Corrosion Fatigue Crack Propagation Behavior of 2205 Duplex Stainless Steel in Aqueous Medium

Abstract:

Experiments of fatigue crack propagation were performed of 2205 duplex stainless steel (2205 DSS) in the deionized water, 3.5%(mass fraction) NaCl solution and atmosphere respectively. It found that in the aqueous solutions the crack growth rate and the fatigue life of the steel was greatly enhanced and reduced respectively. Correspondingly, the influencing degree of the three media can be described respectively as the following two rankings: (da/dN)deionized water>(da/dN)NaCl>(da/dN)atmosphere for fatigue crack propagation rate and Natmosphere>NNaCl>Ndeionized water for fatigue crack propagation life. The fracture morphology revealed that hydrogen induced cracking is the main reason for the deterioration of the fatigue performance of the duplex stainless steel in aqueous medium. Compared with the deionized water, Cl has a stronger corrosion effect, and more defects can be induced in the front of the main crack, therewith which develop into secondary cracks, resulting in the longer the effective crack length and the smaller the driving force of crack propagation.

Key wordsduplex stainless steel ; deionized water ; 3.5%NaCl solution ; corrosion fatigue ; crack growth

YANG Bin1,2, CHEN Ji1, GENG Yue1, CUI Xiao1, GUI Wanglin1
1 School of Mechanical Engineering, Liao Ning Shihua University, Fushun 113001, China
2 NCS Testing Technology Co., Ltd., Shanghai 201103, China
 Cite this article:
YANG Bin, CHEN Ji, GENG Yue, CUI Xiao, GUI Wanglin. Corrosion Fatigue Crack Propagation Behavior of 2205 Duplex Stainless Steel in Aqueous Medium. Corrosion Science and Protection Technology[J], 2018, 30(2): 182-186 doi:10.11903/1002.6495.2017.084

Due to the diversity of material-environment systems and the interaction of environment and cyclic loads, the corrosive fatigue behavior of metallic materials is very complicated. In the complex environment of the marine and nuclear power industry, corrosion fatigue of equipment materials is one of the major failure modes [1,2,3]. In corrosion fatigue, the crack propagation life accounts for 90% of the total fatigue life of corrosion [4]. Studying the fatigue crack propagation behavior of metallic materials in aqueous solution is of great significance for its safe use. 2205 Duplex Stainless Steel (DSS) has gained more and more applications in shipbuilding and transportation due to its excellent corrosion resistance, high strength and economical practicality [5]. At present, research on duplex stainless steels mainly focuses on the corrosion resistance [6,7], heat treatment process and heat treatment on the corrosion performance [8,9,10,11,12,13] and its weldability, weld performance The study [14,15,16,17] and so on, the study of corrosion fatigue behavior of duplex stainless steel is still rare [18]. In this paper, the 2205 DSS specimens were subjected to fatigue crack growth experiments in air, deionized water and 3.5% (mass fraction) NaCl solution environments. The influence of the media environment on the fatigue crack propagation behavior and the fracture mechanism were studied.

1 Experimental method
The experimental material is 2205DSS (S32205) hot-rolled plate provided by Yaang Pipe Industry Co., Limited with a thickness of 10 mm. Its chemical composition (mass fraction, %) is: C 0.015, Mn 1.38, Si 0.34, P 0.025, S 0.001, Cr 21.59, Ni 5.15, Mo 0.72, Cu 0.26, N 0.22, Fe balance; its yield strength and tensile strength are 600 and 805 MPa, respectively, and elongation is 36%; two ferrite and austenite phases in steel are about the same. 1:1.
The standard single-edge notched three-point bending SE (B) specimen was selected and processed in accordance with GB/T6398-2000 “Mechanical Materials Fatigue Crack Growth Rate Test Method”. The sample size is shown in Figure 1. The crack propagation direction is perpendicular to the rolling direction. . Fatigue tests were performed on an Instron 8872 electro-hydraulic servo fatigue tester. With sinusoidal alternating load, stress ratio R=0.1. In the initial load range ΔP = 9000 N, pre-cracking starts and the force is gradually reduced to ΔP = 3500 N. The force reduction rate is approximately 10% to 20% for each stage, and the total length of the pre-crack is approximately 2 mm. The crack opening displacement was measured with a COD gauge, and the crack length a was measured with a flexibility method. According to the seven-point incremental polynomial method in Appendix A of GB/T6398-2000, the data was fitted to obtain the fatigue crack propagation rate da/dN and the stress intensity factor amplitude ΔK. During the fatigue test, the sample was immersed in the environmental tank and the solution medium was circulated with a constant flow pump. The solution flow rate was 1.0 to 1.2 mL/min. Finally, the morphology of the fractures was analyzed by NOVA NanoSEM430 scanning electron microscope (SEM).
img 1 - Corrosion Fatigue Crack Propagation Behavior of 2205 Duplex Stainless Steel in Aqueous Medium

