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Duplex 2205 stainless steel (DSS) has good corrosion resistance due to its typical duplex structure, but the increasingly harsh CO2-containing oil and gas environment results in varying degrees of corrosion, especially pitting, which seriously threatens the safety and reliability of oil and natural gas applications. gas development. In this work, an immersion test and an electrochemical test are used in combination with laser confocal microscopy and X-ray photoelectron spectroscopy. The results showed that the average critical temperature for pitting 2205 DSS was 66.9 °C. When the temperature is higher than 66.9℃, the pitting breakdown potential, the passivation interval and the self-corrosion potential are reduced, the size passivation current density is increased, and the pitting sensitivity is increased. With a further increase in temperature, the radius of the capacitive arc 2205 DSS decreases, the surface resistance and charge transfer resistance gradually decrease, and the density of donor and acceptor carriers in the film layer of the product with n + p-bipolar characteristics also increases, the content of Cr oxides in the inner layer of the film decreases, increases the content of Fe oxides in the outer layer, the dissolution of the film layer increases, the stability decreases, the number of pits and the pore size increase.
In the context of rapid economic and social development and social progress, the demand for oil and gas resources continues to grow, forcing oil and gas development to gradually shift to the southwestern and offshore areas with more severe conditions and environment, so the operating conditions of downhole tubing become more and more severe. . Deterioration 1,2,3. In the field of oil and gas exploration, when the increase in CO2 4 and salinity and chlorine content 5, 6 in the produced fluid, ordinary 7 carbon steel pipe is subject to serious corrosion, even if corrosion inhibitors are pumped into the pipe string, corrosion cannot be effectively suppressed steel can no longer meet the requirements of long-term operation in harsh corrosive CO28,9,10 environments. The researchers turned to duplex stainless steels (DSS) with better corrosion resistance. 2205 DSS, the content of ferrite and austenite in the steel is about 50%, has excellent mechanical properties and corrosion resistance, the surface passivation film is dense, has excellent uniform corrosion resistance, the price is lower than that of nickel-based alloys 11, 12. Thus, 2205 DSS is commonly used as pressure vessel in corrosive environment, oil well casing in corrosive CO2 environment, water cooler for condensing system in offshore oil and chemical fields 13, 14, 15, but 2205 DSS can also have corrosive perforation in service.
At present, many studies of CO2- and Cl-pitting corrosion 2205 DSS have been carried out in the country and abroad [16,17,18]. Ebrahimi19 found that adding a potassium dichromate salt to a NaCl solution can inhibit 2205 DSS pitting, and increasing the concentration of potassium dichromate increases the critical temperature of 2205 DSS pitting. However, the pitting potential of 2205 DSS increases due to the addition of a certain concentration of NaCl to potassium dichromate and decreases with increasing NaCl concentration. Han20 shows that at 30 to 120°C, the structure of 2205 DSS passivating film is a mixture of Cr2O3 inner layer, FeO outer layer, and rich Cr; when the temperature rises to 150 °C, the passivation film dissolves. , the internal structure changes to Cr2O3 and Cr(OH)3, and the outer layer changes to Fe(II,III) oxide and Fe(III) hydroxide. Peguet21 found that stationary pitting of S2205 stainless steel in NaCl solution usually occurs not below the critical pitting temperature (CPT) but in the transformation temperature range (TTI). Thiadi22 concluded that as the concentration of NaCl increases, the corrosion resistance of S2205 DSS decreases significantly, and the more negative the applied potential, the worse the corrosion resistance of the material.
In this article, dynamic potential scanning, impedance spectroscopy, constant potential, Mott-Schottky curve and optical electron microscopy were used to study the effect of high salinity, high Cl– concentration and temperature on the corrosion behavior of 2205 DSS. and photoelectron spectroscopy, which provides the theoretical basis for the safe operation of the 2205 DSS in oil and gas environments containing CO2.
The test material is selected from solution treated steel 2205 DSS (steel grade 110ksi), and the main chemical composition is shown in Table 1.
The size of the electrochemical sample is 10 mm × 10 mm × 5 mm, it is cleaned with acetone to remove oil and absolute ethanol and dried. The back of the test piece is soldered to connect the appropriate length of copper wire. After welding, use a multimeter (VC9801A) to check the electrical conductivity of the welded test piece, and then seal the non-working surface with epoxy. Use 400#, 600#, 800#, 1200#, 2000# silicon carbide water sandpaper to polish the work surface on the polishing machine with 0.25um polishing agent until the surface roughness Ra≤1.6um, and finally clean and put in the thermostat.
