Comprehensive Protection of Steel Storage Tanks: Cathodic Protection Solutions from the Inside Out

The continuous growth of oil and gas extraction and processing is accompanied by the intensive construction of tank farms designed for the storage of crude oil and petroleum products. Steel tanks are exposed to corrosion: on the external side of the bottom – due to the action of ground electrolyte, and on the internal side – due to the impact of process water across the entire tank floor or bottom water acting on the floor and first shell ring of the tank.

Four Main Methods of Corrosion Protection for Storage Tanks

• Insulating coating on the tank surface to prevent interaction with the external corrosive environment;
• Use of corrosion-resistant metals during construction;
• Reducing the aggressiveness of the corrosive environment by various chemical reagents;
• Application of cathodic protection for steel tanks. 

Cathodic protection is also referred to as active protection. It includes:
Impressed current cathodic protection (ICCP) – in cathodic protection, the “negative” terminal of the power source is connected to the tank bottom, giving it a negative electric potential, turning the metal into a cathode.
Sacrificial anode cathodic protection (SACP) – creation of a galvanic pair by connecting a metal with a more negative potential to the tank bottom. In such a pair, the tank bottom metal becomes the cathode, and the more electronegative metal – the anode – corrodes instead. 

Internal Surface Protection for Storage Tanks

Internal protection is primarily provided by sacrificial anodes made of magnesium, aluminum, or zinc, which are placed on the tank floor and walls at the water-contact level. This method is autonomous and does not require power connection or additional equipment. PSS Corporation manufactures several types of tank anodes.

Round magnesium storage tank anode PSS-PRM-20
The PSS-PRM magnesium anode is cylindrical with a recess at the top to increase surface area and current output during the initial operation phase. A steel sleeve is pressed into the center for contact with the bottom. Anodes are placed in concentric rings with spacing equal to twice the radius of one anode’s effective range. 

Aluminum storage tank anode PSS-PAKR
PSS-PAKR anodes are laid horizontally on the tank bottom as a welded string of rods. Each consists of a steel rod core and an anode alloy shell. Cross-sections can be round or trapezoidal and are laid in rings. Polyethylene rings and strips insulate them from direct contact with the tank floor. 

Aluminum and zinc trapezoidal weld-on anodes
In tanks storing oil and petroleum products, only the bottom and the first ring in the environment of bottom water released from the stored oil are protected. For the protection of such tanks, the use of zinc anodes P-KOC is recommended, as unlike magnesium and aluminum anodes, zinc anodes do not spark when hitting the steel surface. Thus, fire and explosion safety requirements are met.
Trapezoidal weld-on anodes, in addition to vertical steel tanks, are used for the protection of horizontal tanks, the underwater part of ballast tanks, compartments, tanks, sea chests, ice boxes, and other structures and equipment operating in seawater. 

Solutions for Storage Tank Bottom Protection

Both galvanic and impressed current cathodic protection methods are used for external surfaces. A wide range of schemes exists. The method is chosen based on site-specific data. Galvanic protection is effective only in low-resistivity soils (up to 50 Ohm·m). In high-resistivity soils, this method is insufficient. Impressed current cathodic protection is more complex, requiring a CP station with a DC power source, but is less dependent on soil resistivity and has an unlimited power resource.

Impressed Current Cathodic Protection for Vertical Storage Tank Bottoms

The most effective method is active cathodic protection using a long anode groundbed laid under the tank bottom. This is applicable during tank construction or major repair.
For existing tanks, deep groundbeds installed outside the tank park are used. The CP system must ensure protective potential levels across the entire tank bottom surface.

Impressed current anodes produced by PSS Corporation meet the requirements for tank bottom protection, are made of oil- and petrol-resistant materials, have a long service life, and are environmentally safe. When arranging cathodic protection of the bottom, the anode groundbeds can be located in the ground next to the tank (for tank farm cathodic protection), or directly under the bottom. Since the service life of a vertical steel tank is at least 50 years, they are usually designed with a margin. The standard scheme to ensure the specified service life is to install two independent anode groundbed loops, each with a service life of at least 25 years. The second loop is commissioned after the first loop's working life expires. In high-resistivity soils, an anode bed activator is used, which reduces soil resistance. The cable outputs of the anode groundbeds have individual project lengths.

EPM flexible linear anode is laid directly under the tank bottom.
PSS-ICPA polymer anode is used when installing the anode field outside the tank boundaries.

PSS-ALCC-2 anode ladder corrosion control sensor is installed to monitor the residual corrosion rate and protection level on the tank bottom.

PSS-EDB-2P bimetallic reference electrode. It is recommended to use long-life bimetallic reference electrodes made from chemically pure copper and titanium plates. Standard copper sulfate reference electrodes are not suitable, as they require regular refilling with copper sulfate.

Anode Junction Box. The cables of each tank are routed to a terminal box located outside the tank bund.

Transformer rectifier units (TRUs), which serve as power supplies for tanks, ensure remote control and automatic maintenance of the required values of protective potentials and protective current. The power supplies are equipped with remote monitoring systems. The TRUs are located outside the explosive zone, preferably in the power transformer substation.

Sacrificial Anode Cathodic Protection for Storage Tank Bottoms

The main task when designing sacrificial protection of tanks is to determine the required number of anodes and their service life. The number of anodes must ensure the minimum allowable current density depending on the specific electrical resistance of the soil and the transition resistance of the insulation. The main advantages of this method are the simplicity of installation and replacement of anodes, cost-effectiveness, and no need for a power supply. Sacrificial protection of tanks is carried out using single, grouped concentrated, and grouped distributed anodes.

When arranging sacrificial protection of the external surfaces of tanks, PSS-PM-U magnesium prepackaged anodes are used. The prepackaged anode is an anode with a steel core and a cable output; the anode is packaged in a bag filled with anode bed activator. PSS-PM-U anodes are manufactured in different sizes: PM-5U, PM-10U, PM-20U.

In addition to the anodes themselves, test stations are used to record protection potential values. Test stations may include BDRM units.