Corrosion Failure Analysis of Aluminum Alloy Flange of Current Transformer in Substation
Substation current transformer (CT) is an important equipment of substation. It mainly undertakes the function of current conversion and electrical isolation, provides current for various instruments and relay protection, and isolates secondary system from high voltage to ensure the safety of person and equipment. Therefore, the normal operation of CT directly affects the safety and stability of substation transmission and transformation. On-the-spot investigation shows that the corrosion of CT pressure release film flange is serious in many substations in coastal areas, which directly affects the normal operation of CT and poses a serious threat to the safe and stable operation of substations.
CT pressure release film flange is made of aluminium alloy. Aluminum alloys are widely used in substation equipment components [1-3] because of their excellent electrical and thermal conductivity, small proportion, good processing performance and low price. The natural formation of oxide film on aluminium alloy in air can protect the matrix to a certain extent, but for a slightly harsh environment, the corrosion problem can not be ignored [4,5]. Current research shows that the common corrosion forms of aluminium alloy are intergranular corrosion, pitting corrosion, SCC, galvanic corrosion, crevice corrosion and layered corrosion, etc. [6,7]. However, the current research on aluminium alloy corrosion is mainly aimed at aluminium alloy [8-10] used in aviation field, and the research on corrosion and failure of aluminium alloy used in substation, especially for CT flange is comparative.
Aiming at the actual corrosion failure of CT pressure release film aluminium alloy flange in a 220 kV substation along the southeast coast of China, this work carried out analysis and research. The composition, mechanical properties, corrosion morphology and corrosion products of the material were analyzed by means of scanning electron microscopy (SEM), energy spectrum analysis (EDS), X-ray diffraction (XRD) analysis and electrochemical analysis. Based on the analysis of actual service environment and electrochemical corrosion behavior of aluminium alloy flange, the causes and mechanism of corrosion failure are studied, and suggestions are given to improve the safe operation ability of CT in substation.
1 Experimental method
The chemical composition of CT pressure release film flange was analyzed by direct reading spectrometer (BRUKER Q4 TASMAN). The hardness of CT pressure release film flange was measured according to GBT 231.1. The measuring equipment was SCTMA Brinell hardness tester. Three points were selected to measure the hardness, and then the mean value was taken as the hardness of the alloy.
ZEISS EVO18 SEM was used to observe the surface corrosion morphology of CT pressure release film flange failure specimens, and EDS was used to analyze the corrosion products. The corrosion morphology of the failure specimens after rust removal (50 mL phosphoric acid + 20 g chromium trioxide plus distilled water disposed in 1000 mL solution and rust removal at 80 for 6 min) was observed by SEM. The corrosion products on the surface of scraped failure samples were analyzed by D8ADVANCE XRD.
The Zahner-Zennium electrochemical workstation was used for electrochemical testing. The three-electrode system was adopted. The working surface of the sample was grinded to 1000 The scanning rate of potentiodynamic polarization was 0.5 mV/s and the scanning range was (+500 mV). The test temperature is room temperature, and the test medium is deionized water and 3.5% sodium chloride solution, respectively. Three groups of parallel experiments were conducted under each condition.
2. Analysis results of corrosion failure
2.1 Macroscopic Corrosion Morphology
In this study, CT pressure release film aluminium alloy flange was used in a substation off the southeast coast of China for about 2 years. Fig. 1 is the macroscopic corrosion morphology of CT pressure release film aluminum alloy flange under actual working conditions. It can be seen that the aluminum alloy flange has obvious corrosion, a large number of white rust and obvious cracks at the bolt fastening force. The flange was removed for further observation. Figure 2 shows the macroscopic corrosion morphology of the aluminium alloy flange with CT pressure release film. It can be seen that the surface of the aluminium alloy flange is covered with a large number of white corrosion products. The distribution of corrosion products is uneven, and there are many rust marks in some areas (Figure 2a). There are three obvious cracks on the surface of the aluminium alloy flange, all of which are located in bolts. The three cracks are basically symmetrically distributed in three locations of six bolt holes. It can be observed that the longitudinal direction of the aluminum alloy flange is also penetrated by the cracks (Fig. 2d). It is obvious from Fig. 2B that there are a large number of white corrosion products piles near the bolt hole and the propagation position of the cracks. Product. From the macro morphology observation, it was found that the corrosion of CT pressure release film aluminium alloy flange was serious in coastal environment, and the corrosion cracks occurred at the bolt fastening point, which penetrated the sample.
