Electrochemical Corrosion Behavior of 254SMo Stainless Steel in Simulated Condensate of Blast Furnace Gas
Anti-corrosion performance of 254SMo super-austenitic stainless steel in simulated condensate of blast furnace gas of a power plant was studied by means of electrochemical impedance spectroscopy, polarization curve and critical pitting temperature measurement. The results show that the corrosion resistance of 254SMo stainless steel decreases and the passive current density increases as the test temperature rises. When the test temperature is lower, a very small hysteresis loop appears on the circular polarization curve of the stainless steel, which means the strong re-passivation ability of the passive film. As the test temperature reaches to 65 ℃, a large hysteresis loop appears on the circular polarization curve of stainless steel, indicating the pitting damage of the surface passivation film. The critical pitting temperature of stainless steel in simulated condensate is 62 ℃.
Key words： blast furnace gas simulated condensate chloride ion 254SMo SS corrosion
Cite this article:
ZHOU Bin, JIN Zhihao, GE Honghua.
Electrochemical Corrosion Behavior of 254SMo Stainless Steel in Simulated Condensate of Blast Furnace Gas. Corrosion Science and Protection Technology[J], 2018, 30(2): 163-166 doi:10.11903/1002.6495.2017.096
Due to the need of water saving, energy saving and emission reduction, blast furnace gas in many steel works was gradually changed from wet dust removal to dry dust removal. However, the corrosion of the gas network of blast furnace gas increased significantly. The main reason is that dry dedusting cannot remove a large amount of water-soluble chlorine-containing substances and other acidic substances in blast furnace gas. These substances remain in the net gas and are transported along the pipeline. Under the appropriate temperature and pressure, the water vapor in the gas condenses. At the same time, a large amount of chlorine-containing substances and acidic substances are dissolved in the high-chlorine, strong-acidic condensate, resulting in the occurrence of condensate pipes and 316L stainless steel expansion joints with severe corrosion [1,2,3,4].
The corrosion of the 316L stainless steel in this condensate shows significant pitting characteristics , which is directly related to the high concentration of Cl- in the condensate. Jafarian et al.  believe that the pitting damage of stainless steel caused by Cl- can be mainly divided into pitting induction period, pitting corrosion metastable growth and stable development period. The transition from metastable pitting to continued growth to steady-state pitting is constrained by many factors, among which medium temperature is an important factor. As the temperature of the medium increases, the accumulation of Cl- on the surface of the stainless steel and the amount of chemisorption increase, resulting in the destruction of the passivation film on the stainless steel surface and the formation of more active sites, so that the surface of the stainless steel oxide at a relatively low potential Local dissolution can occur, leading to cracking of the passivation film and pitting corrosion.
254SMo super austenitic stainless steel is widely used due to its excellent resistance to pitting corrosion. Marconnet et al.  studied the chemical composition and electronic structure of passivation films formed in glucose oxidase solutions of 304L and 254SMo stainless steels. Anderko et al.  established an extensive database of repassivation potentials for six alloys including 254SMo at different temperatures in different concentrations of oxalate-containing Cl-solutions. Bojinov et al.  studied the passivation and dissolution characteristics of 316L, 254SMo, 904L, and 654SMo stainless steels in a 0.5 mol/L sulfate solution at a pH of 2 and found that the difference in dissolution rates was due to Cr in the metal matrix. The content of Mo is different. Gustaf et al.  placed 316, 317, 904L, 254SMo, 3RE60, and 2324 stainless steel in extremely harsh environments (55 °C, pH 2.3, and [Cl-] = 350 mg/L) for a period of time and found that in addition to 254SMo In addition, pitting and crevice corrosion occurred in all other stainless steels.
In this paper, the simulated solution of high-chlorine strong acid condensate was used as the experimental medium to study the corrosion resistance and corrosion of 254SMo super austenitic stainless steel, and to analyze its feasibility for the expansion joint of blast furnace gas pipeline.
1 Experimental method
The experimental material was 254SMo stainless steel. The chemical composition (mass fraction, %) is: C ≤ 0.02, Si ≤ 1.00, Mn ≤ 1.00, P ≤ 0.03, S ≤ 0.01, Ni (17.5~18.5), Cr (19.5~20.5), Cu (0.5~1 ), Mo 6, Fe balance. A 254SMo stainless steel plate was machined into test pieces with a working surface of 1 cm x 1 cm, wires were welded on the back of the test pieces, and electrodes were made by encapsulating the non-working surface with epoxy resin. Before the experiment, the surface of the stainless steel electrode was sanded to bright with different types of sandpaper, then degreased with alcohol and rinsed with deionized water.
