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An offshore wind turbine is set in a highly corrosive marine environment. Its base structure is completely immersed in the seawater. Conventional corrosion prevention methods use both a cathodic protection and a coating method to enhance corrosion protection to protect the structure.
Cathodic corrosion protection is an electrochemical process applying the principles of electrochemical cells transforming a metal material into a cathode. There are two types of cathodic protection: “applied current cathodic protection” and “passive galvanic cathodic protection”. The development and application of “cathode anti-corrosion” technology has been around for more than 100 years. Due to its stability, simple construction, and low maintenance cost, this anti-corrosion method, seen in harbor structures, is widely used in Taiwan Strait and many other countries. Combined with a coating method, it works even more effectively. In recent years, because of a few benefits, such as low density, high electricity price, and greater current generated per unit etc., an aluminum alloy (with trace amounts of zinc, mercury, indium, and other elements) has replaced the commonly used metal, zinc. And it has become the main material used for “sacrificial anodes” in seawater. The “sacrificial anode” process uses a highly active metal as an anode material, welded on to the surface of steel materials, to form a protection potential difference to protect against corrosion by continuously consuming the anode metal, as shown in Figure 2. The sacrificial anode metal will gradually dissolve and release into the marine environment protecting the structure against corrosion. Once the metal particles have been released, they may settle in the surrounding waters and affect the ecology.
Al3+ ions generated by anodic dissolution may coprecipitate with calcareous deposits, and they may result in hydroxysulfate, which will possibly deposit into the final precipitate. The existence of aluminum in the acid-soluble portion of the sediment may have significant environmental influences. Hence, using aluminum sacrificial anodes in a natural environment, especially to protect the facilities near the coast, which will increase the amount of deposited metals. There haven’t been thorough studies or discussions about the impact of “aluminum anodes” on the environment in comparison to “zinc anodes”.
With the anti-corrosion work on marine structures gradually switching to aluminum sacrificial anodes, a large amount of aluminum has been released into the ocean and caused the aluminum concentration levels to rise above normal.
The existence of aluminum in the acid-soluble portion of the sediment may have significant environmental influences. Hence, we do not recommend aluminum sacrificial anodes in a natural environment, especially to protect the facilities near the coast, which will increase the amount of deposited metals.
Aluminum is a common metal mineral. It makes up about 8.1% of the earth’s crust, and it is normally not defined as a pollutant. However, in most parts of the world, aluminum has severely limited the growth and existence of plant species. Since the late 1970’s, the research range on the toxicity of aluminum has been extended to natural habitats, including forests and aquatic areas. The toxicity of aluminum is closely connected to pH levels. Metal is soluble under acidic conditions (pH <5.5) in soil and water. It is also biodegradable. But it is relatively harmless under neutral conditions (pH 5.5–7.5). Reduction or death of invertebrates in forests and water, even the reproduction of fish and amphibians, are directly related to aluminum pollution. It also affects birds and mammals through the food chain indirectly. In the aquatic environment, aluminum acts as a toxic agent on gill-breathing animals, such as fish and invertebrates, by causing loss of plasma and hemolymph ions leading to osmoregulatory failure. In fish, the inorganic monomeric species of aluminum reduce the activities of gill enzymes important in the active uptake of ions. Aluminium may easily be absorbed and interfere with important metabolic processes in mammals and birds.
Chih-Chung Wen, Han-Jen Tsai, Hsiu-Chen Yeh, Global Warming and Coastal Environmental Changes Research Lab., Department of Safety, Health and Environmental Engineering, Hungkuang University, Taichung, Taiwan
Shih-Yen Hsu, Department of Water Resources Engineering and Conservation, Feng Chia University, Taichung, Taiwan
Journal of Environmental Protection, Vol. 11 No. 8, August 2020, 622-635
Download original document: “Environmental Impact Assessment of Sacrificial Anode Method in Taiwan Strait”
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