Fig.1 Macroscopic corrosion morphology of CT pressure release film aluminum alloy flange under actual operating conditions
Fig.2 Macroscopic corrosion morphology analysis of CT pressure release film aluminum alloy flange: (a) entirety, (b) crack 1, (c) crack 2, (d) crack 3
2.2 Corrosion Environment Analysis
The service environment of corrosion failure CT pressure release film aluminium alloy flange was analyzed. It was used in a substation near the southeast coast of China. The atmospheric environment in this substation was typical high humidity and salinity environment along the Southeast coast, with an average annual temperature of 21.5 C and an average annual humidity of 78%. There was a large amount of Cl-, Cl-in the atmospheric environment with an average annual deposition of 159 mg/m2a and Cl-in it. It is one of the important causes of atmospheric corrosion of Al and Al alloys [11-14]. In addition, there are many factories near this substation, which emit S-containing gases, resulting in serious industrial pollution. The deposition of SO2 is 62.7 ug/m3. The atmospheric pollution component is the main factor accelerating atmospheric corrosion of metals [12, 15, 16]. In addition, CT pressure release film aluminium alloy flange is a stress structural component, which needs to be fixed by fastening bolts in the service process. There is a risk of SCC. Through analysis, it can be seen that the atmospheric corrosion environment of aluminium alloy flange with corrosion failure is relatively bad, and the concentration of Cl-and SO2 is relatively high. There are environmental and mechanical factors leading to the corrosion of aluminium alloy flange and SCC.
2.3 Composition and Hardness of Materials
2.3.1 The material composition of CT pressure release film aluminium alloy flange was analyzed by direct reading spectrometer. The main chemical composition of the material was (mass fraction,%): Si 0.23, Fe 0.23, Cu 0.25; Mn 0.12, Mg 1.46, Cr 0.06, Zn 4.85, Ti 0.02, Al margin. Comparing with 7A05 aluminium alloy composition (mass fraction,%) (Si 0.25, Fe 0.25, Cu 0.2, Mn 0.15-0.4, Mg 1.1-1.7, Cr 0.05-0.15, Zn 4.4-5.0, Ti 0.02-0.06, Al margin), the content of Zn, Mg and Cu in CT pressure release film aluminium alloy flange is consistent with that of 7A05 aluminium alloy, and the two components are basically the same. Among them, Fe is an impurity element with relatively few components; Si is mainly used to improve the fluidity; Mn, Cr and Ti are mainly used to refine grains and inhibit the harmful effects of Fe .
2.3.2 The Brinell hardness of aluminum alloy flange with CT pressure release film was measured. Three points (Fig. 3) were selected to measure the Brinell hardness. The Brinell hardness was 130.48, 127.69 and 128.44HBW, respectively. The average Brinell hardness of the alloy was 128.87HBW, and the hardness was more uniform.
Through comparative analysis, the composition of CT pressure release film aluminum alloy flange is basically consistent with that of 7A05 aluminum alloy. 7A05 aluminium alloy belongs to the Al-Zn-Mg alloy in 7xxx alloy system. It is a high strength hard aluminium alloy. It has the advantages of low density, good processability and excellent welding performance, but its corrosion resistance is not high. There is SCC risk in service process. SCC is one of the important reasons leading to its failure [18-20].
