and corrosion reaction is more likely to occur in austenite phase. 316L flange has a single austenite structure, and the surface can form a uniform passivation film. Assuming that the formation rate of passivation film of austenitic phase in 316L flange and that of 2205 flange is the same, the thickness of passivation film on austenite phase surface of two materials can be assumed to be consistent, while that of ferrite phase surface of 2205 flange may be thicker, which leads to the thickness of passivation film on the whole surface of 2205 flange is thicker than that of 316L flange, The interference ability of corrosion resistant ions is stronger.
- (1) There are obvious passivation zones in the anodic polarization curves of 2205 flange and 316L flange in 5% HF solution, and the passivation zone of 2205 flange is wider and the dimensional passivation current density is lower. The impedance curve of 2205 is higher than that of 2205.
- (2) The passive film formed on the surface of 2205 flange and 316L flange in 5% HF solution shows the same semiconductor type characteristics, but the number of defects in the passive film of the former is less.
- (3) 2205 flange has better hydrofluoric acid corrosion resistance than 316L flange, which is mainly related to the high content of Mo and Cr in 2205 flange, less defects of passive film and easy repair of passive film.
The authors have declared that no competing interests exist.
Author: PANG Xuguang1, LIANG Ping1, ZHANG Yunxia2, SHI Yanhua1, ZHAO Yan1, LIU Feng1 (Electrochemical Corrosion Behavior of Stainless Steels 2205 and 316L in Hydrofluoric Acid[J]. 2016, 28(6): 537-542 https://doi.org/10.11903/1002.6495.2016.038)
Source: Network Arrangement – China 2205 Flange Manufacturer – Yaang Pipe Industry (www.metallicsteel.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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