A Priston (P4000A) electrochemical workstation with a three-electrode system was used. A platinum electrode (Pt) with an area of 1 cm2 served as the auxiliary electrode, a DSS 2205 (with an area of 1 cm2) was used as a working electrode, and a reference electrode (Ag/AgCl) was used. The model solution used in the test was prepared according to (Table 2). Before the test, a high-purity N2 solution (99.99%) was passed for 1 h, and then CO2 was passed for 30 min to deoxygenate the solution. , and CO2 in the solution was always in a state of saturation.
First, place the sample in the tank containing the test solution, and place it in a constant temperature water bath. The initial setting temperature is 2°C, and the temperature rise is controlled at a rate of 1°C/min, and the temperature range is controlled. at 2-80°C. Celsius. The test starts at a constant potential (-0.6142 Vs.Ag/AgCl) and the test curve is an It curve. According to the critical pitting temperature test standard, the It curve can be known. The temperature at which the current density rises to 100 μA/cm2 is called the critical pitting temperature. The average critical temperature for pitting is 66.9 °C. The test temperatures for the polarization curve and the impedance spectrum were chosen to be 30°C, 45°C, 60°C and 75°C, respectively, and the test was repeated three times under the same sample conditions to reduce possible deviations.
A metal sample exposed to the solution was first polarized at a cathode potential (-1.3 V) for 5 min before testing the potentiodynamic polarization curve to eliminate the oxide film formed on the working surface of the sample, and then at an open circuit potential of 1 h until the corrosion voltage will not be established. The scan rate of the dynamic potential polarization curve was set to 0.333mV/s, and the scan interval potential was set to -0.3~1.2V vs. OCP. To ensure the accuracy of the test, the same test conditions were repeated 3 times.
Impedance spectrum testing software – Versa Studio. The test was first carried out at a steady open-circuit potential, the amplitude of the alternating disturbance voltage was set to 10 mV, and the measurement frequency was set to 10–2–105 Hz. spectrum data after testing.
Current time curve testing process: select different passivation potentials according to the results of the anodic polarization curve, measure the It curve at constant potential, and fit the double logarithm curve to calculate the slope of the fitted curve for film analysis. the mechanism of formation of the passivating film.
After the open circuit voltage stabilizes, perform a Mott-Schottky curve test. Test potential scan range 1.0~-1.0V (vS.Ag/AgCl), scan rate 20mV/s, test frequency set to 1000Hz, excitation signal 5mV.
Use X-ray photoelectron spectroscopy (XPS) (ESCALAB 250Xi, UK) to sputter test the composition and chemical state of the surface passivation film after 2205 DSS film formation and perform measurement data peak-fit processing using superior software. compared with databases of atomic spectra and related literature23 and calibrated using C1s (284.8 eV). The morphology of corrosion and the depth of pits on the samples were characterized using an ultra-deep optical digital microscope (Zeiss Smart Zoom5, Germany).
The sample was tested at the same potential (-0.6142 V rel. Ag/AgCl) by the constant potential method and the corrosion current curve was recorded with time. According to the CPT test standard, the polarization current density gradually increases with increasing temperature. 1 shows the critical pitting temperature of 2205 DSS in a simulated solution containing 100 g/L Cl– and saturated CO2. It can be seen that at a low temperature of the solution, the current density practically does not change with increasing testing time. And when the temperature of the solution increased to a certain value, the current density increased rapidly, indicating that the rate of dissolution of the passivating film increased with the increase in the temperature of the solution. When the temperature of the solid solution is increased from 2°C to about 67°C, the polarization current density of 2205DSS increases to 100µA/cm2, and the average critical pitting temperature of 2205DSS is 66.9°C, which is about 16.6°C higher than than the 2205DSS. standard 3.5 wt. % NaCl (0.7 V)26. The critical pitting temperature depends on the applied potential at the time of measurement: the lower the applied potential, the higher the measured critical pitting temperature.
Pitting critical temperature curve of 2205 duplex stainless steel in a simulated solution containing 100 g/L Cl– and saturated CO2.
