Electrochemical corrosion behavior of 2205 and 316L stainless steel in hydrofluoric acid

The electrochemical behaviors of 2205 duplex stainless steel and 316L stainless steel in 5% (volume fraction) HF solution were investigated by potentiodynamic polarization and electrochemical impedance. The semiconductor characteristics of the passive films on the surfaces of the two kinds of stainless steel were analyzed by Mott schokkty curves. The results show that both kinds of stainless steel can be passivated in hydrofluoric acid solution, and the passivation range of 2205 duplex stainless steel is wider, and the passivation current density is lower. The surface passivation film of 2205 duplex stainless steel shows higher passivation film resistance and charge transfer resistance, and its anti hydrofluoric acid corrosion performance is better than that of 316L stainless steel, which is mainly related to such factors as high content of Mo and Cr in 2205 duplex stainless steel, less defects of surface passivation film, and easy repair of passivation film.

Abstract:

1. Preface

Hydrofluoric acid is a kind of reducing acid, which can be used as catalyst for alkylation oil production, as well as cleaning agent for silicate scale and iron oxide scale. Hydrofluoric acid has a strong corrosiveness. At present, the main ways to reduce the corrosion of hydrofluoric acid are reasonable selection of materials, use of organic coatings, addition of corrosion inhibitors, etc. [4], in order to extend the service life of equipment or equipment.
Monel alloy steel is the best anti hydrofluoric acid corrosion material at present, but the price of this material is expensive. It is unrealistic to use Monel alloy in large-scale petrochemical plant or whole set of equipment. In many cases, some parts involving a small amount of hydrofluoric acid mostly use carbon steel, but this increases the risk of equipment leakage and 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 stainless steel materials in petrochemical plants, both of which have good corrosion resistance and are widely used [4,5]. At present, there are many researches on the corrosion resistance of the two solutions in the solution containing Cl -, bromoacetic acid, sulfuric acid and so on [6,7], while there are few researches on the corrosion resistance of hydrofluoric acid [8,9]. Therefore, this paper compares the anti hydrofluoric acid corrosion ability of the two by electrochemical method, hoping to provide reference for engineering application.

2. Experimental method

See Table 1 for chemical composition of 2205 duplex stainless steel (2205 DSS) and 316L stainless steel (316L SS). The corrosion medium is 5% (volume fraction) HF. The solution is made up of chemical pure hydrofluoric acid and deionized water.
Table 1 Chemical compositions of 2205 DSS and 316L SS (mass fraction / %)

Steel C Si Mn S P Ni Cr Mo Fe
316L 0.023 0.635 1.11 0.003 0.026 10.7 16.5 2.05 Bal.
2205 0.025 0.60 1.16 0.001 0.026 5.80 22.50 3.00 Bal.

2205 DSS and 316L SS are working electrodes. The test area of the two samples is 1 cm2. After the sample is polished to 1000 × sandpaper, wipe the surface with ethanol, put it into a dryer for drying, and wait for the test.
The electrochemical test was completed by the parstat 2273 electrochemical workstation. The three electrode system was used in the test, with the platinum plate as the auxiliary electrode and the saturated calomel electrode (SCE) as the reference electrode. During the test, the working electrode was polarized at – 1.3 V for 3 min to remove the oxide film formed on the surface of the sample in the air.
The polarization curves of 2205 DSS and 316L s s in 5% HF solution were measured by potentiodynamic polarization method. The scanning potential range started from – 0.25 V relative to the open circuit potential, and ended when the potential reached the overpassivation potential. The scanning rate was 0.5 MV / s. The electrochemical impedance test was carried out under Ecorr potential. The amplitude of disturbance voltage was 10 MV, and the frequency range was 105 ~ 10-2 Hz. The impedance data was fitted by zsimpwin3.10 software. During the test, the solution temperature shall be controlled at (25 ± 2) ℃. The corrosion morphology after polarization was observed by Leica q500mw metallographic microscope.
The surface of 2205 DSS and 316L SS was polarized at 0.4 V constant potential for 2 h to form a passive film. Then the Mott Schottky curve of the passive film was tested. The test frequency was 1000 Hz, the excitation signal was 5 mV, and the potential scanning range was – 0.20 ~ + 1.1V.

