Corrosion Behavior of Materials Used for Surface Gathering and Transportation Pipeline in an Oilfield
In recent years, with the expansion of oil and gas field exploitation, the development of natural gas industry and the construction of gathering and transportation pipelines, oil pipelines are bound to be affected by internal environment and media in the process of transporting crude oil, resulting in more and more serious corrosion.
Corrosion of gathering and transportation pipelines will not only cause pipeline damage, oil and gas leakage, cause safety accidents, cause environmental pollution, endanger people’s health, damage property safety, but also bring great hindrance to the development of oil and gas fields, at the same time lead to adverse social impact.
Carbonamide flooding is to inject urea solution into the formation. The urea solution is hydrolyzed to form non-condensate gases CO2 and NH3 at a certain temperature. CO2 can be fully dissolved in crude oil, generally increasing the volume by 10%~100%. The result not only increases the elastic energy of the formation, but also greatly reduces the resistance in the process of crude oil flow, thus enhancing the oil recovery. However, carbon dioxide dissolves in water and forms carbonate, which has a corrosive effect on metal materials, especially on iron and steel materials. On the premise of the same pH value, the carbonate formed by the aqueous solution containing CO2 gas is more corrosive than that of hydrochloric acid , which reduces the service life of the gathering pipeline and causes huge economic losses. Therefore, it is of great significance to study the corrosion types and causes of gathering and transportation pipelines in the process of carbon-amide composite flooding and to put forward economical and feasible anti-corrosion measures.
The action mechanism of carbon-amide composite flooding is mainly due to the role of CO2, so CO2 corrosion is the main factor in the process of carbon-amide composite flooding. The corrosion mechanism of steel in CO2 environment is relatively complex, and its corrosion is mainly affected by temperature, pressure, flow rate, medium and material. The main product of carbon dioxide corrosion of steel, FeCO3, is deposited on the metal surface to form a corrosion product film . The protective property of the corrosion product film has a very important influence on the corrosion rate. When the corrosion product film is formed on the metal surface, the corrosion rate often decreases by 1-2 orders of magnitude, which has been generally accepted by people [4-6].
Ikeda et al.  studied the effect of Cr content and temperature on CO2 corrosion behavior. The results show that the materials with different Cr contents exhibit different temperatures of maximum corrosion rate. With the increase of Cr content, the temperature corresponding to the maximum corrosion rate moves towards high temperature. However, the corrosion rate at the peak decreases. Masamura et al.  put forward a new concept, that is, Cr equivalent. It is believed that because C and N form Cr23C6 and C r2N precipitation with Cr2 in the material, the content of effective Cr decreases, which reduces the C O2 corrosion resistance of stainless steel. The partial pressure of CO2 is one of the important factors affecting CO2 corrosion. It will directly affect the pH value of solution and the concentration of dissolved CO2 series compounds in solution, thus affecting the corrosion rate [9-10]. The results of literature [11-13] show that under the same other conditions, the higher the partial pressure of CO2, the higher the corrosion rate of materials. For carbon steel, the higher the partial pressure of CO2, the higher the corrosion rate of the material.
1. Experimental method
At present, the surface gathering pipeline in this oilfield is made of 20G steel, L245 steel and 16Mn steel, which are commonly used in pressure vessel. For CO2 corrosion environment, 5Cr steel and 316L stainless steel have good corrosion resistance. Therefore, the above five kinds of materials are selected for corresponding evaluation and comparative study in order to clarify the compound flooding of carboamide. The corrosion severity and trend of surface gathering pipeline and equipment in different temperature and CO2 partial pressure corrosion environment. Table 1 shows the chemical composition of 20G steel, L245 steel, 16Mn steel, 5Cr steel and 316L stainless steel.
Table 1 Comparison of chemical constituents of five tested steels
Table 2 shows the test conditions for corrosion rate measurement. The dimension of weight-loss corrosion specimen is 50mm x 10mm x 3mm. Before the test, the surface of the sample is polished step by step with 320, 600, 800, 1200 sand paper, washed, degreased with acetone, and measured after cold air drying and weighed. Then, the specimens are insulated on a special test stand, divided into two layers, the upper layer is placed in the gas environment, the lower layer is placed in the liquid environment, and placed in the corrosive medium in the autoclave. TFCZ-35/250 magnetic drive reactor is selected as the experimental device.