On fig. 2 shows ac impedance plots of the 2205 DSS in simulated solutions containing 100 g/L Cl- and saturated CO2 at various temperatures. It can be seen that the Nyquist diagram of the 2205DSS at various temperatures consists of high-frequency, mid-frequency and low-frequency resistance-capacitance arcs, and the resistance-capacitance arcs are not semicircular. The radius of the capacitive arc reflects the resistance value of the passivating film and the value of the charge transfer resistance during the electrode reaction. It is generally accepted that the larger the radius of the capacitive arc, the better the corrosion resistance of the metal substrate in solution27. At a solution temperature of 30 °C, the radius of the capacitive arc on the Nyquist diagram and the phase angle on the diagram of the impedance modulus |Z| Bode is the highest and 2205 DSS corrosion is the lowest. As the solution temperature increases, the |Z| impedance modulus, arc radius and solution resistance decrease, in addition, the phase angle also decreases from 79 Ω to 58 Ω in the intermediate frequency region, showing a wide peak and a dense inner layer and a sparse (porous) outer layer are the main features of an inhomogeneous passive film28. Therefore, as the temperature rises, the passivating film formed on the surface of the metal substrate dissolves and cracks, which weakens the protective properties of the substrate and deteriorates the corrosion resistance of the material29.
Using the ZSimDeme software to fit the impedance spectrum data, the fitted equivalent circuit is shown in Fig. 330, where Rs is the simulated solution resistance, Q1 is the film capacitance, Rf is the resistance of the generated passivating film, Q2 is the double layer capacitance, and Rct is the charge transfer resistance. From the results of fitting in table. 3 shows that as the temperature of the simulated solution increases, the value of n1 decreases from 0.841 to 0.769, which indicates an increase in the gap between the two-layer capacitors and a decrease in density. The charge transfer resistance Rct gradually decreased from 2.958×1014 to 2.541×103 Ω cm2, which indicated a gradual decrease in the corrosion resistance of the material. The resistance of the solution Rs decreased from 2.953 to 2.469 Ω cm2, and the capacitance Q2 of the passivating film decreased from 5.430 10-4 to 1.147 10-3 Ω cm2, the conductivity of the solution increased, the stability of the passivating film decreased, and the solution Cl-, SO42-, etc.) in the medium increases, which accelerates the destruction of the passivating film31. This leads to a decrease in the film resistance Rf (from 4662 to 849 Ω cm2) and a decrease in the polarization resistance Rp (Rct+Rf) formed on the surface of the duplex stainless steel.
Therefore, the temperature of the solution affects the corrosion resistance of DSS 2205. At a low temperature of the solution, a reaction process occurs between the cathode and the anode in the presence of Fe2 +, which contributes to the rapid dissolution and corrosion of the anode, as well as the passivation of the film formed on the surface, more complete and higher Density, greater resistance charge transfer between solutions, slows down the dissolution of the metal matrix and exhibits better corrosion resistance. As the temperature of the solution increases, the resistance to charge transfer Rct decreases, the rate of reaction between ions in the solution accelerates, and the rate of diffusion of aggressive ions accelerates, so that the initial corrosion products are again formed on the surface of the substrate from the surface of the metal substrate. A thinner passivating film weakens the protective properties of the substrate.
On fig. Figure 4 shows the dynamic potential polarization curves of 2205 DSS in simulated solutions containing 100 g/L Cl– and saturated CO2 at various temperatures. It can be seen from the figure that when the potential is in the range from -0.4 to 0.9 V, the anode curves at different temperatures have obvious passivation regions, and the self-corrosion potential is about -0.7 to -0.5 V. As the density increases current up to 100 μA/cm233 the anode curve is usually called the pitting potential (Eb or Etra). As the temperature rises, the passivation interval decreases, the self-corrosion potential decreases, the corrosion current density tends to increase, and the polarization curve shifts down to the right, which indicates that the film formed by DSS 2205 in the simulated solution has active activity. content of 100 g/l Cl– and saturated CO2, increases sensitivity to pitting corrosion, is easily damaged by aggressive ions, which leads to increased corrosion of the metal matrix and a decrease in corrosion resistance.
It can be seen from Table 4 that when the temperature rises from 30°C to 45°C, the corresponding overpassivation potential decreases slightly, but the passivation current density of the corresponding size increases significantly, indicating that the protection of the passivating film under these conditions increases with increasing temperature. When the temperature reaches 60°C, the corresponding pitting potential decreases significantly, and this trend becomes more evident as the temperature rises. It should be noted that at 75°C a significant transient current peak appears in the figure, indicating the presence of metastable pitting corrosion on the sample surface.
