Corrosion Behavior of Nickel-based Alloy 718 in H2S/CO2 Environment

With the development of oil and gas exploration, more and more acidic oil and gas fields have emerged. Such oil and gas fields generally contain CO2, H2S and Cl-and other corrosive media, which are easy to cause severe corrosion of oil casing and seriously threaten the safe production of oil and gas fields. The effective anticorrosive measures commonly used in the world are the use of anticorrosive materials [1]. Among them, nickel-based anticorrosive alloys have become the focus of material selection because of their excellent comprehensive anticorrosive properties. Foreign research and application practice also show that the use of nickel-based anticorrosive alloy pipe is a safe and relatively economical way to solve the problem of corrosion in high H2S/CO2 gas fields. It can meet the anticorrosion and production requirements of gas fields [2].
As an important material for downhole tools and equipment for high acidity oil and gas, the research on corrosion resistance of nickel-based alloy 718 has always been the focus of attention in the development and performance improvement of the alloy. In recent years, the research on 718 Alloy at home and abroad has focused on the relationship between structure and properties under different heat treatment processes [4,5], micro-junction. The effect of structure on pitting corrosion and hydrogen storage capacity [6,7,8], the corrosion rule under the change of environmental factors (temperature, Cl-, stress state, etc.) [3,9,10,11], etc., are seldom reported in the literature on the corrosion electrochemistry and corrosion resistance of 718 Alloy under the condition of high H2S/CO2 content in the actual gas field production water environment. In this paper, the corrosion behavior of nickel-based alloy 718 in simulated environment of oil and gas wells was studied by means of corrosion weightlessness test, potentiostatic polarization and electrochemical impedance. The H2S/CO2 corrosion resistance of 718 Alloy was determined, and the corrosion behavior differences under different corrosion systems were compared, so as to provide a reference for the application of nickel-based alloy 718 in high-acid oil and gas wells. The theoretical basis and data support.

1 Experimental method

The material used in the experiment is 718 nickel-base alloy pipe treated by solid solution and aging, which is supplied by Yaang Pipe Industry Co., Ltd. Its chemical composition (mass fraction,%) is C 0.033, Si 0.14, Mn 0.065, P 0.0024, S 0.0006, Cr 18.96, Mo 3.28, Fe 18.67, Ni remainder.
The dimension of weight loss specimens under high temperature and high pressure corrosion is 50 mm x 10 mm x 3 mm. Three parallel specimens are selected for each group of simulation experiments. The electrochemical test specimens are disc-shaped specimens of 15 mm x 3 mm. The surface of the sample was ground to 1200
The medium of corrosion experiment is to simulate the formation produced water of an oil field. The specific composition (g. L-1) is NaHCO 3 0.26, Na2SO 4 0.636, CaCl 2 23.06, MgCl 2.221, NaCl 173.958, KCl 12.646. The test conditions are to simulate the corrosive conditions of a certain oilfield under harsh production conditions. The four corrosive environments are: H2S partial pressure 0.5 MPa, CO2 partial pressure 2.0 MPa, temperature 180 C; H2S partial pressure 0.5 MPa, CO2 partial pressure 2.0 MPa, temperature 200 C; H2S partial pressure 0.5 MPa, CO2 partial pressure 2.0 MPa, temperature 220 C; H2S partial pressure 2.0 MPa, CO2 partial pressure 2.0 MPa, and CO2 partial pressure 2.0 MPa, respectively. The experimental period was 720 h at 220 C. TFCZ5-35/250 magnetically driven reactor was used to simulate high temperature and high acid corrosion environment. Corrosion hangers are insulated from each other on a special test stand and placed in the corrosive medium of the kettle. Before the experiment, high-purity N2 2 h was injected to remove oxygen, then H2S and CO2 were injected to raise temperature and pressure to the set value. At the end of the experiment, the samples were taken out, the corrosive medium was washed away with clean water, and dehydrated with anhydrous ethanol. Two samples were taken out and put into the cleaning solution (10% nitric acid solution, immersed at 60 for 20 min) to remove the corrosion product film. After rinsing with clean water and drying with alcohol, the corrosion rate was calculated by weighing with FR-300MKII electronic balance (accuracy 0.0001 g). Another sample was observed by JSM-5800 scanning electron microscopy (SEM) and analyzed by OXFORD ISIS energy dispersive spectrometer (EDS).

