Calculation and application of temporary cathodic protection for seawater storage tanks

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The corrosion of storage tanks in seawater is mainly electrochemical corrosion, that is, a microcurrent is generated during the corrosion process. According to electrochemical principles, the cathode of a corrosion cell does not corrode, while only the anode corrodes. Therefore, by making the metal to be protected act as the cathode, metal corrosion can be prevented; this method is called cathodic protection. Influenced by international oil prices, state-owned enterprises, private enterprises, and especially local port authorities are increasingly investing in projects to build crude oil storage facilities. During the construction of large storage tanks, hydrostatic testing is an essential procedure. Due to geographical and economic factors, most oil storage projects use seawater for these tests. To reduce the corrosion of steel plates caused by seawater during testing, temporary cathodic protection must be installed in relevant areas of the tanks. To determine the type and quantity of cathodic protection, standards such as GB50393-2008 were used. Taking a 100,000-cubic-meter storage tank as an example, the required amount of cathodic protection was calculated and applied in actual projects, with the results verifying the accuracy of these calculations. When performing hydrostatic testing with seawater on large storage tanks, the sacrificial anode method is generally used.

1. Basic parameters of the storage tank
1.1 Parameters for temporary cathodic protection on the inner surface of the tank
1.3 Tank diameter: 80 meters;
1.4 Maximum water level inside the tank during seawater filling for testing: 20.2 meters;
1.5 Geographical conditions: The sea water is clean, not at an estuary, and has a relatively high salt content, resulting in strong corrosiveness toward steel.
1.6 Duration of temporary cathodic protection: Up to 65 days.

2. Design technical specifications
2.1 Potential of the medium during cathodic protection period: -0.85V to -1.10V (relative to the Cu/CuSO4 reference electrode);
2.2 Protection area: Lower surface of the floating compartment, inner surface of the tank bottom plate, and inner surface of the tank walls;
2.3 Duration of cathodic protection: 65 days.
2.4 Protection effect: The tank body shows no rusting in seawater, and any existing yellow rust on the steel plates is removed. This can be visually checked during drainage. However, if the tank walls are not promptly flushed with fresh water after drainage, oxidation rust will still form on them in contact with the atmosphere, and this rust can be wiped away using a cloth. A small amount of calcium-magnesium compounds form on the lower surface of the floating compartment and the inner surface of the tank bottom plate.

3. Design calculations
3.1 Calculation of the protection area
3.2 Protection current
3.2.1 Selection of current density: The testing period for this crude oil storage tank is around 65 days. Unlike marine structures that operate in seawater for decades, it is acceptable to use a lower initial current density, allowing cathodic polarization to gradually form a calcium-magnesium protective layer, similar to a layer of paint. This polarization process can take anywhere from one month to half a year to achieve the desired protection potential, with little impact on the protection effect. However, since the ballasting test for storage tanks lasts only a few dozen days, the polarization process must be fast, so that the area affected by seawater is immediately protected. Therefore, the current density for protection is set as follows:
3.2.2 Calculation of protection current

4. Selection of anodes
The selected anodes must meet the following criteria:
(1) The overall polarization current required for the entire tank is very high, so the current generated by each individual anode must be maximized.
(2) The duration of temporary cathodic protection is short, usually 1–3 months, or at most half a year. Thus, the anodes chosen should be efficient to minimize unnecessary waste.
(3) The chemical composition of the anodes must be suitable for the cathodic protection polarization rate in this seawater environment.
(4) Throughout the temporary protection period, the materials used for cathodic protection must meet the actual needs of the tank construction, ensuring that excessive calcium-magnesium compounds do not form and cover the welds inside the tank, making them difficult to clean. Therefore, the selected anodes and their chemical compositions are as follows:
(1) Inner surface of the tank bottom plate and lower surface of the floating compartment: Model HCAJ-D/10
(2) Tank wall: Model HCAJ-D/22
(3) Chemical composition of the anodes: HC – special anodes for temporary cathodic protection of steel storage tanks.

5. Calculation of the contact resistance of the selected anodes and the current they generate
0.85 – anode utilization factor. Based on these calculations:
(1) Each anode on the lower surface of the floating compartment generates a current of 10.20A, with a service life of 77 days.
(2) Each anode on the inner surface of the tank bottom plate generates a current of 10.20A, with a service life of 77 days.
(3) Each anode on the inner surface of the tank walls generates a current of 20.27A, with a service life of 85 days. Based on these calculations, each anode can meet the requirements of 65 days of temporary cathodic protection.

6. Calculation of the number of anodes
IP12: Current generated by each block-shaped aluminum-alloy sacrificial anode installed on the inner surface of the tank bottom plate.

7. Actual application results
In the Tianjin ** Petroleum Storage Base project, seawater was used for hydrostatic testing of the storage tanks. Cathodic protection was carried out using aluminum-magnesium-indium-cadmium alloy anodes, based on the calculations mentioned above. After the water-filling tests, it was found that these anodes provided effective protection, significantly reducing corrosion of the steel plates caused by seawater.