Therefore, with an increase in the temperature of the solution, the amount of oxygen dissolved in the solution decreases, the pH value of the film surface decreases, and the stability of the passivating film decreases. In addition, the higher the temperature of the solution, the higher the activity of aggressive ions in the solution and the higher the rate of damage to the surface film layer of the substrate. Oxides formed in the film layer easily fall off and react with cations in the film layer to form soluble compounds, increasing the likelihood of pitting. Since the regenerated film layer is relatively loose, the protective effect on the substrate is low, which increases the corrosion of the metal substrate. The results of the dynamic polarization potential test are consistent with the results of impedance spectroscopy.
On fig. Figure 5a shows It curves for 2205 DSS in a model solution containing 100 g/L Cl– and saturated CO2. The passivation current density as a function of time was obtained after polarization at various temperatures for 1 h at a potential of -300 mV (relative to Ag/AgCl). It can be seen that the passivation current density trend of 2205 DSS at the same potential and different temperatures is basically the same, and the trend gradually decreases with time and tends to be smooth. As the temperature gradually increased, the passivation current density of 2205 DSS increased, which was consistent with the results of polarization, which also indicated that the protective characteristics of the film layer on the metal substrate decreased with increasing solution temperature.
Potentiostatic polarization curves of 2205 DSS at the same film formation potential and different temperatures. (a) Current density versus time, (b) Passive film growth logarithm.
Investigate the relationship between passivation current density and time at different temperatures for the same film formation potential, as shown in (1)34:
Where i is the passivation current density at the film formation potential, A/cm2. A is the area of the working electrode, cm2. K is the slope of the curve fitted to it. t time, s
On fig. 5b shows logI and logt curves for 2205 DSS at different temperatures at the same film formation potential. According to the literature data,35 when the line slopes K = -1, the film layer formed on the surface of the substrate is denser and has better corrosion resistance to the metal substrate. And when the straight line slopes K = -0.5, the film layer formed on the surface is loose, contains many small holes and has poor corrosion resistance to the metal substrate. It can be seen that at 30°C, 45°C, 60°C, and 75°C, the structure of the film layer changes from dense pores to loose pores in accordance with the selected linear slope. According to the Point Defect Model (PDM)36,37 it can be seen that the applied potential during the test does not affect the current density, indicating that the temperature directly affects the measurement of the anode current density during the test, so the current increases with increasing temperature. solution, and the density of 2205 DSS increases, and the corrosion resistance decreases.
The semiconductor properties of the thin film layer formed on the DSS affect its corrosion resistance38, the type of semiconductor and the carrier density of the thin film layer affect the cracking and pitting of the thin film layer DSS39,40 where the capacitance C and E of the potential thin film layer satisfies the relation MS, the space charge of the semiconductor is calculated in the following way:
In the formula, ε is the permittivity of the passivating film at room temperature, equal to 1230, ε0 is the vacuum permittivity, equal to 8.85 × 10–14 F/cm, E is the secondary charge (1.602 × 10–19 C); ND is the density of n-type semiconductor donors, cm–3, NA is the acceptor density of p-type semiconductor, cm–3, EFB is the flat-band potential, V, K is Boltzmann’s constant, 1.38 × 10–3. 23 J/K, T – temperature, K.
The slope and intercept of the fitted line can be calculated by fitting a linear separation to the measured MS curve, applied concentration (ND), accepted concentration (NA), and flat band potential (Efb)42.
On fig. 6 shows the Mott-Schottky curve of the surface layer of a 2205 DSS film formed in a simulated solution containing 100 g/l Cl- and saturated with CO2 at a potential (-300 mV) for 1 hour. It can be seen that all thin-film layers formed at different temperatures have the characteristics of n+p-type bipolar semiconductors. The n-type semiconductor has solution anion selectivity, which can prevent stainless steel cations from diffusing into the solution through the passivation film, while the p-type semiconductor has cation selectivity, which can prevent the corrosive anions in solution from passivation crossings The film comes out on the surface of the substrate 26 . It can also be seen that there is a smooth transition between the two fitting curves, the film is in a flat band state, and the flat band potential Efb can be used to determine the position of the energy band of a semiconductor and evaluate its electrochemical stability43. .
