Effect of tensile stress on pitting corrosion resistance of 2205 duplex stainless steel tube

2205 duplex Stainless steel tube has good mechanical properties and resistance to stress corrosion, so it has been widely used. Due to the frequent service in harsh environments such as oil fields and oceans, pitting corrosion occurs in 2205 duplex steel, and the effect of stress on the pitting performance of stainless steel pipes cannot be ignored. At present, most of the research on pitting corrosion performance of stainless steel pipes mainly focuses on the influence of temperature, Surface roughness, heat treatment, solution ion concentration and other factors [1-6], while the effect of stress on pitting corrosion performance of duplex steel is relatively less.

Parkins proposed [7] that stress plays an important role in membrane rupture, repassivation, and pitting formation. Chen found [8] that residual stresses exacerbate the formation of tiny pitting pits. Zhang et al [9] studied the pitting behavior of Fe-20Cr under compressive stress in 3.5% (mass fraction, later) NaCl solution, and found that compressive stress promoted pore growth. However, Li and Cheng studies have shown [10] that when the load is small, micro-deformation and stress can inhibit pitting corrosion. It can be seen that the effect of stress on pitting corrosion of stainless steel pipes has not been clearly understood. The critical pitting temperature of 2205 duplex stainless steel pipe is about 60 °C [11]. The influence of stress on the pitting behavior of duplex stainless steel pipe has not been reported in the upper and lower characteristic range of critical pitting temperature. For this reason, the dynamic potential polarization characteristics of 2205 duplex Stainless steel tubes under different tensile stresses are compared with 40 °C below the critical pitting temperature and 3.5% NaCl solution at 60 °C near the critical pitting temperature [12]. Combined with electrochemical impedance, the influence of tensile stress on the electrochemical properties of pitting corrosion of Stainless steel tubes was analyzed.

1 Experiment 

1.1 sample The material is a 2205 duplex Stainless steel tube sample. The tensile mechanical properties at room temperature are shown in Figure 1. The yield strength is 620 MPa and the tensile strength is 810 MPa. The sample was machined, cut, and ground into a tensile specimen having a tensile parallel dimension of 20 mm x 5 mm x 2 mm. Before the experiment, the sample was sanded to 1500# with sandpaper, and after ultrasonic cleaning with acetone, it was passivated for 30 min in 20% (mass fraction) HNO3 at 60 ° C, and then sealed with silicone glue to make the Contact area with the solution 1 cm 2 .  

1.2 Electrochemical test The tensile stress was applied by Shimadzu EHF2000 hydraulic servo fatigue testing machine, and the stresses were 0, 140, and 540 MPa, respectively. According to Fig. 1, the tensile stress applied is in the elastic deformation load region of the duplex Stainless steel tube. The electrochemical impedance and potentiodynamic polarization tests in a 3.5% NaCl solution were carried out on a CS350 electrochemical workstation using a three-electrode system: a 2205 duplex steel sample as the working electrode, a calomel electrode as the reference electrode, and a Pt electrode. For the counter electrode. The NaCl solution was heated to (40 ± 2) and (60 ± 2) ° C by a water bath, respectively. Electrochemical impedance (EIS) tests were performed at open circuit potential (OCP) and 600 mV (both potentials relative to SCE) with a frequency range of 100,000 to 1 Hz. The potentiodynamic polarization measurement range is -400 to 1200 mV, and the scanning rate is 0.5 mV/s.

2 Results and analysis Effect of stress on dynamic potential polarization of 2205 duplex steel Figure 2 is a plot of potentiodynamic polarization of 2205 duplex steel under different stresses in a 3.5% NaCl solution at 40 °C and 60 °C. As shown in Fig. 2a, when the stress is applied in the 40 °C solution, the breaking potential of the sample decreases with the increase of the stress, and the breaking potential at 0, 140, 540 MPa is about 0.9, 0.7, and 0, respectively. At the same time, the tensile stress increases the current density in the anode region of the Stainless steel tube. As shown in Fig. 2b, when the temperature was raised to 60 ° C, the breakdown potential under stress-free conditions was lowered from 0.9 V to 0.4 V. According to the literature, this temperature is near the critical pitting temperature of 2205 duplex steel, so the breaking potential is significantly reduced. At this temperature, the tensile stress is increased to 540 MPa, which also significantly reduces the breakdown potential. At the same time, the current density in the passivation region exhibits a transient peak. The appearance of this peak indicates the formation of metastable pitting during the passivation process. Although there is a transient peak at 140 MPa, the breakdown potential is still high, about 0.8 V, indicating that the pitting resistance is still strong under this condition.