Three-electrode system was used for electrochemical test. The reference electrode was saturated calomel electrode (SCE) and the auxiliary electrode was graphite. The test equipment is Wuhan Coste CS electrochemical workstation. The starting potential measured by polarization curve is – 300-500 mV (relative to self-corrosion potential), and the ending potential is the potential corresponding to the current density of the anode about 1A/cm2, and the scanning speed is 0.3333 mV/s. The electrochemical impedance spectroscopy (EIS) test frequency is 105~5 *10-3 Hz, and the measured signal amplitude is 10 mV sinusoidal wave. Two kinds of gas media were continuously injected into the test solution during the same corrosion weight loss experiment, one was H2S/CO2 mixture, the other was CO2 single filling, and the temperature was constant at 70 C.

2 Results and discussion

2.1 Weightlessness of High Temperature and High Pressure Corrosion
Table 1 shows the calculation results of uniform corrosion rate of nickel-based alloy 718 under different high temperature and high pressure conditions. It can be seen that when CO2 partial pressure is 2.0 MPa and H2S partial pressure is 0.5 MPa, the average corrosion rate of nickel-based alloy 718 at three temperatures does not differ much. When the temperature rises to 220 C, the corrosion rate after 720 H chemical immersion is only 0.0009 mm/a. However, when the partial pressure of H2S reaches 2.0 MPa, the corrosion rate also changes obviously at 220 C and increases to 0.0014 mm/a, which indicates that the structure and composition of passive film formed on the surface of nickel-based alloy 718 may change under this condition, and the protective effect on the matrix may be weakened. Referring to NACE SP 0775-2013 standard [12], the uniform corrosion degree of nickel-based alloy 718 is only slightly corroded in high temperature simulated severe formation water H2S/CO2 environment. At the same time, under different corrosion conditions, 718 samples did not show obvious local corrosion. It can be seen that 718 Alloy has good resistance to H2S/CO2 uniform corrosion and local corrosion under simulated conditions.
20190728154544 67816 - Corrosion Behavior of Nickel-based Alloy 718 in H2S/CO2 Environment

Table 1 Uniform corrosion rates of nickel-based alloy 718 under different conditions

Fig. 1 is the micro-SEM morphology of corrosion hanging specimens of nickel-based alloy 718 under different corrosion conditions. It can be found that when the partial pressure of CO2 is 2.0 MPa and that of H2S is 0.5 MPa, the difference of corrosion product film on the surface of samples caused by temperature change is very small. The corrosion products are formed on the surface of the sample at 180 C, but the product layer is very thin, and the scratches left by sandpaper grinding are still visible. With the increase of temperature, the amount of corrosion products on the surface increases slightly. Scratches are covered by electron microscopy, but the structure of passivation film becomes loose. When CO2 partial pressure is 2.0 MPa and H2S partial pressure is 2.0 MPa, it can be observed that the polishing marks on the surface of the sample have almost been completely covered, and there are granular corrosion products. Energy spectrum analysis shows that these granular substances are Ni, Cr and Fe sulfides.

20190728155003 14799 - Corrosion Behavior of Nickel-based Alloy 718 in H2S/CO2 Environment

Fig.1 Surface morphologies of nickel-based alloy 718 under different conditions: (a) H2S: 0.5 MPa, CO2:2.0 MPa, 180 ℃; (b) H2S: 0.5 MPa, CO2: 2.0 MPa, 200 ℃; (c) H2S: 0.5 MPa, CO2: 2.0 MPa, 220 ℃; (d) H2S: 2.0 MPa, CO2: 2.0 MPa, 220 ℃