According to the MC curve fitting results shown in Table 5, the outgoing concentration (ND) and the receiving concentration (NA) and the flat band potential Efb 44 of the same order of magnitude were calculated. The density of the applied carrier current mainly characterizes point defects in the space charge layer and the pitting potential of the passivating film. The higher the concentration of the applied carrier, the easier the film layer breaks and the higher the probability of substrate corrosion45. In addition, with a gradual increase in the temperature of the solution, the ND emitter concentration in the film layer increased from 5.273×1020 cm-3 to 1.772×1022 cm-3, and the NA host concentration increased from 4.972×1021 to 4.592×1023. cm – as shown in fig. 3, the flat band potential increases from 0.021 V to 0.753 V, the number of carriers in the solution increases, the reaction between ions in the solution intensifies, and the stability of the film layer decreases. As the temperature of the solution increases, the smaller the absolute value of the slope of the approximating line, the greater the density of carriers in the solution, the higher the rate of diffusion between ions, and the greater the number of ion vacancies on the surface of the film layer. , thereby reducing the metal substrate, stability and corrosion resistance 46,47.
The chemical composition of the film has a significant effect on the stability of metal cations and the performance of semiconductors, and the change in temperature has an important effect on the formation of a stainless steel film. On fig. Figure 7 shows the full XPS spectrum of the surface layer of a 2205 DSS film in a simulated solution containing 100 g/L Cl– and saturated CO2. The main elements in films formed by chips at different temperatures are basically the same, and the main components of the films are Fe, Cr, Ni, Mo, O, N, and C. Therefore, the main components of the film layer are Fe, Cr, Ni, Mo, O, N and C. Container with Cr oxides, Fe oxides and hydroxides and a small amount of Ni and Mo oxides.
Full XPS 2205 DSS spectra taken at various temperatures. (a) 30°С, (b) 45°С, (c) 60°С, (d) 75°С.
The main composition of the film is related to the thermodynamic properties of the compounds in the passivating film. According to the binding energy of the main elements in the film layer, given in table. 6, it can be seen that the characteristic spectral peaks of Cr2p3/2 are divided into metal Cr0 (573.7 ± 0.2 eV), Cr2O3 (574.5 ± 0.3 eV), and Cr(OH)3 ( 575.4 ± 0. 1 eV) as shown in Figure 8a, in which the oxide formed by the Cr element is the main component in the film, which plays an important role in the corrosion resistance of the film and its electrochemical performance. The relative peak intensity of Cr2O3 in the film layer is higher than that of Cr(OH)3. However, as the solid solution temperature increases, the relative peak of Cr2O3 gradually weakens, while the relative peak of Cr(OH)3 gradually increases, which indicates the obvious transformation of the main Cr3+ in the film layer from Cr2O3 to Cr(OH)3, and the temperature of the solution increases.
The binding energy of the peaks of the characteristic spectrum of Fe2p3/2 mainly consists of four peaks of the metallic state Fe0 (706.4 ± 0.2 eV), Fe3O4 (707.5 ± 0.2 eV), FeO (709.5 ± 0.1 eV ) and FeOOH (713.1 eV) ± 0.3 eV), as shown in Fig. 8b, Fe is mainly present in the formed film in the form of Fe2+ and Fe3+. Fe2+ from FeO dominates Fe(II) at lower binding energy peaks, while Fe3O4 and Fe(III) FeOOH compounds dominate at higher binding energy peaks48,49. The relative intensity of the Fe3+ peak is higher than that of Fe2+, but the relative intensity of the Fe3+ peak decreases with increasing solution temperature, and the relative intensity of the Fe2+ peak increases, indicating a change in the main substance in the film layer from Fe3+ to Fe2+ to increase the temperature of the solution.
