Analysis of internal corrosion causes of product oil pipelines before they are put into production
Corrosion failure analysis of a west-located product pipeline before putting into operation was carried out in terms of analyzing the inspection corrosion data and the pipeline conditions as well as the macro-morphology observation, microstructure examination, analysis of chemical composition and constituents of corrosion products by means of SEM, EDS and XRD. The results showed that due to the large variation of the pipeline elevation and the insufficient cleaning up of the hydrotest water, thus there existed residual water at some low points of the pipeline. Where, water-line corrosion might occur as a result of oxygen concentration difference cell formed between the upper and lower parts of the water-line. In addition, water-line corrosion was accelerated due to sealed with pressurized air. Therefore, as countermeasure, it is proposed that the pipeline should be completely cleaned up and even dried up by air blowing after hydrotesting, and for the pipeline which was not immediately put into operation, a nitrogen injection sealing should be adopted to avoid such corrosion.
|Key words： products pipeline internal corrosion water-line corrosion|
|Meng LIU1, Youwen JIANG1, Shuo HAN2, Xin LV3, Aiping REN4, Wenhui LIU1, Bingchuan YAN5, Xinhua CHEN1|
Corrosion leakage occurred when the product oil pipeline in a certain province of Northwest China was put into production. The results of magnetic flux leakage test showed that some internal pipe sections had serious internal corrosion, and most of the corrosion pipe sections were located at the relatively low point of the pipeline elevation. In order to ensure the safety of the pipeline, the operating unit carried out more than 70 exchange operations, resulting in huge economic losses. In this paper, the corrosion corrosion severity is taken as the research object, and its macroscopic corrosion morphology and corrosion characteristics are analyzed. Scanning electron microscopy, energy spectrum analysis and X-ray diffraction surface analysis techniques are used to analyze the material, corrosion product composition and phase structure of the pipeline, combined with the pipeline. The working conditions, explore the causes of corrosion, and propose corresponding anti-corrosion measures to prevent the occurrence of similar pipeline corrosion.
1 Experimental method
The statistical results of pipeline leakage magnetic detection show that the internal corrosion defect density is as high as 210.8/km, and the corrosion thinning depth is greater than 72% of the wall thickness of 20%. The pipeline has serious internal corrosion. 87.4% of the internal corrosion points are concentrated at the 4-8 o’clock position of the pipeline, showing obvious water corrosion characteristics. In the inner corrosion point, 2873 is concentrated on 58 pipe joints, and there is obvious local concentration.
2.2 Materials and microstructure
The chemical composition of the pipeline is shown in Table 1. As can be seen from Table 1, compared with the standard GB/T 9711-2011 , the main component content of the pipeline is qualified, and there is no excessive phenomenon of harmful components such as P and S. .
Table 1 Chemical compositions of the pipeline (mass fraction / %)
The results of pipeline metallographic structure and inclusion composition analysis are shown in Fig. 2. As can be seen from Fig. 2a and b, the microstructure of the product oil pipeline is mainly composed of ferrite and pearlite, and the grain size is 10~40 μm. Between the inclusions is mainly a spherical oxide containing Al, Ca, Mg and Si. The size of individual oxides is large, and the distribution of inclusions is uneven, and concentration occurs at individual locations. As shown in Figures 2c and 2d, the position of inclusions is usually the location where pitting is easy to initiate, and the size of inclusions is large and localized. Conducive to the initiation of pitting .
Fig.3 Microstructure (a), small oxide inclusion (b), big oxide inclusion (c) and concentrated inclusions (d) of L245MB pipeline
The metallographic structure of the straight weld is shown in Figure 3. The microstructure of the weld zone is also composed of ferrite and pearlite. The grain size and inclusion distribution are not significantly different from the substrate, and there is no obvious slag inclusion or For other welding defects, the microstructure of the weld position is basically the same as that of the substrate, indicating that the corrosion resistance is not much different from that of the substrate, and the weld and the peripheral position do not become the preferential corrosion areas.
Fig.3 Microstructure (a) and inclusions (b) for the weld zone of the pipeline
2.3 Corrosion characteristics and corrosion product analysis
The pipe section with more internal corrosion points is cut along the 3 o’clock and 9 o’clock positions. The corrosion morphology of the surface is shown in Figure 4a. It can be seen that there are black corrosion products on the pipe surface, and some of the positions, especially the bottom black products, fall off. The yellow-brown corrosion product is exposed, and the corrosion products in the pitting strip are mostly black, and the corrosion products in the pitting pit are partially peeled off. Corrosion is a symmetrical pitting, mainly concentrated in the strip within 5 cm, and there is obvious corrosion along the waterline. After removing the rust layer with a wire brush, the typical corrosion morphology is shown in Fig. 4b. It can be seen that the corrosion can be divided into two regions, the upper part is a strip-shaped pitting pit and a local corrosion zone, and the pitting is intermittent. The distribution of the lower part is the uniform corrosion thinning zone, and there is local corrosion, and the corrosion is symmetrically distributed, which is a typical waterline corrosion [4, 5, 6].
