Corrosion Failure Mechanism of 2507 Duplex Stainless Steel Circulation Pump Impeller

Abstract

The circulation pump in a distillation column is a core device in a material circulation system, and its stable operation is crucial for the production process. The impeller of the circulation pump is prone to failure due to long-term contact with corrosive media, and subjected to a large amount of material erosion, which severely challenges the safety control of the distillation reaction system. Focusing on the corrosion failure phenomenon of circulation pump impellers, the failure mechanism was studied by means of macroscopic inspection, chemical composition analysis, metallographic examination, scanning electron microscopy (SEM), and energy dispersive spectrometer (EDS). Results indicated that the corrosion of circulation pump impellers was the result of the combined effects of surface wear, cavitation, and halogen element corrosion. The medium in contact with the impeller contained chloride ions, fluoride ions, and solid particles. During circulation pump operation, a low-pressure zone formed at the inlet, generating numerous water vapor bubbles. These bubbles burst in the high-pressure zone, creating highly localized impact forces. Combined with the abrasive action of solid particles on the impeller surface, this led to the destruction of the passivation film and the formation of numerous small pits. These corrosion pits and the surrounding environment formed micro-galvanic corrosion cells with small anodes and large cathodes. Under the accelerated corrosion caused by fluoride and chloride ions, the corrosion process towards the inner wall of the impeller intensified, ultimately leading to impeller failure. This study clarified the corrosion failure mechanism and its root causes in the 2507 duplex stainless steel circulation pump impeller and proposes corresponding improvement recommendations, providing a scientific basis for preventing similar issues from occurring in the future.

1. Introduction

As a duplex stainless steel with high Cr and Mo content, 2507 steel combines the advantages of both ferritic and austenitic stainless steels, offering excellent mechanical properties and corrosion resistance; thus, it is widely used in desulfurization, wastewater treatment plants, chemical plants, and other harsh corrosive environments rich in H₂S and dilute sulfuric acid [1,2,3,4]. Research on 2507 duplex stainless steel primarily focuses on the effects of high chloride and sulfide environments on its corrosion and passivation behavior. Li et al. [5] investigated the corrosion behavior of 2507 duplex stainless steel in acidic, high-chloride environments. Their study found that with increasing Cl⁻ concentration, the corrosion potential (Ecorr) shifted negatively, the corrosion current density (Icorr) increased, and the polarization resistance (Rp) decreased. Zhu et al. [6] investigated the corrosion resistance of 2507 duplex stainless steel in SO₂-polluted seawater. Their results indicated that when the NaHSO₃ concentration was below 10 mmol/L, an increase in NaHSO₃ concentration led to a higher density of point defects in the passive film and a decrease in corrosion resistance. Feng et al. [7] studied the corrosion and passivation behavior of 2507 duplex stainless steel in electrolyzed seawater anti-fouling environments. Their results indicated that the primary corrosion form in this environment was pitting. As the concentration of NaClO increased, the shielding performance of the passive film decreased, the number of pitting holes increased, and the corrosion resistance declined. Zhang et al. [8] investigated the corrosion behavior of 2507 super duplex stainless steel (SDSS) in H₂S-free and H₂S-containing acidic environments. Their results show that in N₂ environments, with an increase in potential, the dissolution of Fe increases, resulting in an increase in current density. The presence of H₂S reduced the corrosion resistance of 2507 SDSS. Under the synergistic effect of H₂S, the formation of pits in the ferrite region of the phase interface showed an increasing trend to expand to the ferrite region. Li [9] and Nugroho et al. [10] have shown that stainless steel is prone to form pitting pits in chloride and sulfide environments. The shape and depth of pitting pits affect stress distribution in the local area, which in turn affects the stress corrosion cracking and corrosion fatigue crack initiation of the material. However, there are currently few reports on corrosion failure cases and corresponding microscopic failure mechanisms of 2507 duplex stainless steel materials and their key components during service. As 2507 duplex stainless steel is commonly used in corrosive environments, such as marine engineering, chemical industries, and oil and gas industries, failures of key components in these fields could lead to severe safety incidents. Therefore, accurate analysis of such failures’ causes is crucial for ensuring the long-term stable operation of equipment and systems.

