B10 cu-ni alloy is an internationally recognized cu-ni alloy with excellent seawater corrosion resistance. It is widely used in marine engineering at home and abroad. B10 CU-NI alloy not only has excellent physical conductivity of cu alloy, but also has good corrosion resistance and biological fouling resistance in flowing seawater. The corrosion behavior of cu-ni alloy has been studied extensively at home and abroad. The electrochemical corrosion behavior of B10 cu-ni alloy in NaCl solution has been studied. It is considered that the corrosion of B10 cu-ni alloy is controlled by the process of anodic reaction. The results show that a layer of passivation film is formed on the surface of B10 cu-ni alloy in seawater, and the corrosion resistance of B10 cu-ni alloy is improved. Therefore, the factors affecting the passivation film will affect its corrosion resistance. At present, the research on the corrosion resistance of B10 cu-ni alloy mainly focuses on the influence of natural environment variables (such as temperature, Ph and salinity) on the corrosion process of small samples, however, there are few researches on the corrosion resistance of the pipeline under the condition of actual pipe flow. In this paper, the erosion corrosion of B10 cu-ni alloy pipe in flowing seawater was studied by using a self-made simulating erosion corrosion test machine for artificial seawater pipe.

1. Test materials and methods 

1. 1 The domestic B10 copper-nickel alloy pipe was used as the test material, the outer diameter was 12mm, the wall thickness was 1mm, the length was 80mm. The chemical composition of the B10 Cu-ni alloy pipe is shown in Table 1.

Before the test, the sample is placed in anhydrous alcohol and vibrated by ultrasonic wave to remove the impurity and oil on the surface. The samples were cleaned and dried in an oven at 120 °C for 10 minutes. FA2004N analytical balance (accuracy 0.1 mg) was used to weigh the sample before testing. The test medium is artificial sea water. 1. 2 The test method is to install the treated samples on the self-made pipe erosion testing machine and set the flow velocity of the flowing sea water as 1. 5 MS, 2. 0m/s, 2. 5 MS, 3. 0m/s and 3. 5 m/s, the scouring time is 12h, 24h, 48h, 96h and 192h respectively. After washing, the impurities on the surface of the sample are cleaned by anhydrous ethanol. After the sample is cleaned, it is put into the oven at 120 °C and dried for 10 min. The samples were cut into small samples with a surface area (the part in contact with sea water) of 1mm2 on a wire cutting machine. The electrochemical properties of the alloy were tested on the CHI660D electrochemical workstation with three electrode system, Saturatedcalomel electrode (SCE) as reference electrode, graphite as auxiliary electrode and electrolyte solution as analytical reagent, the test temperature is room temperature. The test frequency of AC impedance spectrum is 0. 1hz ~ 100khz, AC excitation signal amplitude 5MV, linear polarization scan rate 5mv/s, open circuit potential (OCP) test time 900s. The microstructure of the Alloy was observed by JSM-5610 scanning electron microscope under different conditions.

In the initial stage of the test, the increase rate of the alloy’s mass loss is high, and in the later stage of the test, the increase rate of the alloy’s mass loss is low. This is because the B10 copper-nickel alloy pipe is completely exposed in the artificial sea water at the initial stage of the erosion and has no protective measures, that’s why it corrodes faster in the early stages. However, with the increase of scouring time, the oxide film, or passivation film, gradually formed on the surface of the alloy, which reduced the mass transfer and charge transfer rates of the CATHODIC and anodic reactions on the alloy surface, the rate of corrosion reaction on the surface of the alloy is reduced. Fig. 2 shows the corrosion rate of B10 CU-NI alloy pipe with different simulated seawater velocity. As can be seen from figure 2: B 10 copper-nickel Alloy pipe in the flow rate of 3. The corrosion rate at 0 m/s is higher than that at other flow rates, and the flow rate is 1. The corrosion rate is the minimum at 5 m/s, so the seawater flow rate is 1. 5m/s and 3. 0 m/s, and compared the results with the experimental data. 