The experimental medium is simulated test fluid [1,2] prepared from the main components of the condensate from the cold end of the blast furnace gas in a plant. The Cl- content is 13.2%, the SO42- content is 1.175%, the Fe3+ content is 9%, and the Ca2+ content 0.064%, pH 2.0.
Electrochemical impedance spectroscopy and polarization curve tests were performed on a PARSTAT 2273 electrochemical workstation. The saturated calomel electrode was used as a reference electrode and the platinum electrode was used as an auxiliary electrode. The test frequency range of the electrochemical impedance spectroscopy is 105~10-2 Hz and the amplitude is 5 mV. The polarization curve was measured with a scan rate of 1 mV/s; the starting potential of the cyclic polarization curve was a relative autocorrosion potential of -0.25 V, scanning in the direction of the anode, when the anodic polarization current density reached 1 x 10-2 A/. At cm2, reverse scan to a relative autocorrosion potential of -0.2 V. The critical pitting temperature of stainless steel was measured using the ASTM standard  with an applied potential of 700 mV. The starting temperature of the test was 30 °C. The experiment was terminated when the polarization current density of the electrode reached 100 μA/cm2. Observe that there is no gap between the electrode and the encapsulating resin before conducting the experiment. All the electrochemical tests in the paper were repeated more than 3 times. All the potentials in the paper are relative to the saturated calomel electrode (SCE). The surface morphology of the metal was observed using the German ZEISS LSM 700 laser confocal microscope.
2 Results and Discussion
2.1 Electrochemical Impedance Spectroscopy of 254SMo Stainless Steel in Simulated Liquid
Figure 1 shows the Nyquist diagram of a 254SMo stainless steel electrode soaked in a simulated solution at different temperatures for 1 h. It can be seen that as the temperature increases, the resistance value of the 254SMo stainless steel gradually decreases, the stability of the surface passivation film decreases, and the corrosion resistance of the stainless steel decreases. The Nyquist plot of the 254SMo stainless steel in solution exhibits double-capacitor-resistant arc features, where the high-frequency capacitive arc corresponds to the charge transfer process, and the low-frequency capacitive-resistance arc corresponds to the membrane resistance and membrane capacitance at the electrode surface . When the temperature of the solution rises to 65 °C, the radius of the arc of the impedance of the stainless steel electrode decreases significantly. This may be due to the sharp decrease of the electrode impedance value due to the formation of steady state pitting. In general, when the temperature rises, the surface layer of the stainless steel becomes unstable, and corrosion damage is likely to occur. This may be due to the loss of Cr in the passivation film, which results in a decrease in the protective effect of the film on the substrate.
Fig.1 Nyquist plots of 254SMo stainless steel in simulated condensate for 1 h at 30 ℃, 50 ℃ (a) and 65 ℃ (b)
2.2 Effect of Temperature on Polarization Behavior of 254SMo Stainless Steel
Fig. 2 shows the potentiodynamic polarization curves of 254SMo stainless steel electrodes in simulated liquids at different temperatures. It can be found that the anodic polarization curve of 254SMo stainless steel in the solution at 30 and 50 °C has an obvious passivation zone, and the passivation current density is small, 28 and 81 mA/cm2 at 0.6 V potential, respectively; when the temperature of the solution rises At 65 °C, the anodic polarization current density of the stainless steel electrode at a potential of 0.6 V rapidly increased to 2338 mA/cm2, apparently in a non-deactivating state, and pitting corrosion was found on the surface of the stainless steel at the end of the test (Fig. 3). This is because O2 in the solution is more easily adsorbed on the oxide film when the temperature is lower. As the temperature increases, the dissolved oxygen concentration decreases and the adsorption equilibrium between the dissolved oxygen and the adsorbed oxygen on the surface of the passivation film is changed. At the same time, the increase of temperature exacerbates the thermal movement of adsorbed oxygen on the surface of the passivation film, resulting in the partial desorption of adsorbed oxygen, resulting in a decrease in the rate of oxygen reduction on the surface of the electrode and a decrease in the pH value of the localized area on the surface of the passivation film, thereby affecting passivation. Film stability. In addition, the self-corrosion potentials of the stainless steel electrodes in the 30, 50, and 65 °C solutions were 392, 422, and 279 mV, respectively. The corrosion rate of stainless steel in the solution at 65 °C increases significantly and the corrosion potential decreases significantly. It also shows that the temperature increase has a depolarizing effect on the stainless steel electrode.