Fig.3 Brinell hardness measuring points
2.4 Microscopic Corrosion Morphology
Microscopic corrosion morphology and crack morphology of aluminium alloy flange before and after rust removal were observed by SEM, as shown in Fig. 4-6. From the micro-corrosion morphology of CT pressure release film aluminium alloy flange before rust removal in Fig. 4, it can be seen that the surface of flange is covered by a large number of corrosion products, and the corrosion product layer is relatively dense (Fig. 4a). There are small cracks in the corrosion product layer in some areas (Fig. 4b). The corrosion products adhere well on the surface of the sample.
Fig. 5 shows the micro-corrosion morphology of CT pressure release film aluminium alloy flange after rust removal. After rust removal, it can be seen that there are a large number of pitting pits on the surface of aluminium alloy. The distribution and size of pitting pits are uneven (Fig. 5a). Some areas have a tendency of pitting interconnection (Fig. 5b). Combined with environmental analysis, the atmospheric Cl-concentration of the invalid aluminium alloy flange is relatively high. Cl-will cause pitting corrosion on the surface of aluminium alloy flange [11-14].
Fig. 6 is the crack morphology of aluminium alloy flange with pressure release film on failure CT. From the figure, it can be seen that there are a lot of corrosion products in the crack of aluminium alloy flange, which accumulate in the crack (Fig. 6a), and are relatively loose (Fig. 6b). Figure 7 shows the fracture morphology of CT pressure release film aluminum alloy flange, which has brittle characteristics. The cleavage and quasi-cleavage morphologies can be observed, and there are obvious river-like morphology and secondary cracks. At the same time, obvious intergranular cracking morphology and fracture characteristics can be seen in fracture morphology observation (Figure 8). According to the observation of macro and micro corrosion morphology of invalid aluminium alloy flange, the crack occurs in the fastening force part of the flange. It is explained that the crack is caused by corrosion and stress together. Corrosion and stress affect each other and cause SCC of the aluminium alloy flange to fail.
Fig.4 Micro-corrosion morphologies of CT pressure release film aluminum alloy flange: (a) 200×, (b) 2000×
Fig.5 Micro-corrosion morphologies of CT pressure release film aluminum alloy flange after rusting: (a) 50×, (b) 200×
Fig.6 Crack morphologies of CT pressure release film aluminum alloy flange: (a) 200×, (b) 2000×
Fig.7 Micrograph of corrosion fractures
Fig.8 Micrograph of intergranular cracking
2.5 Composition of Corrosion Products
2.5.1 EDS analysis Figure 9 is EDS analysis of corrosion products of aluminium alloy flange with CT pressure release film. The corrosion products of aluminium alloy flange surface (point A) and crack interior (point B, point C) are analyzed by EDS. From the graph, it can be seen that the elements of corrosion products of alloy flange surface and crack interior are basically the same, with slightly different contents. The corrosion products contain a certain amount of O, N, S, P, Cl and other elements. It is shown that there are corrosive media containing O, S, Cl and other elements in the working environment of the aluminium alloy flange, and the existence of Cl-can easily cause pitting corrosion of the aluminium alloy [11-14], which is consistent with the observation results of the micro-corrosion morphology of the flange (Fig. 5).
2.5.2 XRD analysis figure 10 is the XRD spectrum of corrosion products of Al alloy flange with CT pressure release film. From XRD analysis, it is concluded that the corrosion products of Al alloy flange in this coastal environment are mainly AlPO4, AlCl3, MgCl2 and ZnAl2S4. Based on the XRD analysis of corrosion products and EDS analysis, it is known that the main environmental impact factors of corrosion failure of aluminium alloy flange are Cl and S, which cause the corrosion of aluminium alloy and lead to SCC under stress.
