The principle of cathodic protection of aluminum alloy sacrificial anode
Pure aluminum cannot be used as a sacrificial anode material because the passive film potential on its surface is relatively positive. By adding an alloy to prevent the formation of this oxide film, the activity of the aluminum anode can be maintained. Aluminum anode alloys contain zinc, cadmium, indium, mercury, tin, and the like. Zinc acts as the initial activation of the aluminum anode, and other alloys maintain the long-term activity of the aluminum anode. Some formulations add manganese, silicon, and titanium to balance activity and self-corrosion. Both iron and copper have an adverse effect on the current capacity and potential of the aluminum anode. Adding a small amount of silicon (0.11%) can eliminate the adverse effects of iron. The aluminum anode needs chloride ions to activate. When the chloride ion content is lower than the chloride ion content in seawater (3.5% seawater, the chloride ion content is 35000mg/L), the capacitance of the aluminum anode will decrease and the potential will become positive. When the ion content is lower than 4200mg/L, the aluminum anode potential reaches -1060mV (CSE), barely acceptable potential, and the capacitance is significantly reduced. The capacitance of aluminum anodes decreases with decreasing current output density.
The performance of the above-mentioned aluminum anode is the performance under the condition of an ambient temperature of 25°C, and the capacitance of the aluminum anode increases with the increase of temperature until it reaches 70°C, and then decreases rapidly. The potential becomes positive gradually with increasing temperature (25~100°C). Since the work of the anode requires chloride ions, aluminum anodes are mainly used for cathodic protection of metal structures in seawater environments or the inner bottom of crude oil storage tanks, and cannot be used in soil and freshwater environments with low chloride ion content. According to DNVRP401, capacitance decreases with temperature, you can refer to the formula: Z=2500－27 (T—— 20) (T is the anode working temperature, ℃). In salt water, the current capacity may be reduced to half. Aluminum anodes are fixed directly to the structure to be protected without the need for fillers.