2.2 Electrochemical corrosion behavior
Figure 2 shows the polarization curves of nickel-based alloy 718 in saturated CO2 and H2S/CO2 solutions at 70 C. It can be seen from the graph that under the conditions of CO2 and H2S/CO2 corrosion, there is obvious Tafel region in the strong polarization region of the cathode, which indicates that the cathode reaction is completely controlled by activation. Compared with the anodic polarization curves of nickel-based alloy 718 under the conditions of CO2 and H2S/CO2 corrosion, it is found that there are great differences between them. The anodic polarization curves of carbon dioxide corrosion have almost no activation-passivation transition zone, but go directly into the passivation zone. The corrosion potential increases in the shorter passivation platform zone, and the corrosion current density is very small and the display is almost unchanged. 。 But when the anode potential reaches about 0.15 V, the current density increases rapidly. Some studies [13,14] show that the corrosion resistance of nickel-based corrosion-resistant alloys is affected by many factors, such as passivation film on the surface of materials, structure and stability of corrosion product film. The appearance of passivation zone shows that nickel-based alloy 718 has good passivation ability in saturated CO2 solution. When the material surface polarizes to a certain potential, a dense and well-covered corrosion product film will be formed, which separates the metal surface from the solution medium and prevents the matrix from further corrosion. However, there are a lot of activities in the solution. Sex Cl-, whose ion radius is small and easily penetrates the passivation film, will destroy the local passivation film on the metal surface before the potential reaches the passivation potential, and then dissolve the metal surface with a large anode current density, resulting in a relatively short passivation platform. In addition, the absence of the activation-passivation transition zone on the polarization curve. Because the potential of cathode reaction is higher than that of passivation, the polarization curve of cathode and anode intersects in the passivation region [15]. When the anode potential continues to increase to the critical pitting potential, the dissolution-repair dynamic equilibrium of the passive film is destroyed, the passive film is broken down and begins to break down, and the material begins to pit at the potential, and the polarization current increases rapidly. However, under H2S/CO2 corrosion conditions, the anodic polarization curves show multiple activation-passivation transition phenomena, which indicates that the passivation film of nickel-based alloy 718 in the corrosion system is extremely unstable and the protective effect on the matrix is weakened. Table 2 shows the fitting results of corrosion potential, corrosion current density and Tafel slope of nickel-based alloy 718 under different corrosion conditions. It can be seen from the table that with the addition of H2S gas, the corrosion potential shifts from – 161.46 mV (vs. SCE) to – 603.25 mV (vs. SCE), and the corrosion current density increases from 1.8851*10-7 A/cm2 to 3.2341*10-6 A/cm2, an order of magnitude. It is generally believed that the more positive the self-corrosion potential or the smaller the self-corrosion current, the better the corrosion resistance of the material. Therefore, nickel-based alloy 718 is more susceptible to corrosion in the presence of H2S.

20190728155732 92948 - Corrosion Behavior of Nickel-based Alloy 718 in H2S/CO2 Environment

Fig.2 Polarization curves of nickel-based alloy 718 under different corrosion conditions

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Table 2 Electrochemical parameters of nickel-based alloy 718 under different corrosion conditions

The AC impedance spectra of nickel-based alloy 718 in different corrosion systems are shown in Fig. 3. The specific equivalent circuit is shown in Fig. 4, where Rs is solution resistance, Cdl is double layer capacitance between metal matrix/corrosion product film and solution, Rt is charge transfer resistance between metal matrix/corrosion product film, Qf and Rf are film capacitance and film resistance respectively, W represents Warburg impedance of diffusion process, namely diffusion resistance. N is the dispersion index, indicating the extent to which the film capacitance Qf deviates from the ideal capacitance (when n = 1, it is the ideal capacitance). In the electrochemical process, the change of Rt value can reflect the corrosion trend of materials. Charge transfer resistance Rt characterizes the difficulty of charge transfer through the interface between electrodes and electrolyte solution. Usually, the smaller the Rt value, the easier the charge transfer process and the larger the Rt value, the more difficult the passive film is to be penetrated. The more difficult the process is [16].

