Electrochemical corrosion behavior of 254SMO stainless steel in simulated condensate of blast furnace gas
Due to the need of water saving, energy saving and emission reduction, many iron and steel plants’ blast furnace gas gradually changed from wet dedusting to dry dedusting, but then the corrosion of blast furnace gas pipe network increased obviously. The main reason is that the dry dedusting can not remove a large number of water-soluble chlorine containing substances and other acid substances in the blast furnace gas, which are left in the clean gas and transported out with the pipeline. Under the appropriate temperature and pressure, the water vapor in the gas condenses, and a large number of chlorine containing substances and acid substances are dissolved in it to form a high chlorine strong acid condensate, The pipeline and 316L stainless steel expansion joint where the condensate appears are severely corroded [1,2,3,4].
The corrosion of 316L stainless steel in the condensate shows obvious pitting characteristics , which is directly related to the high concentration of Cl – in the condensate. Jafarian et al.  think that the pitting damage of stainless steel caused by Cl – can be mainly divided into pitting induction period, pitting metastable growth and stable development period. Whether metastable pitting can continue to grow into stable pitting is restricted by many factors, among which medium temperature is an important factor. With the increase of medium temperature, the accumulation of Cl – and the amount of chemical adsorption on the surface of stainless steel increase, which leads to the destruction of passive film on the surface of stainless steel and the formation of more active points, so that the oxide on the surface of stainless steel can be locally dissolved at a relatively low potential, leading to the rupture of passive film and pitting corrosion.
254SMO super austenitic stainless steel is widely used because of its excellent pitting resistance. Marconnet et al.  studied the chemical composition and electronic structure of the passivation film of 304L and 254SMO stainless steel in glucose oxidase solution. Anderko et al  established a detailed database of the re passivation potential of six alloys including 254SMO in CL solution with different concentrations of oxyacid radical at different temperatures. Bojinov et al.  studied and compared the over passivation dissolution characteristics of 316L, 254SMO, 904L and 654smo stainless steel in 0.5mol/l sulfate solution with pH 2. It was found that the difference of dissolution rate was due to the different content of Cr and Mo in the metal matrix. Gustaf et al.  put 316, 317, 904L, 254SMO, 3re60 and 2324 stainless steel in extremely harsh environment (55 ℃, pH 2.3 and [Cl -] = 350 mg / L) for a period of time, and found that all stainless steel except 254SMO had pitting corrosion and crevice corrosion.
In this paper, the corrosion resistance and corrosion law of 254SMO super austenitic stainless steel were investigated by using the simulated liquid of high chlorine and strong acid as the experimental medium, and the feasibility of its application in the expansion joint of blast furnace gas pipeline was analyzed.
The experimental material is 254SMO stainless steel. Its 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 residual. 254SMO stainless steel plate is processed into a test piece with a working surface of 1cm × 1cm, and the back of the test piece is welded with wires, and the non working surface is sealed with epoxy resin to make electrodes. Before the experiment, the surface of stainless steel electrode was polished with different types of sandpaper, then degreased with alcohol, and washed with deionized water for standby.
The experimental medium is a simulated test liquid [1,2] prepared according to the main components of the cold end pipe condensate of blast furnace gas in a certain plant, in which the content of Cl -, SO42 -, Fe3 +, Ca2 + and pH is 13.2%, 1.175%, 9%, 0.064% and 2.0 respectively.
The electrochemical impedance spectrum and polarization curve were measured on the parstat 2273 electrochemical workstation. The saturated calomel electrode was used as the reference electrode and the platinum electrode as the auxiliary electrode. The frequency range of EIS is 105~10-2 Hz, and the amplitude is 5 mV. The scanning speed of 1 mV / S is used for polarization curve test; the starting potential of cyclic polarization curve is – 0.25 V relative self-corrosion potential, scanning towards the anode direction. When the anode polarization current density reaches 1 × 10-2 A / cm2, the reverse scanning reaches – 0.2 V relative self-corrosion potential. The critical pitting temperature of stainless steel is measured by ASTM standard , the applied potential is 700 MV, the initial temperature of the test is 30 ℃, when the polarization current density of the electrode reaches 100 μ A / cm2, the experiment can be finished. Before the test, the experiment can be carried out only when there is no gap between the electrode and the resin. All the electrochemical tests were repeated more than three times, and all the potentials were relative to the saturated calomel electrode (SCE). The metal surface morphology was observed by Zeiss lsm700 laser confocal microscope.
Results and discussion
EIS of 254SMO stainless steel in simulated solution
Fig. 1 is the Nyquist diagram of 254SMO stainless steel electrode immersed in simulated solution at different temperatures for 1 h. It can be seen that with the increase of temperature, the impedance of 254SMO stainless steel decreases gradually, the stability of passivation film decreases, and the corrosion resistance of stainless steel decreases. The Nyquist diagram of 254SMO stainless steel in solution shows the characteristics of double capacitive arc, in which the high frequency capacitive arc corresponds to the charge transfer process, and the low frequency capacitive arc corresponds to the film resistance and film capacitance on the electrode surface . When the temperature of solution rises to 65 ℃, the radius of impedance arc of stainless steel electrode decreases greatly, which may be due to the formation of steady-state pitting, resulting in the sharp decrease of electrode impedance. Generally speaking, when the temperature rises, the surface film layer of stainless steel becomes unstable and easy to corrode and destroy. It may be that the loss of Cr in passivation film leads to the decrease of the protective effect of the film on the matrix.