3. Experimental results

3.1 open circuit potential curve
Figure 1 shows the open circuit potential curve of 2205 DSS and 316L SS in 5% HF solution. It can be seen that with the extension of corrosion time, the open circuit potential of both kinds of stainless steel moves to the positive direction. This is mainly because after the sample is immersed in the solution, the fresh metal surface begins to form passive film gradually. When the time reaches 2200s, the potential fluctuation range is very small, and the values tend to be stable at + 0.03 V and – 0.13 V, respectively. The potential of the former is higher than that of the latter, indicating that the passivation film formed on the surface of 2205 DSS has a more stable trend.

img 1 1 - Electrochemical corrosion behavior of 2205 and 316L stainless steel in hydrofluoric acid

Fig.1 Open circuit potentials of 2205 DSS and 316L SS in 5%HF solution

3.2 potentiodynamic polarization curve
Figure 2 shows the potentiodynamic polarization curves of 2205 DSS and 316L SS in 5% HF solution. It can be seen that the cathode part of the two polarization curves shows hydrogen evolution reaction, while the anode part has obvious passivation behavior, indicating that both materials can be passivated in this medium, but there is an activation passivation transition zone in the anode polarization curve of 316L SS, while there is no such zone in 2205 DSS, which directly enters the passivation state, and the passivation current density of 2205 DSS is smaller, indicating that the material is passivated. The passivation of material is easier than 316L SS.

img 2 1 - Electrochemical corrosion behavior of 2205 and 316L stainless steel in hydrofluoric acid

Fig.2 Potentiodynamic polarization curves of 2205 DSS and 316L SS 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 DSS in this solution is significantly higher than that of 316L SS. Therefore, from the perspective of thermodynamics, 2205 DSS shows a better anti hydrofluoric acid corrosion trend; from the perspective of kinetics, the self-corrosion current density of 2205 DSS in this solution is significantly lower than that of 316L SS, and the anti hydrofluoric acid corrosion ability is stronger. At the same time, it can be seen that the passivation range of 2205 DSS is much wider than that of 316L SS, and the over passivation potential of 2205 DSS is higher. Moreover, the current density corresponding to the over passivation potential of 2205 DSS is about 1.2502 × 10-5 A / cm2, and the current density corresponding to the over passivation potential of 316L SS is about 2.3823 × 10-5 A / cm2. All these parameters show that 2205 DSS has stronger anti hydrofluoric acid corrosion performance than 316L SS.

Table 2 Fitted results of potentiodynamic polarization curves of 2205 DSS and 316L SS

Steel Ecorr
mVSCE
Icorr
μAcm-2
Passive current densityμAcm-2 Pitting potentialmVSCE Passive potential rangemVSCE
316L -205 34.2 -20.7 488 329~488
2205 -38.3 5.6 -11.3 850 150~850

Figure 3 shows the macromorphology of 2205 DSS and 316L SS after polarization in 5% HF solution. It can be seen that pitting occurred on the surfaces of both kinds of stainless steels, but the number of pitting on 2205 DSS surface is much less than that on 316L SS, which is consistent with the law reflected by polarization curve.

img 3 1 - Electrochemical corrosion behavior of 2205 and 316L stainless steel in hydrofluoric acid

Fig.3 Metallography images of 2205 DSS (a) and 316L SS (b) in 5%HF solution after polarization

3.3 electrochemical impedance curve
FIG. 4A and B are the electrochemical impedance spectra of 2205 DSS and 316L SS in 5% HF solution, respectively. It can be seen that the Nyquist curves of both of them show capacitive arc characteristics, but the real and imaginary values of the Nyquist curves of 2205 DSS are much higher than those of 316L SS.

img 4 1 - Electrochemical corrosion behavior of 2205 and 316L stainless steel in hydrofluoric acid

Fig.4 Nyquist (a, b) and Bode (c, d) plots of passive films formed on 2205 DSS and 316L SS in 5%HF solution

