The microstructure,corrosion resistance and corrosion morphology of the cold-rolled LDX2101 stainless steel with varying degrees of deformation were characterized by means of optical microscope, and potentiodynamic polarization, and electrochemical impedance spectroscopy. The result showed that: with the increase of stress, the stability of the passivation film on the steel surface was reduced, thus its corrosion resistance decreased obviously. The increasing of stress may also cause the change of corrosion morphology, and thus selective corrosion could clearly observed. As the increase of stress, the pitting preferentially occurred on α/γ phase boundary or inclusions, and then extended to ferrite phase, and thereby the corrosion morphology may elongate along the direction of tensile stress. Due to the action of tensile stress, inclusions located at the surface or subsurface are also involved in the formation of pitting corrosion, and therewith, the pitting corrosion extended to the α phase located at the substrate surface or even deeper.
1 Mechanical and Electrical Design and Research Institute of Shanxi Province, Shanxi Institute for Quality Supervision and Inspection of Machinery Products, Taiyuan 030000, China 2 PLA Military Representative Office in Shengyang Aircraft Industries (Group) Co., Ltd., Shengyang 110850, China
Cite this article: LV Yingxi, DONG Zhi, ZANG Xu. Effect of Bending Stress on Stress Corrosion Property of 2101 Lean Duplex Stainless Steel. Corrosion Science and Protection Technology[J], 2018, 30(2): 172-176 doi:10.11903/1002.6495.2017.154
Corrosion is a phenomenon in which the material and the surrounding environment are chemically or electrochemically reacted and destroyed. It is a spontaneous process. Stainless steel is a corrosion resistant alloy invented in the early 20th century and has been widely used in petroleum, chemical, construction and defense fields. In recent years, due to Ni’s high price and lack of resources, it has been unable to meet the needs of the stainless steel industry. Therefore, economical duplex stainless steel with Mn and N instead of Ni has become the focus of stainless steel in the future , in which LDX2101 combines volume fraction The similar α- and γ-phases have excellent mechanical properties and corrosion resistance. They are alternatives to 304 austenitic stainless steels  and are widely used as structural materials in many harsh environments [3,4,5]. In the actual production and long-term use, the corrosion of the material is mostly stress corrosion. Generally, the greater the force of the component, the more serious the corrosion caused in the corrosive environment. Stress corrosion failure often has no obvious sign beforehand [6,7,8,9]. Once a crack forms, it will rapidly expand and fail. Some bursts of long-term use of chemical equipment are mostly caused by stress corrosion [10,11,12], which shows that the damage caused by it is extremely serious. LDX2101 economical duplex stainless steel contains many alloy elements and is unevenly distributed in two phases. The structure is complex, and the corrosion rates of α-phase and γ-phase are different under stress corrosion. There are many researches related to its corrosion resistance. [13,14,15]. However, LDX2101 lacks a systematic analysis of its surface morphology, corrosion initiation sites, and the regularity of corrosion in each phase under the influence of stress and corrosion media. In order to provide a deeper understanding of the stress corrosion behavior of LDX2101 and to provide guidance for avoiding the occurrence of stress corrosion in practical applications, this paper will study the stress corrosion of LDX2101, a cold rolled economical duplex stainless steel in a corrosive environment, through a specific stress device. behavior. 1 Experimental method The experimental material is the economical duplex stainless steel LDX2101 cold-rolled steel plate produced by Yaang Pipe Industry Co., Limited The sample size is 150 mm×15 mm×1.6 mm, as shown in Fig. 1a. The main chemical composition (mass fraction, %) is: C 0.02, Si 0.50, Mn 5.46, P 0.03, S 0.0017, Cr 21.76, Ni 1.50, Mo 0.16, N 0.14, Fe balance. In order to study the stress corrosion behavior under different degree of deformation, a specific device was introduced into a 3.5% (mass fraction) NaCl solution to apply different magnitudes of bending stress at the center of the cold-rolled LDX2101 specimen, ie, tensile stress, and the specimen remained during the experiment. The constant stress value is shown in Figures 1b and c. The variation of the maximum bending stress in the center depends on the maximum bending deflection, and the maximum bending deflection data for bending times 1, 2, and 3 are 0.36, 0.38, and 0.75 y/mm, respectively. The greater the y value of the bending deflection, the greater the bending stress to be subjected to, ie, the greater the degree of deformation.