Fig.4 Macro corrosion morphologies of the pipeline with corrosion products (a) and after corrosion products mechanically removed (b)
The SEM morphology and EDS analysis of corrosion products in different corrosion areas are shown in Fig. 5. It can be seen that the corrosion products are mainly loose granular, honeycomb and needle-like . The black corrosion products on the surface of the pitting area are mainly oxides of Fe, and the composition is close to Fe2O3. The corrosion products in the pits contain sediment impurities such as Si and Ca, and some of the yellow-brown corrosion products exposed at the bottom of the black corrosion products contain a small amount. Cl.
Fig.5 SEM images (a1~c1, a2~c2) and EDS results (a3~c3) for different corrosion areas of the pipeline: (a) pitting surface, (b) pitting zone where surface products peeled off, (c) bottom of the pipeline where surface products peeled off
The SEM morphology and EDS analysis of the corrosion products in the pitting area of the pipeline are shown in Figures 6a-d. The thickness of the corrosion product layer varies from a few hundred micrometers to several tens of micrometers due to the shedding of the corrosion products. The corrosion products in the inner part of the pitting pit are thin, and its composition is shown in Figure 6c. The outer layer of the corrosion product is rich in oxygen and the inner layer is rich in iron. The corrosion product around the pitting pit is covered by a layer of sediment impurities containing Ca and Mg as shown in Fig. 6b. The composition analysis of Fig. 6d shows that the outer layer of the corrosion product is rich in Fe, the inner layer is rich in O, and the innermost layer is rich. Fe.
Fig.6 SEM images (a, b, c, f) and EDS results (d, e, g, h, i) for corrosion cross sections of different corrosion areas: (a) corrosion pit, (b) area around the corrosion pit, (c) partial enlarged view of the circled area in Fig.6a, (d) EDS analysis for α area in Fig.6c, (e) EDS analysis for β area in Fig.6c, (f) partial enlarged view of Fig.6b, (g) EDS analysis for α area in Fig.6f, (h) EDS analysis for β area in Fig.6f, (i) EDS analysis for γ area in Fig.6f
The black corrosion product and the yellow corrosion product of the bottom corrosion surface layer are scraped off with a wooden board, and then the phase analysis is performed. The XRD spectrum of the bottom corrosion layer is shown in Fig. 7a, and it can be seen that the black corrosion product is mainly composed of γ-Fe2O3, Fe3O4 and a small amount of SiO2. Composition. The yellow product is mainly composed of α-FeOOH, γ-FeOOH and a small amount of Fe3O4. The corrosion products are mainly oxides of Fe, similar to the atmospheric corrosion products of carbon steel . The XRD spectrum of the pitting area is shown in Fig. 7b. The corrosion products in the pitting area are mainly γ-Fe2O3, α-FeOOH, γ-FeOOH and a small amount of deposit CaCO3.
Fig.7 XRD spectrum for different corrosion products: (a) black and yellow corrosion products, (b) corrosion products at the pitting zone
2.4 Analysis of corrosion causes
In the construction of long-distance oil and gas pipelines, water is generally used as a pressure test medium for strength test. After pressure test, the test water is properly cleaned. Based on the pipeline, the water needs to be considered. After the pressure test of crude oil and refined oil pipelines, there is usually no Drying requirements, in the case of large fluctuations in pipeline elevation, the test pressure water tends to remain at the relatively low point of the elevation, forming a corrosive environment [11, 12, 13]. The elevation data indicates that the pipeline has a large elevation fluctuation and the corrosion leakage point is at the low point of the relative elevation. Corrosion statistics also show that the corrosion is mainly concentrated at the 4~8 o’clock position at the bottom of the pipeline. Therefore, the corrosion of the pipeline is mainly due to the pressure test water. Residual at the low point of the pipeline, forming a corrosive environment, eventually leading to pipeline corrosion failure.