20190728161018 64982 - Corrosion Behavior of Nickel-based Alloy 718 in H2S/CO2 Environment

Fig.3 EIS plots of nickel-based alloy718 in CO2 (a) and H2S/CO2 (b) corrosion systems

20190728161245 60587 - Corrosion Behavior of Nickel-based Alloy 718 in H2S/CO2 Environment

Fig.4 Equivalent circuits for nickel-based alloy718 in CO2 (a) and H2S/CO2 (b) corrosion systems

Fig. 3A shows the electrochemical impedance spectra of nickel-based alloy 718 in saturated CO2 solution system. It can be found that the impedance spectra at the self-corrosion potential show a single capacitive arc reactance characteristic and a time constant, indicating that the anode process is the control step of the whole reaction process and is the electrochemical charge transfer process. High frequency capacitive arc resistance symbolizes the relaxation process of the double layer between the electrode surface and the liquid film. This may be due to the dense and stable passive film on the material surface in CO2 system, and the less influence of corrosive medium ions on the damage of the film. For EIS atlas under saturated H2S/CO2 environment (Fig. 3b), capacitive arc reactance appears in the high frequency part of Nyquist curve, while linear section appears in the low frequency region, i.e. Warburg impedance related to diffusion process, which shows that the corrosion reaction process is controlled by diffusion. This indicates that the corrosion reaction process is characterized by metal dissolution reaction on the substrate surface and ion penetration. The diffusion reaction of the overcorrosion product film acts together. When the passive film structure formed on the material surface is not compact enough and there are many defects, it can not hinder the transfer of electrons in the liquid phase and lose the effective protection of the matrix. The migration of these defects in the passive film leads to the diffusion of electrochemical impedance spectroscopy, so the control step of the whole electrode process is from electrochemical charge transfer. The mass transfer process from a process to a substance [16].
Fitting results of the corresponding element parameters of nickel-based alloy 718 in different corrosion systems of CO2 and H2S/CO2 under self-corrosion potential are listed in table 3. It can be seen that with the addition of H2S gas in the corrosion system, the resistance Rf decreases obviously and the protectivity of passive film decreases. The polarization resistance R is calculated by subtracting the real part of_0 from the real part of__ infinity. It can also be seen that under the condition of H2S/CO2 corrosion, the value of polarization resistance R of 718 Alloy decreases, the dynamic resistance of electrochemical corrosion decreases [17], and the corrosion rate increases, which is consistent with the measurement results of polarization curve.
20190728161421 43007 - Corrosion Behavior of Nickel-based Alloy 718 in H2S/CO2 Environment

Table 3 Fitted results of EIS measured in different corrosion systems for nickel-based alloy 718

3 Conclusion

(1) The corrosion rate of nickel-based alloy 718 is similar under simulated high temperature and high pressure H2S/CO2 corrosion environment. When the partial pressure of H2S increases to 2.0 MPa, the corrosion rate is only 0.0014 mm/a. According to the standard, its corrosion degree belongs to slight category, so it has excellent uniform corrosion resistance in harsh H2S/CO2 environment.
(2) The anodic polarization curve of nickel-based alloy 718 in saturated CO2 corrosion system has obvious passivation zone, and the corrosion reaction is controlled by the anodic process. With the addition of H2S in the system, the anodic polarization curve shows multiple activation-passivation transition, and the protective effect of passivation film decreases; the polarization resistance of the fitted sample is much larger than that of H2S. / Polarization resistance under CO2 corrosion condition.

Source: China 718 Pipes Manufacturer – Yaang Pipe Industry Co., Limited (www.ugsteelmill.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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Summary
corrosion behavior of nickel based alloy 718 in h2s co2 environment - Corrosion Behavior of Nickel-based Alloy 718 in H2S/CO2 Environment
Article Name
Corrosion Behavior of Nickel-based Alloy 718 in H2S/CO2 Environment
Description
The corrosion behavior of nickel-based alloy 718 in simulated environment of oil and gas wells was studied by means of corrosion weightlessness test, potentiostatic polarization and electrochemical impedance.
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