Fig.1 Size of stress corrosion specimen (a), schematic diagram of stress corrosion specimens (b) and stress corrosion test device (c)
The CS350 electrochemical workstation and the CorrTest test software were used to collect experimental data. The sample has a working area of 1 cm2. First, the open circuit potential test was performed for about 30 minutes. After the open circuit potential was stabilized, pitting potential and electrochemical impedance spectroscopy tests were performed. From the open circuit potential of -200 mV, scanning was started at a rate of 0.5 mV/s until the pitting potential, and a sine wave with an amplitude of 10 mV was used as the disturbance signal. The test frequency range was 105 to 10-2 Hz, from high frequency to low frequency. Scan, measured data using Zview software analysis, and select the appropriate equivalent circuit to fit. Three sets of parallel experiments were performed for each set of experiments. The surface of the sample was etched with 0.3 g K2S2O5+20 mL HCl+80 mL H2O corrosion reagent. The surface of the sample was wiped with absolute ethanol and acetone, and the microstructure and corrosion morphology were observed using an AxioScope Al-type Cie’s photon microscope. In order to better observe the corrosion surface morphology of the sample, the sample was cut longitudinally from the position near the center, lightly rubbed off with sandpaper for more than 3 mm, avoiding the impact of wire cutting on the corrosion pit, and the profile of the etching pit. Observe and analyze. 2 Results and Discussion 2.1 Microstructure The optical microstructure of the cold-rolled LDX2101 is shown in Fig. 2. The island-like austenitic phase is evenly distributed in the darker ferrite matrix phase. No obvious precipitates are found, and two phases are The microstructure is elongated along the cold rolling direction. Compared with the solid solution sample, due to the relatively large amount of cold rolling deformation, the grain refinement is more serious, while the austenite phase belongs to the hard phase, so part of the austenite phase in the cold rolling process is broken due to deformation. The ferrite and austenite phases did not change significantly.
Fig.2 Micrograph of the cool-rolled sample of LDX2101
2.2 Electrochemical corrosion The LDX2101 samples with different deformation levels (deflection y values) were subjected to electrochemical potentiometric scanning polarization curves in 3.5% NaCl solution. The results are shown in Fig. 3, and the corresponding main characteristic values in the polarization curves are shown in Table 1. With the increase of the applied potential, the oxide film on the surface of the metal continues to grow to form a stable passivation film. At the same time, the metal anode dissolves until the applied potential reaches the breakdown potential and pitting occurs. It can be seen that the Ecorr and Icorr values of the polarization curves of all samples are similar and there is no significant difference, indicating that the four samples have similar corrosion tendencies in the 3.5% NaCl solution, and all of them have obvious passivation regions and can all be in the metal. Different degrees of passivation film are formed on the surface of the material. Among them, the passivation zone of the cold-rolled specimen was the widest and stable, and the Epit reached 396 mV. As the deflection y value increases, that is, the degree of deformation increases, the passivation zone gradually decreases, and the pitting potential Etit gradually decreases. When the deflection y value is 0.75, the pitting potential Epit decreases obviously, only 196 mV, and the passivation area dimension of the passivation current density also increases, which means that with the increase of stress, the material surface forms a passive film to maintain the passivation state. More and more difficult.
Fig.3 Electromagnetic potential polarization curves of LDX2101 subjected to different bending deflection in 3.5%NaCl solution
The electrochemical impedance test results of LDX2101 in 3.5% NaCl solution with different deformation degree (deflection y value) are shown in Fig. 4. In the Nyquist diagram, each curve shows an unfinished capacitive reactance arc, and the radius of the arc represents the impedance. It is generally believed that the larger the radius of the capacitive arc, the better the corrosion resistance of the material. In contrast, the radius of capacitive arc of the cold-rolled sample without force is the largest, that is, the surface forms a stable passivation film, which has the largest resistance value, and has good corrosion resistance and passivation performance. With the increase of deflection, the arc radius of capacitive reactance gradually decreases, and the impedance value gradually decreases, which greatly reduces the stability of the surface passivation film and is consistent with the trend of the pitting potential of the polarization curve. The radius of the capacitive reactance arc with the deflection y value of 0.36 is slightly smaller, and as the deflection increases until the maximum deformation amount, the capacitive reactance arc of the low frequency band exhibits a nearly linear change characteristic, and the radius of the capacitive reactance is significantly reduced, indicating the surface passivation film thereof. Gradually deteriorated, corrosion resistance decreased significantly. In the Bode diagram, the curves are similar in motion, and the high-frequency region of the impedance spectrum is composed of capacitive reactance arcs, which indicates that whether the material is stressed or not does not change the corrosion mechanism of the passivation film, ie, the reaction mechanism of each sample electrode is basically similar [16, 17]. In general, as the stress (deflection y value) increases, the passivation zone becomes narrower and the passivation current density (Ipass) increases, which decreases the stability of the passivation film on the surface of the sample. The pitting potential gradually decreases and the corrosion resistance decreases. decreasing gradually.