The characteristic spectral peaks of Mo3d5/2 mainly consist of two peak positions Mo3d5/2 and Mo3d3/243.50, while Mo3d5/2 includes metallic Mo (227.5 ± 0.3 eV), Mo4+ (228.9 ± 0.2 eV) and Mo6+ ( 229.4 ± 0.3 eV), while Mo3d3/2 also contains metallic Mo (230.4 ± 0.1 eV), Mo4+ (231.5 ± 0.2 eV) and Mo6+ (232, 8 ± 0.1 eV) as shown in Figure 8c, so the Mo elements exist in the over three valence state of the film layer. The binding energies of the characteristic spectral peaks of Ni2p3/2 consist of Ni0 (852.4 ± 0.2 eV) and NiO (854.1 ± 0.2 eV), as shown in Fig. 8g respectively. The characteristic N1s peak consists of N (399.6 ± 0.3 eV), as shown in Fig. 8d. Characteristic O1s peaks include O2- (529.7 ± 0.2 eV), OH- (531.2 ± 0.2 eV) and H2O (531.8 ± 0.3 eV), as shown in Fig. The main components of the film layer are (OH- and O2 -), which are mainly used for the oxidation or hydrogen oxidation of Cr and Fe in the film layer. The relative peak intensity of OH- increased significantly as the temperature increased from 30°C to 75°C. Therefore, with an increase in temperature, the main material composition of O2- in the film layer changes from O2- to OH- and O2-.
On fig. Figure 9 shows the microscopic surface morphology of sample 2205 DSS after dynamic potential polarization in a model solution containing 100 g/L Cl– and saturated CO2. It can be seen that on the surface of the samples polarized at different temperatures, there are corrosion pits of varying degrees, this occurs in a solution of aggressive ions, and with an increase in the temperature of the solution, more serious corrosion occurs on the surface of the samples. substrate. The number of pitting pits per unit area and the depth of corrosion centers increase.
Corrosion curves of 2205 DSS in model solutions containing 100 g/l Cl– and saturated CO2 at different temperatures (a) 30°C, (b) 45°C, (c) 60°C, (d) 75°C c .
Therefore, an increase in temperature will increase the activity of each component of the DSS, as well as increase the activity of aggressive ions in an aggressive environment, causing a certain degree of damage to the sample surface, which will increase the pitting activity. , and the formation of corrosion pits will increase. The rate of product formation will increase and the corrosion resistance of the material will decrease51,52,53,54,55.
On fig. 10 shows the morphology and pitting depth of a 2205 DSS sample polarized with an ultra high depth of field optical digital microscope. From fig. 10a shows that smaller corrosion pits also appeared around large pits, indicating that the passivating film on the sample surface was partially destroyed with the formation of corrosion pits at a given current density, and the maximum pitting depth was 12.9 µm. as shown in Figure 10b.
DSS shows better corrosion resistance, the main reason is that the film formed on the surface of the steel is well protected in solution, Mott-Schottky, according to the above XPS results and related literature 13,56,57,58, the film mainly passes through the following This is the process of oxidation of Fe and Cr.
Fe2+ readily dissolves and precipitates at the interface 53 between the film and solution, and the cathodic reaction process is as follows:
In the corroded state, a two-layer structural film is formed, which mainly consists of an inner layer of iron and chromium oxides and an outer hydroxide layer, and ions usually grow in the pores of the film. The chemical composition of the passivating film is related to its semiconductor properties, as evidenced by the Mott-Schottky curve, indicating that the composition of the passivating film is n+p-type and has bipolar characteristics. The XPS results show that the outer layer of the passivating film is mainly composed of Fe oxides and hydroxides exhibiting n-type semiconductor properties, and the inner layer is mainly composed of Cr oxides and hydroxides exhibiting p-type semiconductor properties.
2205 DSS has high resistivity due to its high Cr17.54 content and exhibits varying degrees of pitting due to microscopic galvanic corrosion55 between duplex structures. Pitting corrosion is one of the most common types of corrosion in DSS, and temperature is one of the important factors influencing the behavior of pitting corrosion and has an impact on the thermodynamic and kinetic processes of the DSS reaction60,61. Typically, in a simulated solution with a high concentration of Cl– and saturated CO2, the temperature also affects the formation of pitting and the initiation of cracks during stress corrosion cracking under the stress corrosion cracking, and the critical temperature of pitting is determined to evaluate the corrosion resistance. DSS. The material, which reflects the sensitivity of the metal matrix to temperature, is commonly used as an important reference in material selection in engineering applications. The average critical pitting temperature of 2205 DSS in the simulated solution is 66.9°C, which is 25.6°C higher than that of Super 13Cr stainless steel with 3.5% NaCl, but the maximum pitting depth reached 12.9 µm62. The electrochemical results further confirmed that the horizontal regions of the phase angle and frequency narrow with increasing temperature, and as the phase angle decreases from 79° to 58°, the value of the |Z| decreases from 1.26×104 to 1.58×103 Ω cm2. charge transfer resistance Rct decreased from 2.958 1014 to 2.541 103 Ω cm2, solution resistance Rs decreased from 2.953 to 2.469 Ω cm2, film resistance Rf decreased from 5.430 10-4 cm2 to 1.147 10-3 cm2. The conductivity of the aggressive solution increases, the stability of the metal matrix film layer decreases, it dissolves and cracks easily. The self-corrosion current density increased from 1.482 to 2.893×10-6 A cm-2, and the self-corrosion potential decreased from -0.532 to -0.621V. It can be seen that the change in temperature affects the integrity and density of the film layer.