The macroscopic corrosion morphology of the pipeline in Figure 4 shows obvious waterline corrosion characteristics. The steel material is partially immersed in the corrosive medium, and the corrosion occurring in the upper part with sufficient air is called waterline corrosion. After the pipeline is pressed, the test water in the low-lying position remains in the pipeline, close to the position on the waterline side, and is in an oxygen-rich state. The potential of the steel surface is positive, mainly the oxygen reduction reaction, and close to the lower side of the waterline. In part, since oxygen exists in the form of dissolved oxygen in water, the concentration of O suddenly decreases, and the potential of the steel surface is negative, mainly causing dissolution of the Fe anode. A so-called “concentration battery” appears near the waterline. The very thin oxygen-rich zone during the corrosion process is the cathode, and the lower part of the waterline is exposed to corrosion as an anode . Corrosion distribution characteristics are also related to the depth of residual water, oxygen concentration gradient, deposition of corrosion products, and the type and content of ions in corrosive media . The pipeline working conditions indicate that after the pipeline is pressure tested, the air-filled pressure (1.2 MPa) is sealed for more than 3 years. The high oxygen content in the gas phase increases the oxygen concentration gradient, increases the difference of the oxygen concentration battery, and further accelerates the corrosion [ 7]. During waterline corrosion, cations and anions combine to form iron hydroxide at the junction of the waterline cathode region and the anode region near the waterline by charge transport and concentration diffusion. Steel corrosion is usually protected under neutral pH conditions. Poor, can be used as a cathode to promote corrosion reaction in the electrochemical corrosion of γ-FeOOH, the shape of which is loose particles or honeycomb as shown in Figures 5b and c. γ-FeOOH can be reduced to Fe3O4 during the corrosion process, and Fe3O4 can be further oxidized to stabilize γ-Fe2O3. In addition, Fe3O4 is easily formed in anoxic regions such as pitting pits. The γ-FeOOH oxidizes over time to a more stable protective α-FeOOH [8,9]. Therefore, the content of α-FeOOH and γ-Fe2O3 in the black corrosion products with longer generation time in Fig. 7a is more, and the content of γ-FeOOH and Fe3O4 is more in the newly formed yellow corrosion products. The surface of the pitting area is relatively stable, and the internal corrosion is high due to the enrichment and acidification of chloride ions in the medium . It is easy to generate new corrosion products. Therefore, the product in the pitting position in Figure 7b contains Stable γ-Fe2O3 and α-FeOOH also contain a small amount of newly formed product γ-FeOOH.
Air-pressurized pressure storage causes corrosion products to rapidly form and accumulate on the surface of the pipe. The corrosion product can protect the substrate on the one hand to reduce the corrosion rate. If the corrosion product film is sufficiently good, the corrosion rate can be greatly reduced. It even prevents the further development of corrosion; on the other hand, the rapid accumulation of corrosion products increases the difference between the oxygen concentration of the coverage area and the surrounding area, especially the waterline area, forming a partial occlusion concentration cell. The rapidly generated corrosion products have poor stability, the porosity of the film layer is high, and the protection is poor. Cl- in the medium easily diffuses into the corrosion surface, and Cl- enrichment and local acidification occur to form autocatalyst. Effect, forming severe pitting . The pitting site forms a large amount of corrosion products that accumulate to form a bulge as shown in Figure 5a.
Part of the corrosion products generated by pitting are diffused to the periphery of the pitting. Because there is no protective measure on the inner wall of the long-distance pipeline, there is often an oxygen-rich atmospheric corrosion product on the surface. The surface cleaning and internal detection may cause the surface to cover the sediment. Impurities, so the cross-sectional structure of the pipeline around the pitting is the upper layer is sediment impurity, the middle layer is the iron-rich corrosion product formed by the diffusion deposition of pitting products, and the bottom layer is the original oxygen-enriched atmospheric corrosion product, as shown in Fig. 6d. The formation of pitting will have a certain inhibition and protection effect on the corrosion of the surrounding location , so a pitting strip with different intervals is formed in the vicinity of the waterline.
On the part of the pipeline far from the waterline, on the one hand, the difference in oxygen concentration gradient becomes smaller, and on the other hand, the corrosion rate is lower due to the protection of the pitting strip zone. As the corrosion continues, the corrosion products continue to form and settle down by gravity, even reaching the bottom of the pipeline, affected by the sedimentation of corrosive products and the corrosion of pipeline microstructure (such as inclusions and pearlite position)  The corrosion morphology of this region is characterized by uniform corrosion and local corrosion as shown in FIG.
3 Conclusions and recommendations
The above analysis shows that the corrosion of the pipeline is caused by the waterline corrosion caused by the residual pressure of the pipeline in the low pressure point. The air-filled pressure storage accelerates the development of waterline corrosion, and the Cl- in the medium also increases the pitting of the pipeline.
To prevent the occurrence of similar corrosion, the following measures are recommended:
- (1) For pipelines with large elevation fluctuations, pressure test should be carried out uniformly. After pressure test, deep sweeping should be carried out. If conditions permit, pipeline drying should be carried out.
- (2) The pipelines that have been delayed in production after completion shall be fully dried, sealed with nitrogen or sealed with corrosion inhibitors, and regularly inspected for corrosion of the inner wall.
The authors have declared that no competing interests exist.
Source: China Steel Pipeline 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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