Figure 4C shows the Bode pattern of 2205 DSS and 316L SS in 5% HF solution. It can be seen that the maximum phase angle of 2205 DSS is about 82 °, and the range of the maximum phase angle is wide, while the maximum phase angle of 316L SS is about 70 °, the range of the maximum phase angle is narrow, and the phase angle of low-frequency segment is also significantly lower than that of 2205 DSS. In addition, it can be seen from Fig. 4D that the impedance mode value of 2205 DSS is also far greater than that of 316L SS. Therefore, these curves of electrochemical impedance can reflect that the corrosion resistance of 2205 DSS is better than that of 316L SS.

img 5 1 - Electrochemical corrosion behavior of 2205 and 316L stainless steel in hydrofluoric acid

Fig.5 Equivalent circuit for the electrochemicalimpedance plots

The electrochemical impedance curves of 2205 DSS and 316L SS are fitted with the equivalent circuit diagrams 5A and B respectively, where RS is the solution resistance, QF is the corrosion product film capacitance, RF is the corrosion product film resistance, QD is the double layer capacitance on the electrode surface, RT is the charge transfer resistance, l is the inductive resistance and RL is the resistance related to the inductive resistance. The pipe fittings results are shown in Table 3. It can be seen that the film resistance of 2205 DSS is about 2.5 times that of 316L SS, and the charge transfer resistance is about 49.5 times that of 316L SS, indicating that the corrosion resistance of 2205 DSS is better than that of 316L SS.

Table 3 Fitted results of electrochemical impedance plots

Steel Rs / Ωcm2 Qf nf Rf / Ωcm2 Qt nt Rt / Ωcm2
316L 2.69 2.369×10-5 1 58.4 8.848×10-5 0.7347 351
2205 3.09 1.740×10-5 1 146.3 3.202×10-5 0.7073 17430

3.4 Mott schokkty curve
In many cases, the semiconductor properties of passive films are closely related to the Pitting Behavior of materials. By testing Mott Schottky curve, we can determine the type of passive film, donor density nd and / or acceptor density Na and other parameters. Specific calculation formulas can be found in reference [4].
Fig. 6 shows Mott-Schokkty curves of passive films formed by constant potential polarization of 2205 DSS and 316L SS at 0.4V for 2 h. It can be seen that in the range of – 0.20 ~ + 0.80 V, the slopes of the pipe fitting lines of 316L SS and 2205 DSS are all positive, indicating that the passivation films on the surfaces of the two kinds of stainless steels are n-type semiconductors. As the potential continues to rise, i.e. in the range of + 0.85 ~ + 1.1V, the slope of the pipe fitting straight line of 316L SS and 2205 DSS is negative, indicating that the two passivation films are p-type semiconductors. This is because the potential area is in the range of passivation, the oxide film dissolves, some high valence oxides are broken down, while Fe, Cr and Mo at the bottom of the film may be corroded to form low valence oxygen. As a result, the oxide composition of the passivation film changes and the semiconductor characteristics of the passivation film change [11].

img 6 - Electrochemical corrosion behavior of 2205 and 316L stainless steel in hydrofluoric acid

Fig.6 Mott-Schottky plots of the passive films formed on2205 DSS and 316L SS

The pipe fittings results of donor density nd and acceptor density Na of 316L SS and 2205 DSS surface passivation film are shown in Table 4. It can be seen that the donor density and acceptor density of 2205 DSS surface passivation film are lower than 316L SS.

Table 4 Fitted values of donor density and acceptor density of passive film for 2205 and 316L SS

Potentialrange Material Slope109 ND / NA1021 cm-3
-0.2~+0.80 V 2205 5.7374 1.576
-0.2~+0.80 V 316L 4.7104 1.920
+0.85~+1.1 V 2205 -1.5657 0.577
+0.85~+1.1 V 316L -1.3313 0.679

