In this study, cathodic protection and electrochlorination were experimentally tested for their effectiveness in controlling corrosion and biofouling. From these results, we observe that corrosion and biofouling control would be possible by using cathodic protection and electrochlorination.
INTRODUCTION
Study background and objective
Nevertheless, what makes the situation worse is that an effective and efficient technique that would enable control of corrosion and biofouling in seawater pipes is still lacking. The main objective of this study was to achieve simultaneous and full control of corrosion and biofouling in seawater pipes.
Study contents
In this context, it is clear that a thorough investigation of effective ways to control corrosion and biofouling in seawater pipes is urgently needed.
LITERATURE REVIEW
Corrosion
- Background theory
- Corrosion control in seawater
- Review of previous study on corrosion control in seawater pipe
The positive dissolution rate (ia), as shown in Fig 2.7, corresponds to the corrosion rate (icorr) a current density term at the corrosion potential31). In the example shown in Fig. 2.8, the corrosion potential of metal M is –0.405 V. The minus sign indicates that the metal is negative with respect to the reference electrode30). The activation polarization curve for the cathodic reaction of hydrogen ions to form hydrogen gas is shown in Fig 2.12.
At this time, the cathodic protection current is applied from the sacrificial anode, as the arrow described in Figure 2.18.
Biofouling
- Background theory
- Biofouling control
- Review of previous study on biofouling control by electrochlorination
The process of biofouling occurs through both physical reactions and biochemical reactions as illustrated in Fig 2.24. Among methods indicated in Fig 2.25, the most economical and widespread method is to use chlorine. Also, HClO produced by chlorine reacts with water, dissolves in the form of hypochlorite (pKa=7.537 at 25℃) according to the formula below. The reaction generation equation is shown in equation 2.1829).
The distribution of HClO, chlorine gas, and hypochlorite ion for each pH level is shown in Figure 2.27. The measurement method for HClO concentration is shown in Table 2.5, and the results of the measurement method investigation are shown in Table 2.6. Conversely, no virus was observed on the shale in either the 3% NaCl solution or natural seawater (see Figure 2.29).
After chlorination, it was observed that growth of diatoms of the mono-species and multiple species was inhibited and that in the case of Navicula, Amphora and natural biofilms, the decrease in cell densities was respectively and 40 % (see Fig 2.30). Compared to the respective controls, there was only 1-8 % chlorophyll in the chlorine-treated mono-species biofilms (Navicula and Amphora) while it was 2-6 % in the natural biofilms (see Fig 2.31). In the case of the bacterial sludge, it appeared that only concentration of residual chlorine at 0.1 mg/L enables bacterial sludge to die out.
Technology to control corrosion and biofouling
- Current technology to control corrosion and biofouling
- Hybrid technology for cathodic protection and electrochlorination
However, due to the increased DC voltage required for complete destruction of marine organisms, the rate of anode consumption and thus the cost of operating the facility increases rapidly. In addition, the effect of the TC anode installed to prevent galvanic corrosion is insufficient, and more and more pipes are damaged by galvanic corrosion. In the case of chlorination facilities, high concentrations of HClO can affect the marine environment even if marine organisms cannot die or are decimated, as K.C.
Ltd chlorination facility, which produces HClO and injects it into the seawater inlet (see Fig 2.34), simply does not consider the feed stream. As a result, until now, facilities equipped with a combined control system that can both prevent the corrosion of the inner surface of pipes and inhibit biofouling are still lacking. Connecting an insoluble anode to the positive pole of the DC power unit and a negative pole to the pipe to apply cathodic protection current to the inner surface of the pipe prevents corrosion and creates an environment where marine organisms can hardly grow due to HClO produced by cathodic protection. current.
However, because the new hybrid technology has too low a HClO concentration produced only by cathodic protection current to decimate marine organisms, an additional DC power supply unit and the insoluble anode and cathode must be installed to electrolyze large amounts of seawater in the cooling water piping system. (see Figure 2.25). The system provides sufficient HClO concentration for the entire system by adding HClO generated by the cathodic protection current and electrochlorination system, while preventing the inner surface of the tube from corroding. New technology can not only prevent the corrosion of the inner surface of pipes by measuring the cathodic protection potential and applying the cathodic protection current, but also monitor the HClO concentration in real time to maintain the minimum HClO concentration.
EXPERIMENTAL METHOD
Laboratory tests
- Materials and specimens
- Test apparatus
- Procedures
Samples used for cathodic protection test in the bowl and a schematic diagram of sample is shown in Fig 3.3. A schematic diagram and overall appearance of the in-cup cathodic protection test apparatus is shown in Fig 3.5. A schematic diagram and the overall appearance of the in-tank cathodic protection test apparatus is shown in Fig 3.6.