Fig.1 Standard specimen for three point bending

img 2 - Corrosion Fatigue Crack Propagation Behavior of 2205 Duplex Stainless Steel in Aqueous Medium

Fig.2 aN curves in three environments for 2205 DSS

img 3 - Corrosion Fatigue Crack Propagation Behavior of 2205 Duplex Stainless Steel in Aqueous Medium

Fig.3 Relationship between stress intensity factor and fatigue crack propagation rate of 2205 DSS in three environments

2 experimental results
Comparison of 2.1 a-N Curve and da/dN-ΔK Curve
The a-N curves of fatigue crack propagation of 2205 DSS specimens under different conditions are shown in Fig. 2. The specimen fractured after the crack propagation of about 14 mm, and the fatigue crack growth life was Natmosphere>N3.5%NaCl>Ndeionized water. The fatigue crack propagation life is about 3.2×105 in air, about 2.8×105 in NaCl solution and about 1.9×105 in deionized water. Compared with the sample in air, the fatigue crack propagation life of 2205 DSS decreased by 12.5% in NaCl solution and decreased by 40.6% in deionized water solution.
Figure 3 is a comparison of the da/dN-ΔK curves for the 2205 DSS sample under different conditions. The solution environment significantly accelerated the fatigue crack propagation of the 2205 DSS specimen. At the same ΔK, the fatigue crack growth rate of the 2205 DSS sample in deionized water is the fastest, (da/dN) deionized water> (da/dN) NaCl> (da/dN) atmosphere. For example, when ΔK=40 MPam1/2, (da/dN)NaCl=3.488×10-4 mm/cycle, which is approximately 1.63 times the steady-state expansion rate in air, while (da/dN)deionized water=5.994×10 -4 mm/cycle, approximately 2.81 times the steady-state expansion rate in air. The increase of the solution environment near the threshold area not only reduces the threshold value, but also increases the critical ΔKc of the transition from the near threshold area to the steady state expansion area. (ΔKc) deionized water> (ΔKc) NaCl> (ΔKc) atmosphere. In the solution environment, the formation of Fe-containing corrosion products at the crack tip can easily cause the closure effect, which can effectively reduce the crack tip ΔK, which may be the reason for the near threshold area to become larger. In the steady state region, the fatigue crack propagation can usually be fitted by Paris formula da/dN=CΔKm. C and m are linear fitting parameters. The results are shown in Table 1. matmosphere>mNaCl>mdeionized water, Catmosphere<CNaCl<Cdeionized water And Cdeionized water is much higher than Catmosphere.
2.2 fracture morphology
Figure 4 shows the fracture morphology of 2205 DSS specimens in air, NaCl solution and deionized water. Figure 4a shows that in the air, the fatigue fracture of the 2205 DSS specimen is characterized by brittle quasi-cleavage fracture. In the near-threshold region, the main crack surface is relatively flat, and the river pattern flows in the crack propagation direction, perpendicular to the main crack surface. A small amount of discontinuous secondary cracks can be found in the direction; in the steady-state extension region, the number of secondary small cracks increases significantly; in the destabilized region, small secondary cracks are connected in a tearing manner to form large cracks, and many can be seen clearly. a continuous tearing edge. Fig. 4b shows that in the NaCl solution, the fatigue fracture of the 2205DSS specimen has brittle quasi-cracking, secondary cracking, tearing and other hydrogen embrittlement features, and a small amount of small pitting can be found on the main crack surface in the near-threshold area. In the pit, a small amount of discontinuous secondary cracks can be found in the direction perpendicular to the main crack surface; the number of pits on the main crack surface in the steady-state expansion zone increases significantly, and the pit pit grows, and the number of secondary small cracks increases significantly; In the destabilization zone, the size of pits on the main crack surface is significantly increased, and secondary small cracks are gradually connected to form a plurality of slender, discontinuous torn edges. Fig. 4c shows that in the deionized water, the fatigue fracture of the 2205 DSS sample shows a quasi-cleavage fracture, the fracture surface is relatively smooth in the near-threshold area, no obvious secondary small cracks are found, and the fracture surface in the steady-state extension area is rough At the centerline of the plane, a clear, thick ridge-like torn edge can be seen throughout the instability zone.
img 4 - Corrosion Fatigue Crack Propagation Behavior of 2205 Duplex Stainless Steel in Aqueous Medium