The operational conditions of the strong circulation pump P-146 in the distillation columns are characterized by a flow rate of 190 m³/h, a head of 13 m, and an operating temperature from 150 to 160 °C. The internal medium of the distillation columns contains 0.01% p-toluidine, 0.01% p-nitrotoluene, 70.78% 2B oil (trichlorotetramethylbenzene), and 29.15% high-boiling substances, along with solid particles such as catalysts and activated carbon, and the environment is alkaline. During the operation of the distillation column, abnormalities in the head and flow of the strong circulation pump were observed, along with an elevated temperature gradient within the tower. Upon inspecting the circulation pump and removing the pump cover, it was found that the impeller was severely corroded. The impeller was made of 2507 duplex stainless steel. To determine the cause of the failure, this study conducted a corrosion failure analysis of the failed impeller. The aims were to ensure the stable operation of the distillation tower’s circulation pump, provide a scientific basis for implementing targeted anti-corrosion measures, and optimize distillation operating conditions. This analysis was crucial for enhancing the operational reliability and production efficiency of the entire production system.

2. Physicochemical Experiments

2.1. Macroscopic Inspection

The failed circulation pump impeller underwent a macroscopic inspection. Figure 1a shows the overall morphology of the impeller, from which it can be observed that the sample surface had many unevenly distributed pits, which were dense and numerous. Figure 1b displays the morphology of a sampled area from Figure 1a, indicating the presence of numerous small pits and interconnected large pits with pitting characteristics, some containing corrosion products. These pits varied in size and depth, appearing in horseshoe and hole shapes, as shown in Figure 1c,d, and displaying characteristics of cavitation and pitting [11].

2.2. Chemical Composition Analysis

A SPECTRO-type photoelectric direct-reading spectrometer was used to perform a chemical composition analysis of the circulation pump impeller material. The measured chemical composition of the material is shown in

Table 1. Chemical composition analysis results of circulation pump impeller (wt.%).

Element	C	Si	Mn	P	S	Cr	Ni	Mo	N
Standard requirements	0.030	0.80	1.20	0.035	0.020	24.0~26.0	6.0~8.0	3.0~5.0	0.24~0.32
Sample composition	0.015	0.46	0.52	0.023	0.016	24.89	7.26	3.83	0.28

. From data in Table 1, it is evident that the chemical elements of the impeller material all complied with the requirements of ASTM A240/A240M-20 for 2507 duplex stainless steel.

2.3. Metallographic Structure Analysis

Figure 2a shows the metallographic structure of the material at the pit location on the circulation pump impeller at the sampling position indicated in Figure 1b. In the figure, it can be observed that the microstructure consisted of gray and black phases, uniformly distributed in a band-like pattern with clear phase boundaries and no other structures [12,13,14]. The chemical compositions of the gray and black phases were analyzed by energy dispersive spectrometer (EDS), as shown in

Table 2. Chemical composition of different phases (wt.%).

Phase Type	Fe	Cr	Mo	Ni	N	Si	Phase Category
Gray	67.31	21.30	3.32	7.10	0.21	0.76	α phase
Black	64.56	24.32	5.67	4.53	0.27	0.65	γ phase

. These data reveal that the black phase was a chromium–molybdenum-rich phase, with a chromium content of 24.32% and a molybdenum content of 5.67%, which was identified as the α phase; the gray phase was a nickel–iron-rich phase, with a nickel content of 7.10% and an iron content of 67.31%, identified as the γ phase. Figure 2b presents the X-ray diffraction (XRD) pattern of the material at the erosion pit of the centrifugal pump impeller. It can be observed that the XRD spectrum only has characteristic diffraction peaks of the α and γ phases, with no characteristic diffraction peaks of other precipitated phases. The characteristic diffraction peaks at 2θ of 43.21° and 64.93° correspond to the (110) and (200) crystal planes of the α phase, respectively, and the characteristic diffraction peaks at 2θ of 43.36°, 51.52°, and 74.36° correspond to the (111), (200), and (220) crystal planes of the γ phase, respectively. Using Image-Pro Plus 6.0 image analysis software [15], the content ratios of the α and γ phases were determined to be 50.38% and 49.62%, respectively, with an α/γ phase ratio of approximately 1. The above analysis indicates that the material of the centrifugal pump impeller was consistent with the typical 2507 duplex stainless steel structure. The metallographic structure had not undergone any changes.