The MMO anode was installed inside the tank, and the multi-channel power supply was used for both the measurement of the potential and the application of the cathodic protection current. Potential change of samples placed in the solutions was measured, and the surface condition of sample was inspected. In contrast, in the other two samples, cathodic protection current was applied to maintain the cathodic protection potential at –1,100 m V/SSCE.
Furthermore, it is effective to protect the inside of pipe from corrosion if potential is maintained in the range mostly between –1,300 mV/SSCE and –1,200 mV/SSCE12). Furthermore, to verify the effect of the galvanic corrosion current, samples No.4 and 10 were set in the galvanic corrosion condition by connecting the carbon steel sample with the copper alloy sample. In a test sample applying for cathodic protection, the cathodic protection potential was maintained at –1,100 mV/SSCE; in other cases, to compare the results, the samples were allowed to corrode.
Field test
In the protected samples, after the experiment, inorganic layers were found on the surface of the samples, created by the cathodic protection current that electrolyzed the seawater. Furthermore, as the flow rate increased, the reactant movement and boundary diffusion current density increased, resulting in a higher cathodic protection current being applied to the metal surface. At –900 mV/SSCE cathodic protection potential, little difference between flow rates was observed.
As the cathodic protection potential decreased, the current density difference between the flows increased. At –1200 mV/SSCE cathodic protection potential, the maximum current density of 8.37 mA/cm2 was found at a flow rate of 11 m3/h. There was a tendency for the cathodic protection current to increase as the cathodic protection potential decreased.
The results of cathodic protection test in the beaker showed a more active potential in seawater than in fresh water, and more currents were applied. Protected samples in both freshwater and seawater did not produce any corrosive by-products on the surface of the test sample applied to cathodic protection compared to the naturally corroded test sample. Furthermore, in biofouling control aspect, with an increase in flow rate, HClO concentration increased due to the increase of cathodic protection current.
RESULTS & DISCUSSION
Test results of corrosion control
Based on the polarization resistance results, the corrosion rate and corrosion current density in fresh and sea water were calculated (see Figure 4.2). Thus, the corrosion current density in seawater is about 3.6 times higher than in fresh water. In the activation polarization section, the current density in seawater at 25℃ was 2 times higher than that in fresh water.
In comparison with fresh water at 25 ℃ and at 35 ℃, the current density in the latter was 3 times higher than in the former. The magnitude of the current density also increased with an increase in temperature and flow rate. This layer acts as a barrier against the corrosive environment, which leads to a decrease in the power requirement38) (see Fig. 4.12).
10 mm Figure 4.11 Photo comparison between corroded samples and protected samples in different solutions and flow rates for 7 days. 1 mm Figure 4.18 Macro photographs of the corroded and protected sample after testing in fresh water at different flow rates. 1 mm Figure 4.21 Macro photographs of a corroded and protected sample after testing in seawater at different flow rates.
Test results of biofouling control
The higher the total current applied (i.e. the combination of the cathodic protection current with the electrolysis current), the higher the concentration of HClO produced. This means that the concentration of HClO could be adjusted by additional applied current from the electrochlorination plant when the concentration of HClO generated by the cathodic protection current alone is too low to kill marine organisms. The differences between the various currents were due to the fact that a high cathodic protection current was used to maintain the cathodic protection potential with increasing current.
This is because the large cathodic protection current is used to maintain cathodic protection potential in response to increasing current rates. As shown by the results of cathodic protection in the cistern, under both general and galvanic corrosion conditions, more current was applied in seawater than in fresh water, and no aftereffect of galvanic current was observed in the case of galvanic corrosion. As shown by the results of cathodic protection and electrochlorination, in relation to corrosion control, the higher the cathodic protection current used, the lower the cathodic protection potential was maintained.
Therefore, by using cathodic protection on the pipe, we can prevent the inner surface of the pipe from corrosion. Consequently, it can be seen that HClO produced by cathodic protection and electrochlorination current can control the bioremediation by automatically adjusting the concentration of HClO. When performing corrosion and biofouling control, bearing in mind that the condition of cathodic protection varies depending on the diameter, shape and flow rate of the pipe, and that the efficiency of the electrochlorination facility varies depending on the types of anode and the distance between the anode and the tube. (cathode), additional experiments would be needed to establish an optimal cathodic protection and electrochlorination.
CONCLUSIONS