Table 2 Test conditions for corrosion rate measurement
The uniform corrosion rate W is calculated by weight loss method. See formula (1): (1)
Formula: m is weight loss before and after corrosion, g; S is sample area, cm2; t is sample time, h; Rho is material density, g/cm3. Corrosion degree was determined according to NACE SP 0775-2013 standard .
Before the experiment, CO2 is introduced and the temperature is increased to meet the design requirements. After the experiment, the surface of the sample was washed with distilled water, the corrosive medium was removed, and the anhydrous alcohol was dehydrated and dried to be used. The method of removing the corrosion products on the surface of carbon steel and low alloy steel is to stir the samples into the cleaning solution vigorously until the corrosion products are removed. After drying by cold air, the samples were weighed with FR-300MKII electronic balance (accuracy 0.1 mg) and the weight loss corrosion rate was calculated.
JSM-5800 scanning electron microscopy (SEM) was used to observe the surface corrosion morphology, OXFORD ISIS energy dispersive spectrometer (EDS) was used to analyze the element content, and X’Pert Pro X-ray diffraction (XRD) from Panalytical Company of the Netherlands was used to analyze the composition of the corrosion products on the surface of the samples.
2 Results and discussion
2.1 Corrosion Rate Analysis
Fig. 1 Average corrosion rate trend of five materials in gas and liquid environment with temperature change under 0.5 MPa CO2 partial pressure （ a） gas phase （ b）liquid phase
Fig. 1 shows the trend of corrosion rate with temperature when the partial pressure of CO2 is 0.5 MPa for five materials in gas and liquid environments respectively. It can be seen from the graph that the corrosion rate in the gas phase is much lower than that in the liquid phase. In the gas phase environment, the uniform corrosion rate of all materials has little change with the increase of temperature, all of which are less than 0.2mm/a. In liquid phase environment, the uniform corrosion rates of 20G steel, L245 steel and 16Mn steel decrease first, then increase and then decrease with the increase of temperature, reaching the maximum at 60℃. Except stainless steel, 5Cr steel of low alloy steel exhibits good corrosion resistance when the temperature is 60℃, but when the temperature is above 80℃, the corrosion resistance decreases significantly. The uniform corrosion rate of 316L stainless steel has little change, which is slight corrosion.
Fig. 2 Average corrosion rate of five materials in gas and liquid environment with partial pressure of CO2 at 80℃（ a） gas phase （ b） liquid phase
Fig. 2 shows the trend of uniform corrosion rate of five kinds of materials in gas and liquid environments with the change of partial pressure of CO2 at 80℃. It can be seen from the graph that the uniform corrosion rate in liquid phase environment is much higher than that in gas phase environment. In the gas phase environment, with the partial pressure of CO2
With the increase of CO2 partial pressure, the average corrosion rate of 20G steel, L245 steel, 16Mn steel and 5Cr steel increases with the increase of CO2 partial pressure in liquid environment, and the variation of 5Cr steel is more obvious, and the corrosion resistance of 316L stainless steel is not good. The material is slightly corroded.
2.2 Corrosion Morphology Analysis
The average corrosion rate of different materials under different temperatures and partial pressures of CO2 can be obtained by high temperature and high pressure corrosion weight loss test. Considering the actual working conditions of the oilfield, it can be concluded that when the temperature is 80℃ and the partial pressure of CO2 is 0.5 MPa, 20G can be obtained.
The average corrosion rates of steel, L245 steel, 5Cr steel, 16Mn steel and 316L stainless steel are the maximum under the experimental conditions, so this condition is the most serious corrosion condition in the experimental range. Fig. 3 shows the micro-corrosion morphology of the surface of the five materials after cleaning at 80℃ and 0.5 MPa CO2 test conditions. It can be seen from the figure that the surface of the five materials is slightly corroded in the gas phase environment. In the liquid phase environment, the surface of 20G steel, L245 steel, 16Mn steel and 5Cr steel is rough, uneven, and the corrosion pit is deep. It can be seen that the corrosion morphology is inhomogeneous overall corrosion, while 316L stainless steel only has slight pitting corrosion.
2.3 Analysis of Corrosion Products
Table 3 and Figure 4 are EDS and XRD analysis results of samples cleaned at 80℃ and 0.5 MPa CO2 liquid phase, respectively. According to the analysis of Fig. 3, Table 3 and Figure 4, the content of C, O and Fe in the corrosion products of carbon steel is very high.