Fig.9 EDS analysis of corrosion products of CT pressure release film aluminum alloy flange: (a) test position: (a) crack inside, (b) crack edge, (c) sample surface
Fig.10 XRD analysis of corrosion products of CT pressure release film aluminum alloy flange
3. Analysis and discussion of corrosion failure
Combining with the analysis of actual working conditions and corrosion products (figs. 9 and 10), the atmospheric environment of aluminium alloy flange with failure CT pressure release film is coastal industrial atmospheric environment, which contains a large number of corrosive elements such as Cl and S. The corrosive products also contain more Cl and S elements. Pollution components in atmospheric environment are the main factors for atmospheric corrosion of metals. Cl-is an important factor affecting the corrosion sensitivity of aluminium alloys [11-14]. From the potentiodynamic polarization curve of the effect of 11Cl-on the corrosion of C T pressure release film, it can be seen that a certain concentration of Cl-causes the corrosion current density of aluminium alloys to increase significantly and the corrosion potential to decrease significantly. The accelerated corrosion effect of l-on aluminium alloy is obvious. Cl-can destroy the dense protective layer on the surface of aluminium alloy, and can be adsorbed at the active sites such as defective oxide film or uneven material on the surface of aluminium alloy. The chemical reaction between the adsorbed Cl-and the oxide film will lead to thinning, cracking of partial oxide film and direct corrosion of aluminium , resulting in accelerated local dissolution corrosion of pitting corrosion (Fig. 5). Cl-forms AlCl3 through a series of reactions (Fig. 10). The reaction steps are :
At the same time, SO2 adsorbed in industrial atmosphere dissolves and hydrates on the surface of aluminium to form HSO 3-, which is oxidized to SO 42-, reacts with aluminium alloy and accelerates corrosion of aluminium alloy [11,15].
By observing the macro-morphology of the failed aluminium alloy (Fig. 1 and 2), it can be seen that there are obvious cracks in the fastening force of the flange bolt of the aluminium alloy. The assembly stress can lead to the defect destruction of the local oxide film on the surface of the aluminium alloy, and synergize with Cl-to form pitting corrosion. From the appearance of crack initiation in flange of aluminium alloy in Fig. 12, it can be seen that the crack initiation is at the corrosion defect. Because of the stress concentration in the defect and pitting, pitting becomes the initiation point of crack, and the further action of Cl, S and other elements, the corrosion reaction occurs and the accumulation of corrosion products is formed at the initiation and propagation path of crack (Fig. 2, 6). The accumulation of corrosion products leads to the increase of stress in the crack, and the synergistic effect of stress and corrosion environment leads to the corrosion reaction. Crack initiation and propagation [18-20].
The composition of flange material of failed aluminium alloy is basically the same as that of 7A05 hard aluminium alloy. The 7xxx aluminium alloy is prone to SCC [18-20]. The corrosion and SCC phenomena are obvious in the coastal industrial environment. It is not suitable for substations in coastal industrial environment without protective measures.
Fig.11 Effect of Cl- on CT pressure release film’s corrosion
Fig.12 Fracture macrograph of crack initiation
(1) In coastal industrial atmospheric environment, the corrosion of CT pressure release film aluminium alloy flange is serious, SCC causes its failure, and the corrosion products are mainly Al chloride. Cl-and S2-are the main environmental factors causing corrosion failure. The SCC of Cl-is caused by the combined action of corrosion factors such as Cl-and stress.
(2) Aluminum alloy material resistant to S, Cl and SCC should be used in CT pressure release film flange of coastal industrial environment substation, and 7A05 aluminium alloy should not be used directly without anti-corrosion measures. Protective measures such as S, Cl corrosion resistant coatings, coatings or anti-corrosion grease should be adopted to protect aluminum alloy parts from corrosion.
(3) In order to reduce the risk of SCC, CT pressure release film aluminium alloy parts should keep the stress of bolts relatively uniform when fastening, and under the condition of meeting the service requirements, the stress should not be too large.
The authors have declared that no competing interests exist.
Source: China Flanges Manufacturer – Yaang Pipe Industry Co., Limited (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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