2.4. Scanning Electron Microscopy and Energy Spectrum Analysis

Figure 3a displays the SEM morphology of the centrifugal pump impeller’s surface at the sampling location indicated in Figure 1b. The image reveals the presence of corrosion products and a corrosion scale layer on the impeller’s surface. Figure 3b,c are magnified views of points ① and ② from Figure 3a, respectively. EDS analysis was performed on the corrosion scale layer in Figure 3b,c, and it was observed that in addition to the elements typically found in 2507 duplex stainless steel, the corrosion scales also contained elements such as F, O, and Cl. Figure 3d provides a magnified view of point ③ from Figure 3a. EDS analysis of corrosion products in Figure 3d revealed a higher concentration of F, O, and Cl in these corrosion products.

Figure 4a,b show the SEM morphology of the corrosion pits on the impeller surface at the sampling location indicated in Figure 1b. These images reveal that the corrosion pits did not penetrate the circulation pump impeller. The surface exhibited rough and porous characteristics, with a nearly circular shape, and the pits contained bright white corrosion products resulting from both corrosion and mechanical wear.

Figure 5a–d show EDS spectra of marked positions ①–④ in Figure 4a,b, respectively. It is evident that, in addition to the elements typical of stainless steel, spectra from the edges of the pits (point ①) and the bottoms of the pits (points ② and ③) also reveal the presence of elements such as F, Cl, K, and O, with F and Cl being corrosive elements. The corrosion products inside these pits (position ④) contained a high level of O, as well as Fe, Ca, Na, S, and K, indicating that these corrosion products were primarily metal oxides and some other salt substances.

3. Impeller Failure Analysis

According to the test results mentioned above, the chemical composition of the circulation pump impeller met the standard requirements, and the metallographic examination results revealed a normal 2507 duplex stainless steel microstructure. Numerous pits were found on the surface of the circulation pump impeller, identified as cavitation defects and pitting defects. During the normal operation of the circulation pump, the liquid pressure decreases from the pump inlet to the impeller inlet. When the pressure near the impeller inlet is lower than the saturated vapor pressure of the liquid at the operating temperature, the liquid begins to vaporize and form bubbles. These bubbles move with the liquid into the high-pressure area of the pump, where they burst and release energy. The surrounding liquid quickly fills the original bubble cavity, causing a hydraulic impact on the impeller, which causes the impeller to be impacted and damaged [16], as shown in Figure 6. The breaking of bubbles in the circulation pump also generates noise and vibration, leading to intense cavitation resonance and exacerbating the damage to the impeller. The chemical composition analysis revealed that the corrosion products contained a significant amount of oxygen, indicating that a considerable amount of air was ingested by the pump under actual operating conditions. The pump shaft end seal is an oil-impregnated graphite packing soft seal, and as operating time increases, the radial gap between the shaft sleeve and the packing continuously enlarges. Air enters the low-pressure area at the impeller inlet from the pump shaft under atmospheric pressure, and then enters the high-pressure area of the pump with the medium, impacting the impeller and causing erosion and corrosion damage to the component surface, which is also a crucial reason for the cavitation of the pump impeller.

The corrosion scale layer, corrosion products, as well as the bottom and edges of the pits, contained significant amounts of F and Cl elements. These halogen elements, as active ions, can accelerate metal corrosion, hinder the formation of the passive film, promote the pitting corrosion process, and reduce the material’s corrosion resistance [17,18,19,20]. Due to the presence of solid particles, such as activated carbon, in the tower, which act as catalysts and cause abrasion on the impeller, combined with bubble impact corrosion, the passive film on the surface of the impeller was damaged [21,22,23]. At locations where the passive film was destroyed, the metal surface exhibited higher activity and lower potential, acting as the anode, while the other regions remained passivated with higher potential, forming the cathode, thus creating a corrosion system with a large cathode and a small anode [24,25,26]. Oxidation reactions occurred at the anode, leading to metal dissolution, while in the alkaline environment of the cathode, dissolved oxygen underwent reduction reactions, as shown in Equations (1) and (2). Continuous generation of OH⁻ increased the pH in the cathode region, which then reacted with metal ions to form insoluble metal oxides or hydroxides, as shown in Figure 4b. These products accumulated at the pit opening, creating an occluded zone. Inside the pit, the metal continued to ionize, and to maintain electrical neutrality, large amounts of Cl⁻ and F⁻ from the external medium migrated into the pit, forming chlorides and fluorides. The hydrolysis of these chlorides and fluorides increased the acidity inside the pit, further accelerating the anodic dissolution of the metal [27]. This resulted in autocatalytic corrosion, promoting the corrosion to propagate toward the inner wall of the impeller, as illustrated by Equations (3) and (4). The corrosion mechanism is illustrated in Figure 7.