Fig.2 Polarization curves of 254SMo stainless steel in simulated condensate at different temperatures
Fig.3 Surface morphology of 254SMo stainless steel after measurement of polarization curve in simulated condensate at 65 ℃
2.3 254SMo stainless steel pitting temperature sensitivity
Figures 4 and 5 are the critical pitting temperature test curves of the 254SMo stainless steel in the simulation liquid and the cyclic polarization curves in the different temperature solutions, respectively. From Fig. 4, it can be seen that at a polarization potential of 700 mV (vs SCE), the polarization current density of the stainless steel electrode varies less with temperature at a lower temperature (30-50 °C); as the temperature of the solution rises further High, the electrode polarization current density gradually increased, indicating that the dissolution rate of the passive film increases. For passivated metals, when the dynamic balance of dissolution and repair of the passivation film is achieved, the stability of the passivation film can be maintained; when the passivation film dissolves faster than the film repair speed, the passivation film is destroyed. , And the temperature of the solution can promote this phenomenon. According to the critical pitting temperature test standard , when the solution temperature exceeds a certain value, the polarization current density of the metal electrode will increase sharply. At this time, the passivation film on the metal surface begins to crack to form a tiny etching hole, and the corresponding temperature is The initial temperature of the metal surface passivation film breaks to form the micro-etching hole, that is, the critical pitting temperature. Generally, the temperature corresponding to the polarization current density of 100 μA/cm2 in the test curve is taken as the critical pitting temperature [10, 12,13]. In Figure 4, when the temperature of the solution rises to 62 °C, the polarization current density of the stainless steel electrode reaches 100 μA/cm2. Therefore, the critical pitting temperature of the 254SMo stainless steel in the simulation liquid is 62 °C.
Fig.4 Test curve of critical pitting temperature of 254SMo stainless steel in simulated condensate
The size of the hysteresis loop in the cyclic polarization curve usually reflects the repairability of the passive film on the metal surface. The smaller the hysteresis loop, the stronger the repair ability of the passive film [14,15,16]. As can be seen from Figure 5, when the solution temperature is 30 and 50 °C, a small hysteresis loop appears on the cyclic polarization curve. As the temperature increases, the hysteresis loop area increases and the self-healing ability of the passive film decreases. When the solution temperature rises to 65 °C, the retrace polarization current density at the same potential is significantly greater than the positive sweep polarization current density. Pit corrosion is found on the surface of the sample after the experiment. This is mainly because in the 65°C solution, when the potential of the sample is higher than the pitting potential during the positive scan, serious pitting has occurred. Even if the electrode potential is swept back to the negative potential, it is still difficult to stop immediately. The further expansion of pitting has led to an increase in the polarization current density, at which point the stainless steel passivation film rupture cannot be repaired.
Fig.5 Cyclic polarization curves of 254SMo stainless steel in simulated condensate at different temperatures
(1) In the simulation of the blast furnace gas condensate, as the temperature of the solution increases, the resistance value of the 254SMo stainless steel electrode gradually decreases, and the electrode resistance value drops significantly at 65°C; the anodic polarization curve of the 254SMo stainless steel is obviously obtuse In the zone, the passive current density is lower when the solution temperature is ≤50 °C. When the solution temperature rises to 65 °C, the stainless steel surface is no longer passivated, and the anodic polarization current density at the same potential increases rapidly by 2 orders of magnitude.
(2) The critical pitting temperature test results show that the critical pitting temperature of the 254SMo stainless steel in the blast furnace gas condensate simulation fluid is 62 °C. When the solution temperature is lower, a smaller hysteresis loop appears on the cyclic polarization curve, and the repairability of the passive film is better. When the solution temperature rises to 65°C, a large hysteresis loop appears on the cyclic polarization curve. The passivation film on the stainless steel surface was damaged by pitting.
(3) In the blast furnace gas condensate simulation liquid, the stability of the 254SMo stainless steel passivation film is greatly affected by the temperature, and it is recommended that the temperature should not exceed 60 °C.
The authors have declared that no competing interests exist.
Source: China Pipe Fittings 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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