Table 1 Main characteristics of polarization curves of LD X2101 subjected to bending deflection
Fig.4 Nyquist plots (a) and Bode plots (b) of LDX2101 subjected to different bending deflection in 3.5%NaCl solution
2.3 Corrosion morphology analysis The surface corrosion morphology of LDX2101 specimens with different degree of deformation (deflection y value) after electrochemical etching is shown in Figure 5. The corrosion morphology of the cold-rolled sample is typical pit pit morphology as shown in Figure 5a. The corrosion occurs in the α/γ phase boundary. After the formation of steady-state pitting corrosion, it gradually expands into the ferrite phase. Corrosion Morphology of Duplex Stainless Steel in Cl – Medium . However, the corrosion morphology of the specimen after deformation is different from that of the undeformed specimen is obviously different. Due to the tensile stress, the defect or inclusion on the surface of the specimen also becomes the formation position of pitting corrosion, even when the tensile stress increases to a certain extent. Surface inclusions  also become pitting locations. When the deformation degree is small (as shown in Figure 5b), the corrosion area is prolonged along the stress direction, and the corrosion position is also increased; as the stress increases, not only the corrosion degree increases, but also the surface layer gradually erodes to the subsurface layer in different degrees, such as In Figures 5c and d, a hollow surface morphology appears, which is due to corrosion of the subsurface inclusions.
Fig.5 Macroscopic corrosion micrographs of the LDX2101 subjected to different bending deflection in 3.5%NaCl solution: (a) 0, (b) 0.36, (c) 0.38, (d) 0.75
In order to further analyze the influence of different deflection values on the stress corrosion of LDX2101, the vertical section corrosion morphology of LDX2101 after electrochemical etching (Fig. 6) was analyzed. It can be seen that the corrosion morphology of cold-rolled specimens is a typical pit pit morphology, as shown in Figure 6a, showing a pronounced hemispherical or elliptical shape, pitting pit inner diameter is less than the outer diameter, that pitting pit inside The narrow surface is wide, and the internal boundaries are clearly visible, and there is no particular feature that selectively corrodes a phase. When the stress is small, the morphology of the pit does not change, and the corrosion pit is mainly characterized by the selective corrosion of the ferrite phase, but the width and depth of the pit increase with the increase of the deflection value, and The subsurface layer gradually spreads, and the surface layer and the subsurface layer communicate with each other, presenting a wide and deep irregular corrosion morphology, as shown in Figs. 6c and d. As the deflection value continues to increase, the selective corrosion inside the etch pit becomes more and more obvious. As shown in FIGS. 6 e and f, the corrosion rate of the internal metal is greater than the surface, the corrosion spreads from the surface to the inside of the substrate, and the selective corrosion of ferrite The body phase forms an intermediate hollow corrosion morphology. This is due to the presence of tensile stress, which changes the position of the pitting nucleation, and the direction of the expansion of the corrosion and the stability of the passivation film, so that the location of the corrosion is not only generated in the surface layer, but also the location of the defect in the subsurface layer. The surface corrosion areas are connected together and selectively etched. That is, the corrosion is along the phase boundary, from the substrate to the subsurface layer, and the selective ferrite phase corrosion characteristics are formed in all directions in the etch pit. Increasing, the corrosion morphology changes into a large area and a deep corrosion pit.
Fig.6 Transverse sections of corrosion micrographs of the LDX2101 subjected to different bending deflection in 3.5%NaCl solution: (a, b) 0, (c, d) 0.36, (e, f) 0.75
3 Conclusion (1) As the stress (deflection) increases, the stability of the passivation film on the surface of LDX2101 decreases, the pitting potential gradually decreases, and the corrosion resistance deteriorates. (2) Under the action of tensile stress, the corrosion morphology of LDX2101 surface was changed, showing obvious selective corrosion. As the stress increases, corrosion preferentially occurs in the α/γ phase boundary and inclusions, and gradually spreads to the ferrite phase. The corrosion area is gradually extended along the stress direction; due to the influence of tensile stress, inclusions on the surface or subsurface layer It is also involved in the formation of pitting corrosion so that the corrosion spreads to the α-phase at the sub-surface or deeper.
The authors have declared that no competing interests exist.
(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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