On the contrary, a high concentration of Cl- and a saturated solution of CO2 gradually increase the adsorption capacity of Cl- on the surface of the passivating film with increasing temperature, the stability of the passivation film becomes unstable, and the protective effect on the substrate becomes weaker and the susceptibility to pitting increases. In this case, the activity of corrosive ions in the solution increases, the oxygen content decreases, and the surface film of the corroded material is difficult to quickly recover, which creates more favorable conditions for further adsorption of corrosive ions on the surface. Material reduction63. Robinson et al. [64] showed that with an increase in the temperature of the solution, the growth rate of pits accelerates, and the rate of diffusion of ions in the solution also increases. When the temperature rises to 65 °C, the dissolution of oxygen in a solution containing Cl- ions slows down the cathodic reaction process, the rate of pitting is reduced. Han20 investigated the effect of temperature on the corrosion behavior of 2205 duplex stainless steel in a CO2 environment. The results showed that an increase in temperature increased the amount of corrosion products and the area of shrinkage cavities on the surface of the material. Similarly, when the temperature rises to 150°C, the oxide film on the surface breaks, and the density of craters is the highest. Lu4 investigated the effect of temperature on the corrosion behavior of 2205 duplex stainless steel from passivation to activation in a geothermal environment containing CO2. Their results show that at a test temperature below 150 °C, the formed film has a characteristic amorphous structure, and the inner interface contains a nickel-rich layer, and at a temperature of 300 °C, the resulting corrosion product has a nanoscale structure. -polycrystalline FeCr2O4, CrOOH and NiFe2O4.
On fig. 11 is a diagram of the corrosion and film formation process of 2205 DSS. Prior to use, 2205 DSS forms a passivating film in the atmosphere. After being immersed in an environment that simulates a solution containing solutions with a high content of Cl- and CO2, its surface is quickly surrounded by various aggressive ions (Cl-, CO32-, etc.). ). J. Banas 65 came to the conclusion that in an environment where CO2 is simultaneously present, the stability of the passivating film on the surface of the material will decrease with time, and the formed carbonic acid tends to increase the conductivity of ions in the passivating layer. film and acceleration of dissolution of ions in a passivating film. passivating film. Thus, the film layer on the sample surface is in a dynamic equilibrium stage of dissolution and repassivation66, Cl- reduces the rate of formation of the surface film layer, and tiny pitting pits appear on the adjacent area of the film surface, as shown in Figure 3. Show. As shown in Figure 11a and b, tiny unstable corrosion pits appear at the same time. As the temperature rises, the activity of corrosive ions in solution on the film layer increases, and the depth of the tiny unstable pits increases until the film layer is completely penetrated by the transparent one, as shown in Figure 11c. With a further increase in the temperature of the dissolving medium, the content of dissolved CO2 in the solution accelerates, which leads to a decrease in the pH value of the solution, an increase in the density of the smallest unstable corrosion pits on the SPP surface, the depth of the initial corrosion pits expands and deepens, and the passivating film on the sample surface As the thickness decreases, the passivating the film becomes more prone to pitting as shown in Figure 11d. And the electrochemical results additionally confirmed that the change in temperature has a certain effect on the integrity and density of the film. Thus, it can be seen that corrosion in solutions saturated with CO2 containing high concentrations of Cl- is significantly different from corrosion in solutions containing low concentrations of Cl-67,68.
Corrosion process 2205 DSS with the formation and destruction of a new film. (a) Process 1, (b) Process 2, (c) Process 3, (d) Process 4.
The average critical pitting temperature of 2205 DSS in a simulated solution containing 100 g/l Cl– and saturated CO2 is 66.9 ℃, and the maximum pitting depth is 12.9 µm, which reduces the corrosion resistance of 2205 DSS and increases the sensitivity to pitting . temperature increase.
Post time: Feb-16-2023