$$\mathrm{M}\to {\mathrm{M}}^{\mathrm{n}+}+{\mathrm{n}\mathrm{e}}^{-}$$

(1)

$${\mathrm{O}}_2+{2\mathrm{H}}_2\mathrm{O}+4{\mathrm{e}}^{-}\to 4{\mathrm{O}\mathrm{H}}^{-}$$

(2)

$${\mathrm{M}}^{\mathrm{n}+}+{\mathrm{n}\mathrm{C}\mathrm{l}}^{-}+{\mathrm{n}\mathrm{H}}_2\mathrm{O}\to \mathrm{M}{\left(\mathrm{O}\mathrm{H}\right)}_{\mathrm{n}}+\mathrm{n}\mathrm{H}\mathrm{C}\mathrm{l}$$

(3)

$${\mathrm{M}}^{\mathrm{n}+}+{\mathrm{n}\mathrm{F}}^{-}+{\mathrm{n}\mathrm{H}}_2\mathrm{O}\to \mathrm{M}{\left(\mathrm{O}\mathrm{H}\right)}_{\mathrm{n}}+\mathrm{n}\mathrm{H}\mathrm{F}$$

(4)

Based on the above analysis, the corrosion of the circulation pump impeller resulted from a combination of surface wear, cavitation, and halogen element corrosion, constituting a complex form of corrosion. Impeller corrosion can lead to increased power consumption, decreased efficiency in the circulation pump, and decreased medium flow, and exacerbates the imbalance of rotating components, ultimately causing structural damage to the pump body. This significantly impacts the operational safety and service life of the pump. Focusing on the issue of cavitation that frequently occurs in circulation pump impellers, rounding the impeller inlet to create a more streamlined shape can effectively reduce cavitation [28]. Surface modification techniques such as laser cladding or boronizing the impeller surface can enhance its resistance to cavitation and wear [29,30]. Additionally, lowering the concentration of halide ions in the medium and using stainless steel materials with higher Cr, Mo, and Ni contents can improve the material’s resistance to pitting corrosion [31].

4. Conclusions

This study investigated the corrosion failure mechanism of the 2507 duplex stainless steel impeller in circulation pumps. Through macroscopic examination, chemical composition analysis, metallographic analysis, and corrosion product analysis, the following conclusions were drawn:

(1)

The chemical composition of the circulation pump impeller met standard requirements. The matrix microstructure consisted of ferrite and austenite, and no abnormalities were found, indicating that the impeller’s corrosion was unrelated to the material’s quality.

(2)

External factors such as surface wear, cavitation, and halogen element corrosion were the primary causes of corrosion failure in the circulation pump impeller. As the blades rotated, low-pressure zones formed generating bubbles that collapsed in high-pressure areas. The impact of the bubble collapse combined with the scouring force of internal particles damaged the passive film on the stainless steel surface, creating a large cathode–small anode catalytic corrosion system. Under the accelerated corrosive action of reactive halogen ions, the corrosion rate at the damaged areas of the passive film increased, eventually leading to component failure.

(3)

It is recommended that the impeller inlet be rounded, shaping it to be more streamlined, in order to reduce flow separation, enhance fluid flow efficiency, and improve the impeller’s resistance to cavitation. Wear resistance and pitting corrosion protection of circulation pumps can be achieved through both the appropriate selection of materials and the application of surface engineering techniques.