Nickel-based alloy (Inconel625 N06625) has excellent performance in corrosive environments in seawater
Inconel 625 Introduction:
625 alloy has excellent corrosion resistance and oxidation resistance, good tensile properties and fatigue properties from low temperature to 980°C, and is resistant to stress corrosion in salt spray atmosphere. Therefore, it can be widely used in the manufacture of aerospace engine parts, aerospace structural components and chemical equipment and in situations where it is exposed to seawater and is subject to high mechanical stress.
chemical composition
C: ≤0.10, Mn: ≤0.50, Si: ≤0.50, P: ≤0.015, S: ≤0.015, Cr: 20.0~23.0, Ni: 54.0~60.0, Mo: 8.0~10.0, Ti: ≤0.40, Al: ≤0.40, Fe: ≤5.0, Nb: 3.15~4.15
Foreword
Nickel-based 625 alloy has excellent fatigue strength and resistance to Cl stress corrosion cracking, making it widely used in offshore oil and gas transmission composite pipes [-. The liner of the metal composite pipe is made of metal materials with excellent corrosion resistance, and the base pipe is made of carbon steel materials with excellent stiffness and strength to improve the overall performance of the composite pipe.
1 Experimental materials and methods
The test material Inconel 625 surfacing layer was taken from the 316L/In-conel 625 composite pipe surfacing layer area. The weight loss sample size is 100 mm × 50 mm × 2 mm, and the electrochemical test sample size is 10 mm × 10 mm × 2 mm. The surface of the sample was polished with 1200° sandpaper, ultrasonic treated in anhydrous ethanol solution for 5 min, and dried in the air until use. The test medium is natural seawater in Qingdao Sea, and the test temperature is controlled at 10°C to simulate the seawater temperature at a depth of 200 m. The corrosion weight loss sampling nodes of the samples are 5d, 15d, 30d, 60d and 90d. After the samples are taken out, they are cleaned, the corrosion products are removed and then weighed to calculate the corrosion rate of the samples. The ACM-Gill6 electrochemical workstation was used to test the sample polarization curve and AC impedance spectrum. The 625 alloy overlay layer was used as the working electrode, the saturated calomel electrode (SCE) was used as the reference electrode, and the Pt sheet was used as the auxiliary electrode. The polarization test scanning potential starts from the open circuit potential and scans toward the cathode and anode at a scanning rate of 1mV/s; the AC impedance spectrum test frequency is 10³-10-²Hz, and is perturbed with an amplitude of 10 mV near the open circuit potential. Zeiss ULTRA55 scanning electron microscope/energy spectroscopy was used to observe the macroscopic and microscopic corrosion morphology of the sample surface. The corrosion products are weighed and the corrosion rate of the sample is calculated. The ACM-Gill6 electrochemical workstation was used to test the sample polarization curve and AC impedance spectrum. The 625 alloy overlay layer was used as the working electrode, the saturated calomel electrode (SCE) was used as the reference electrode, and the Pt sheet was used as the auxiliary electrode. The polarization test scan potential starts from the open circuit potential and scans toward the cathode and anode at a scan rate of 1mV/s; the AC impedance spectrum test frequency is 10⁵-10-²Hz, and is perturbed with an amplitude of 10 mV near the open circuit potential. Zeiss ULTRA55 scanning electron microscope/energy spectroscopy was used to observe the macroscopic and microscopic corrosion morphology of the sample surface.
2 Experimental results and discussion
The average corrosion rate of Inconel625 alloy cladding layer immersed in seawater at 10 ℃ for 90 days. The figure shows that the corrosion rate of the 625 alloy cladding layer immersed in seawater for 5 days is 0.0037 mm/a, and then the corrosion rate rapidly decreases to 0.001 mm/a, and as the immersion time increases, the corrosion rate of the cladding layer further decreases. The corrosion rate of the sample after immersion for 90 days was approximately 0.0004 mm/a.
(a) to (c) are the surface morphology of the Inconel625 alloy cladding layer after immersion in seawater at 10°C for 0d, 60d and 90d respectively, showing that no pitting pits appear on the surface of the 625 alloy cladding layer after being immersed in seawater for different times. , the surfacing layer still shows uniform corrosion in seawater. The pits shown in the figure are mainly caused by mechanical polishing (Figure 3(a)). Figure 3(d) Energy spectrum analysis shows that the surface elements of the cladding layer include the basic elements of 625 alloy Ni, Fe, Cr, Ti, Mo, and C. There is no element 0 in the energy spectrum, indicating that no oxide corrosion product layer is formed.
Electrochemical AC impedance spectra of the 625 alloy cladding layer soaked in seawater at 10°C for different times. The impedance spectrum of the 625 alloy cladding layer in seawater also shows the characteristics of a single-stage electrochemical reaction with a time constant, and no passivation film or corrosion product layer appears as time constant. The cladding layer has always been characterized by a pure metal surface and Seawater contact. The impedance modulus value is basically maintained at the level of 10⁵Q·cm² at each time, and it has strong resistance to seawater corrosion.
The impedance spectrum Bode plot is fitted using the equivalent circuit shown. In Figure 5, R is the solution resistance, and R is the charge transfer resistance, which represents the difficulty of the electrochemical reaction on the sample surface. Q. It is a capacitor equivalent component, representing the charge and discharge effect of the double electric layer at the interface between the sample and the corrosion medium. The fitting results are shown in Table 1. It can be seen that the charge transfer resistance and electric double layer capacitance of 625 alloy are similar after immersion for different periods. After soaking for 5-90 days, the charge transfer resistance changes firstly and then decreases. When soaked for 5 days, the charge transfer resistance is the smallest and reaches the maximum at 30 days. Then as the soaking time increases, the charge transfer resistance gradually decreases.
Cathodic polarization and anodic polarization curves of 625 alloy surfacing layer samples soaked for different times. By performing parameter fitting on the polarization curve, the fitting results are shown in Table 2. It can be seen that the self-corrosion potential E of the sample after the 625 alloy cladding layer was immersed in seawater at 10°C for different times. There is a positive trend, and the self-corrosion current density of the sample is i. It decreases slightly with the extension of immersion time; it can be found from the anodic polarization curve that there is no pitting corrosion breakdown during the anodic polarization process of the 625 alloy cladding layer after immersion for different times.
3 Conclusion
(1) The corrosion rate of the 625 alloy cladding layer in seawater at 10°C was initially 0.0037 mm/a, and then decreased to 0.0004mm/a.
(2) The 625 alloy surfacing layer corrodes uniformly in seawater, and no corrosion product layer or passivation layer is formed on the surface.
(3) The 625 alloy surfacing layer has stable corrosion resistance after being soaked in seawater for 90 days.