2. 2. electrochemical testing 

2. 2. 1 change of polarization behavior of potentiodynamic

Fig. 3 shows the polarization curve of B10 cu-ni alloy pipe with scouring time in flowing artificial seawater, where the horizontal coordinate is the logarithm of current density and the vertical coordinate is the potential. Figure 3 a shows b 10 cu-ni alloy pipe at a flow rate of 1. 5 m/s, the polarization curves of the potentiodynamic potentials were obtained after scouring for different time. As can be seen from Fig. 3A, the corrosion potential of B10 cu-ni alloy pipe is relatively stable before 96h, and when the erosion time reaches 96h, the corrosion potential of B10 cu-ni alloy pipe increases and a relatively stable corrosion passivation film is formed. Figure 3 B shows B 10 copper-nickel alloy pipe at a flow rate of 3. The polarization curves of the potentiodynamic potential under the scour of 0 m/s seawater are different from figure 3 A in that the polarization curves of the potentiodynamic potential under the scour of 3. 0 m/s seawater scoured for 192 h before the corrosion potential increased, which indicated that the passivation film formed on the surface of B10 cu-ni alloy pipe was late at higher flow velocity.

 2. The change of AC impedance spectrum

Figure 4 shows an artificial ocean current with a velocity of 1. 5m/s and 3. 0 m/s, the electrochemical impedance of B10 cu-ni alloy tube changes with the increase of scouring time, in which the horizontal coordinate is the real part of the impedance and the vertical coordinate is the imaginary part of the impedance. As can be seen from Fig. 4 A, the artificial sea current velocity is 1. 5m/s, the radius of capacitive arc increases with the increase of the scouring time, which shows that the charge transfer resistance of the alloy surface increases with the increase of the scouring time. When the alloy was washed for 96 h, the ARC resistance radius suddenly increased, which indicated that the surface of the alloy had formed a relatively complete passivation film. As can be seen from Fig. 4 B, the artificial sea current velocity is 3. 0 m/s, after 192 H, the ARC resistance radius of the alloy increases sharply, which indicates that the resistance of the alloy surface increases and the corrosion rate begins to decrease. 2. 3 observation of microcosmic corrosion appearance

Figure 5 shows B10 copper-nickel alloy pipe in 1. 5m/s and 3. 0 m/s flow rate, with the increase of scouring time, the micro-corrosion morphology was observed by SEM. Among them, figure 5A and figure 5B are flow Velocity 1. 5m/s and 3. 0m/s, the micro-scanning photographs of the specimen after being scoured in artificial seawater for 12 hours. As can be seen from figures 5A and 5B, after corrosion in flowing seawater for 12 hours, the abrasive stripes on the surface of the alloy after sanding are still clear. Figure 5 C shows the flow rate of the Specimen 1. 5m/s seawater for 96h, as can be seen from Fig. 5C, the surface of the alloy has formed a relatively complete passivation film, which is denser and has low porosity. Figure 5 D shows the flow rate of the sample 3. 0 m/s seawater for 96 h, the corrosion products were formed and adhered on the surface of the alloy, but the corrosion product film was incomplete. Figure 5 e shows the flow rate of the Specimen 1. 5 m/s seawater for 192H, the surface film of the alloy is more complete and compact. Figure 5F shows the flow rate of the Specimen 3. 0 m/s seawater for 192 H, the surface scratch of the alloy has disappeared, and a dense and uniform passivation film has been formed on the surface, which can effectively reduce the corrosion reaction rate. The variation process of the alloy surface shows that when the simulated flow velocity of seawater is 1. 5 m/s, the corrosion rate of cu-ni alloy is higher at the initial stage than at the later stage. When the simulated ocean current velocity is 3. 0 m/s, the corrosion rate of cu-ni alloy reaches the maximum, and the passivation film is formed after 192 hours of erosion, but the passivation film is unstable and easy to be destroyed.

3. Conclusion 

(1) the corrosion rate of B10 CU-NI alloy pipe increases with the increase of the artificial seawater velocity when the scouring time is the same, but when the artificial seawater velocity reaches 3. 0 M/S, the corrosion rate increases rapidly, the surface of the alloy is seriously damaged, and there are corrosion pits. 

(2) the formation time of passivation film is different with different flow velocity. When the simulated ocean current velocity is 1. 5 m/s, the corrosion rate of B10 CU-NI alloy pipe is higher at the initial stage than that at the later stage. When the simulated ocean current velocity is 3. 0 m/s, the corrosion rate of B10 CU-NI alloy pipe reaches the maximum, and the passivation film is formed after 192 hours of erosion, but the passivation film is unstable and easy to be destroyed. 

Source: Web Collation

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