Fig.1 Nyquist plots of 254SMo stainless steel in simulated condensate for 1 h at 30 ℃, 50 ℃ (a) and 65 ℃ (b)
Effect of temperature on polarization behavior of 254SMO stainless steel
Figure 2 shows the potentiodynamic polarization curve of 254SMO stainless steel electrode in simulated solution at different temperatures. It can be found that in the solution of 30 ℃ and 50 ℃, there is an obvious passivation zone in the anodic polarization curve of 254SMO stainless steel, and the current density of the passivation state is relatively small, which is 28 and 81 MA / cm2 at 0.6V potential respectively; when the solution temperature rises to 65 ℃, the current density of the anodic polarization of the stainless steel electrode increases rapidly to 2338 MA / cm2 at 0.6V potential, which is obviously in the non passivation state, After the test, pitting was found on the stainless steel surface (Fig. 3). However, O2 in the solution is easy to adsorb on the oxide film when the temperature is low. With the increase of temperature, the concentration of dissolved oxygen decreases and the adsorption equilibrium between the dissolved oxygen and the adsorbed oxygen on the surface of the passive film changes. At the same time, the increase of temperature intensifies the thermal movement of adsorbed oxygen on the surface of passivation film, resulting in local desorption of adsorbed oxygen, which leads to the decrease of reduction rate of oxygen on the surface of electrode, the decrease of pH value in the local area of passivation film surface, and then affects the stability of passivation film. In addition, the corrosion potentials of stainless steel electrodes were 392422 and 279 MV at 30, 50 and 65 ℃, respectively. The corrosion rate of stainless steel in 65 ℃ solution increased obviously, while the self corrosion potential decreased obviously, which indicated that the increase of temperature had the effect of anode depolarization on 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 ℃
Temperature sensitivity of pitting corrosion of 254SMO stainless steel
Figure 4 and figure 5 show the critical pitting temperature test curves of 254SMO stainless steel in the simulated solution and the cyclic polarization curves in different temperature solutions, respectively. It can be seen from Figure 4 that at 700 MV (vs SCE) polarization potential, the polarization current density of stainless steel electrode changes little with temperature at a lower temperature (30-50 ℃); with the further increase of solution temperature, the polarization current density of electrode increases gradually, indicating that the dissolution rate of passive film increases. For the passive metal, when the dissolution and repair of the passive film reach a dynamic balance, it can maintain the stability of the passive film; when the dissolution speed of the passive film is greater than the repair speed of the film, the passive film will be damaged, and the rise of solution temperature 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 will start to break to form micro pitting, and the corresponding temperature is the starting temperature of the passivation film on the metal surface to break to form micro pitting, that is, the critical pitting temperature, Generally, the temperature corresponding to the polarization current density of 100 μ A / cm2 in the test curve is taken as the critical pitting temperature [10,12,13]. In Figure 4, when the solution temperature rises to 62 ℃, the polarization current density of the stainless steel electrode reaches 100 μ A / cm2. Therefore, the critical pitting temperature of 254SMO stainless steel in the simulated solution is 62 ℃.
Fig.4 Test curve of critical pitting temperature of 254SMo stainless steel in simulated condensate
Fig.5 Cyclic polarization curves of 254SMo stainless steelin simulated condensate at different temperatures
- (1) In the simulated solution of blast furnace gas condensate, with the increase of solution temperature, the impedance value of 254SMO stainless steel electrode decreases gradually, and the impedance value of electrode decreases greatly at 65 ℃; there is an obvious passivation zone in the anodic polarization curve of 254SMO stainless steel, and the current density of passivation state is small when the solution temperature is ≤ 50 ℃; when the solution temperature rises to 65 ℃, the surface of stainless steel is no longer passivated, At the same potential, the polarization current density of anode increases rapidly by two orders of magnitude.
- (2) The critical pitting temperature test results show that the critical pitting temperature of 254SMO stainless steel in the simulation liquid of blast furnace gas condensate is 62 ℃. When the solution temperature is low, there is a small lag ring on the cyclic polarization curve, and the repair ability of the passive film is better; when the solution temperature rises to 65 ℃, there is a large lag ring on the cyclic polarization curve, and the passive film on the stainless steel surface is damaged by pitting corrosion.
- (3) The stability of 254SMO stainless steel passivation film is greatly affected by temperature in the simulation liquid of blast furnace gas condensate. It is suggested that the temperature should not exceed 60 ℃.
Source: China 254SMO Pipe Fittings Manufacturer – Yaang Pipe Industry Co., Limited (www.pipelinedubai.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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