Fig.4 SEM micrographs showing morphology of fatigue fracture of 2205 DSS in air (a), 3.5%NaCl solution (b) and deionized water (c) environments

3 Analysis and Discussion
Hydrogen-induced cracking is the main reason for the weakening of duplex stainless steel materials in aqueous media [19,20,21]. The fatigue crack propagation of 2205DSS materials in aqueous solution is mainly related to the hydrogen embrittlement of the ferrite phase [22]. In aqueous solution, the cyclic load causes the passivation film on the surface of the sample to crack, and the bare metal can act as a dissolution reaction at the anode. The crack wall can be used as a cathode for hydrogen evolution reaction. The electrochemical reaction is shown in (1)~(3). When the adsorbed hydrogen Hads diffused to the crack tip reaches a critical concentration, the crack tip metal cracks while leaving Fe-containing oxide on the crack surface. This process is repeated and fatigue cracks expand forward.
Anode reaction M→Mz++ze−M→Mz++ze-(1)
Hydrolysis reaction Mz++pH2O→M(OH)z−pp+pH+Mz++pH2O→M(OH)pz-p+pH+(2)
Cathodic reaction H++e−→HadsH++e-→Hads Hads+Hads→H2Hads+Hads→H2(3)
Among them, M represents the major element of the stainless steel matrix Fe, Cr or Ni.
The difference in crack propagation ability between 2205 DSS materials in two aqueous solutions is related to the driving force for crack propagation. The two phases of the duplex stainless steel have inconsistent deformation capabilities and are liable to cause local stress concentration at the phase boundary, generate defects and develop into small secondary cracks, resulting in an increase in the effective crack length, as shown in FIG. 5 . If the crack growth driving force Es is expressed as:
Es=EDL0+LEs=EDL0+L(4)
Among them, ED is the total energy required for crack propagation, the initial length of the main crack is L0, and the crack length caused by the secondary crack increases to ΔL. Compared with the deionized water, the Cl-containing medium has stronger corrosion effect, and it is easy to induce a large number of defects and form more secondary cracks at the front end of the main crack, so that the effective crack length is longer and the driving force for crack propagation is smaller. The fatigue crack growth driving force of the 2205DSS material in 3.5% NaCl solution is lower than that in the deionized water medium, and the expansion speed is slower, which is consistent with the data obtained by the Paris formula.

20180510155039 49291 - Corrosion Fatigue Crack Propagation Behavior of 2205 Duplex Stainless Steel in Aqueous Medium

Table 1 Paris’ equations at different environment

img 5 - Corrosion Fatigue Crack Propagation Behavior of 2205 Duplex Stainless Steel in Aqueous Medium

Fig.5 Comparison of the schematic diagrams of crack propagation in 2205 DSS samples immersed in 3.5% NaCl solution (a) and in deionized water (b)

The difference in media environment caused the fracture mechanism of the 2205 DSS specimen to change. In the 3.5% NaCl solution, Cl- induced more defects in the front of the main crack, resulting in secondary cracks, resulting in a number of small tearing edges; whereas in deionized water, only the leading end of the main crack formed a coarse in the destabilization zone. Ridge-like torn edges.
4 Conclusion
Aqueous solution environment significantly improved the fatigue crack growth rate of 2205 DSS, and reduced its fatigue life. The order of fatigue crack propagation rate was: (da/dN)deionized water>(da/dN)NaCl>(da/dN)atmosphere, fatigue crack growth life sequence For: Natmosphere>NNaCl>Ndeionized water. The two phases have uncoordinated deformability and are prone to stress concentration at the phase boundary, resulting in secondary small cracks and increasing the effective crack length. Compared with deionized water, Cl-has stronger corrosion effect, and can induce more defects and develop secondary cracks at the front of the main crack, so that the effective crack length is longer and the driving force for crack propagation is smaller.

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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