4. Discussion

4.1 Effect of material composition on passivation film
The polarization curves show that both 316L SS and 2205 DSS can form passive films in HF solution. Literature shows that Cr2O3, FeO and NiO are the main passivation films of 316L SS in CL containing solution [12], and the composition of the passivation films in acetic acid solution is the same as that of Cr2O3, FeO and NiO [11]. The passivation film formed on the surface of 2205 DSS in artificial seawater is in the form of Cr2O3, FeO, Fe3O4, MoO2, NiO, etc. [13], and 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% [14]. It can be seen that for 316L SS and 2205 DSS, the main components of the passivation film on the surface of 316L SS do not change significantly with the change of medium. It can be concluded that Cr2O3, FeO and NiO are the main passivation films of 316L SS in hydrofluoric acid medium, while 2205 DSS contains more Cr and Mo than 316L SS, the former is the main forming element of the passivation film, and the latter can enhance the stability of the passivation film [15]. Therefore, the passivation film of 2205 DSS in HF medium contains MoO2 and more Cr2O3 besides FeO and NiO, which makes 2205 DSS table. The surface passivation film is denser and more stable than that of 316L SS, which can be confirmed by the results of AC impedance curve pipe fitting that 2205 DSS shows higher film resistance and charge transfer resistance.
4.2 Effect of passivation film performance
The Mott schokkty curve analysis results in Figure 6 have shown that the donor density and acceptor density of the passivation film on the 2205 DSS surface are smaller than those on the 316L SS under the same film forming potential and film forming time, indicating that the number of defects on the passivation film on the 2205 DSS surface is less, and these defects are often the main places where the corrosion reaction takes place, so the active point or active area where the 2205 DSS surface corrosion takes place is more Less. In addition, the passivation film is also in the process of self repair. Compared with 316L SS, the repair process of 2205 DSS passive film is easier and faster than 316L SS [11]. Therefore, the passivation film on 2205 DSS surface shows lower pitting sensitivity.
4.3 Organizational impact
Microstructure is also an important factor affecting the corrosion resistance of materials. 2205 DSS is composed of ferrite phase and austenite phase. The austenite phase is dispersed in the ferrite phase. The results of TASI et al. [16] show that the corrosion resistance of the passive film formed on the surface of ferrite phase and austenite phase is different. Cheng et al. [17] also showed that in 0.5 mol / L NaHCO3 + 0.5 mol / L NaCl solution, the thickness of the passivation film formed on the surface of ferrite phase is thicker than that on the surface of austenite phase, and the corrosion reaction is more likely to occur on the austenite phase. The microstructure of 316L SS is a single austenite, and a uniform passivation film can be formed on its surface. Assuming that the formation rate of the passivation film formed by the austenite phase in 316L SS is the same as that of the austenite phase in 2205 DSS, the thickness of the passivation film formed on the austenite phase surface of the two materials can be assumed to be the same, while the thickness of the passivation film formed on the ferrite phase surface of 2205 DSS may be thicker, resulting in the thickness of the passivation film on the whole surface of 2205 DSS being thicker than that of 316L SS, and resistant to the interference of aggressive ions.

5. Conclusion

  • (1) Both 2205 DSS and 316L SS have obvious passivation zone in 5% HF solution, and 2205 DSS has wider passivation range and lower passivation current density. The electrochemical impedance curve pipe fitting results show that 2205 DSS has higher passive film resistance and charge transfer resistance.
  • (2) The passivation films of 2205 DSS and 316L SS formed on the surface of 5% HF solution show the same semiconductor type characteristics, but the defects of the former passivation films are less.
  • (3) 2205 DSS has better anti hydrofluoric acid corrosion performance than 316L SS, which is mainly related to the high content of Mo and Cr in 2205 DSS, the few defects of passive film, and the easy repair of passive film.

Source: China Pipe Fitting Manufacturer – Yaang Pipe Industry Co., Limited (www.steeljrv.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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Summary
electrochemical corrosion behavior of 2205 and 316l stainless steel in hydrofluoric acid - Electrochemical corrosion behavior of 2205 and 316L stainless steel in hydrofluoric acid
Article Name
Electrochemical corrosion behavior of 2205 and 316L stainless steel in hydrofluoric acid
Description
The electrochemical behaviors of 2205 duplex stainless steel and 316L stainless steel in 5% (volume fraction) HF solution were investigated by potentiodynamic polarization and electrochemical impedance.
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