The results of EDS analysis of micro-corrosion products on the surface of samples under different simulated conditions after 168 h experiment show that the main corrosion products are carbonate and Fe3C. Among them, Fe3C is not the corrosion product but the residual material of the metal itself after being corroded. In CO2 corrosion environment, the ferrite on the surface of metal matrix is an anode, while Fe3C is a cathode. As the corrosion reaction leads to the dissolution of the anode, the residual Fe3C is deposited together with the corrosion products on the metal surface . Only carbonate composed of Fe and Mn is the real corrosion product. It is a complex salt formed by the reaction of H2CO3 and metal and solution formed by the dissolution of CO2 in water and the exchange and solid solution of cations such as Fe and Mn.
Fig.3 Surface Microstructure of Five Materials after Testing (a) 20G steel ; (b) L245 steel ; (c) 5Cr steel ; (d) 16Mn steel ; (e) 316L stainless steel
Table 3 EDS analysis of corrosion products
The corrosion products of 20G steel, L245 steel and 16Mn steel are mainly FeCO3, mainly CO2 corrosion products (Fig. 4a, 4B and 4d), and 5Cr steel is also FeCO3 in the liquid phase environment of 80℃ and 0.5MPa CO2, and the corrosion products of 20G steel, L245 steel and 16Mn steel are mainly FeCO3.
A small amount of Cr was enriched in the corrosion product film (Fig. 4c), while there was almost no corrosion product on the surface of 316L stainless steel, and a large amount of Cr was enriched in the corrosion product film (Fig. 4e).
Fig.4 XRD Analytical Atlas of the Surface of Five Materials after Testing (a) 20G steel ; (b) L245 steel ; (c) 5Cr steel ; (d) 16Mn steel ; (e) 316L stainless steel
FeCO3 is formed by the reaction of 20G steel, L245 steel, 5Cr steel, 16Mn steel and CO2. The solubility of FeCO3 has a negative temperature coefficient. The corrosion product film formed in the range below 80℃ is very loose and easy to fall off under the action of fluid. Therefore, the surface of the sample is in the active state, and the corrosion rate increases with the increase of temperature. The density of corrosion product film formed above 80℃ increases with the increase of temperature, and the protective effect on metal matrix is enhanced. The corrosion rate of the corrosion product film begins to decrease under the protective effect of the corrosion product film instead of the activated state on the surface of the sample. When the peak corrosion rate is reached at 80℃, it is difficult for FeCO3 to deposit a compact and complete corrosion product film, and the metal matrix suffers from serious overall corrosion due to lack of surface protection.
316L stainless steel is easy to form passivation reaction in electrolyte solution because of the increase of the ratio of Cr to Fe, forming double-layer dense passivation film. The inner layer is chromium-rich oxide layer, and the outer layer is iron-rich oxide layer, which can slow down its corrosion. However, with the increase of temperature, the protective performance of the oxide film formed on 316L stainless steel surface decreases and corrodes. In addition, the existence of MnS inclusions on 316L stainless steel surface makes the passivation film inhomogeneous. Corrosive ions (Cl – and so on) in solution contact with inclusions preferentially, which leads to the dissolution of metal ions and pitting corrosion.
- (1) The average corrosion rates of 20G steel, L245 steel, 5Cr steel, 16Mn steel and 316L stainless steel in liquid phase are much higher than those in gas phase under the design temperature and partial pressure of CO2.
- (2) The average corrosion rate of 20G steel, L245 steel, 5Cr steel and 16Mn steel increases first and then decreases with the increase of temperature at a constant partial pressure of CO2; when the temperature rises to 80℃, the density of corrosion product film increases obviously, the protection effect on matrix increases, and the corrosion rate decreases; when the temperature is below 80℃, the corrosion rate decreases. The product film is easy to be destroyed by fluid, which leads to high corrosion rate. The average corrosion rate is the highest at 80℃. The corrosion morphology is non-uniform and all-round corrosion. The corrosion products are thick and loose, and are coarse grained FeCO3. Pitting corrosion is unavoidable on 316L stainless steel due to irregular MnS inclusions. A large number of corrosive ions contact with internal ions in the pitting area, resulting in increased corrosion and corrosion rate.
- (3) By means of SEM, EDS and XRD analysis, the main corrosion products of 20G steel, L245 steel, 5Cr steel and 16Mn steel in aqueous solution containing CO2 are FeCO3, and the corrosion product film structure is loose. There are almost no corrosion products on the surface of 316L stainless steel.
Source: China Transportation Pipeline Manufacturer – Yaang Pipe Industry Co., Limited (www.steeljrv.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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