Introduction to Sacrificial Anode Cathodic Protection for the Inner Wall of Storage Tanks

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Since the corrosion of the inner walls at the bottom of crude oil tanks and sewage tanks is primarily caused by electrochemical corrosion and bacterial corrosion resulting from crude oil deposits and sewage, and the crude oil deposits and sewage at the tank bottom have a high salt content (mainly S, Cl, HCO, Na, Ca, etc.) as well as high temperatures, their corrosive nature is quite strong. Currently, the sacrificial anode method is widely used for cathodic protection of the inner wall of storage tank bottoms. This approach ensures the safety and reliability of the tanks, requires no dedicated maintenance personnel, and provides excellent protection. Materials commonly used as sacrificial anodes include magnesium and magnesium alloys, zinc alloys, aluminum alloys, etc. The anode blocks are evenly arranged on the inner wall of the storage tank, with the steel plates being directly welded to the anode blocks. Features of sacrificial anode protection: ① Rapid and simple installation, with no corrosion interference. ②It has low investment costs and is cost-effective. ③Safe and reliable, requiring no dedicated management. ④The protective effect is significant. Depending on the properties of the inner wall material, an aluminum alloy anode or a magnesium alloy anode can be used as the anode. When sacrificial anode protection is used for the inner wall, attention should be paid to the influence of temperature. In water media at temperatures of 40–70°C, magnesium anodes are not suitable due to their excessively high corrosion rate. Calculate the required number of anodes based on the protection area, protection duration, and dielectric resistivity, and select the specifications and shape of the anodes. The anodes are evenly distributed in a circular pattern on the bottom plate of the tank, and the anode supports are welded to the bottom plate. Sacrificial anodes are easy to install, and can be replaced taking advantage of the tank cleaning opportunity when they have been consumed to 85% of their initial weight. Design steps for sacrificial anodes on the inner wall of storage tanks: ① Calculate the cathodic protection area (the area submerged in water inside the tank). The area to be protected on the inner bottom wall of the tank is calculated as follows: S = πr² – Area to be protected, where r is the radius of the tank. ② Select the protection current density and calculate the required protection current. The protection current is calculated as: I = SIa, where S is the area to be protected and Ia is the protection current density. ③ Determine the protection period and calculate the total amount of anodes needed. The service life of anodes is given by: T = 0.85W/ωI, where W is the net mass of the anodes in kg, and ω is the anode consumption rate in kg/(A·a). ④ Calculate the number of anodes required based on the number of individual anodes. The number of anodes is given by: N = f·IA/Ia, where IA is the required protection current in amps, Ia is the current output per anode, and AF is a safety factor ranging from 2 to 3 times. The sacrificial anode method is a commonly used cathodic protection technique for storage tanks; it can be arranged in any configuration without concerns regarding power connections. Its potential is limited, so there’s no need to worry about over-protection. Sacrificial anodes can also be made in any shape. Depending on the properties of the inner wall material, an aluminum alloy anode or a magnesium alloy anode can be used as the anode. When sacrificial anode protection is used for the inner wall, attention should be paid to the influence of temperature. In water media at temperatures of 40–70°C, magnesium anodes are not suitable due to their excessively high corrosion rate. Calculate the required number of anodes based on the protection area, protection duration, and dielectric resistivity, and select the specifications and shape of the anodes. The anodes are evenly distributed in a circular pattern on the bottom plate of the tank, and the anode supports are welded to the bottom plate. Sacrificial anodes are easy to install, and can be replaced taking advantage of the tank cleaning opportunity when they have been consumed to 85% of their initial weight. When cathodic protection is applied to steel containers, both external current and sacrificial anode methods can typically be used ； The forced current method is suitable for large water tanks, such as elevated water tanks, seawater storage tanks, boiler feed water tanks, and river water tanks in power stations. Auxiliary anodes can be made of ferrosilicon, platinum-plated titanium, graphite, or lead (not suitable for drinking water); they are suspended from an appropriate position on the top of the tank, or the anodes can be fixed by drilling holes in the tank wall. In water with high resistivity, a copper-core continuous platinum-titanium wire anode should be used to achieve a uniform current distribution. When using sacrificial anode protection, the anodes should be fixed directly to the inner wall of the tank, 600 mm below the lowest water level, and distributed as evenly as possible. The following points should be noted when carrying out sacrificial anode installation: 1. Clean the tank and remove rust to meet the standards required for coating application ； 2. Weld the anode block to the inner side of the tank bottom and to the wall plate at a height of 800 mm from the lower inner wall, ensuring a secure weld and removing any residue. 3. For coating application, insulating tank coating is used for the inner bottom plate and wall panels up to 1 meter in height, while conductive coating is used for other areas. During construction, only the body of the anode block needs to be exposed; the welding leads need to be welded. The solder joints, the lower surface of the anode block, and the bottom plate of the tank all require coating.