How to effectively solve the problem of oil storage tank anticorrosion? Comparison of sacrificial anode and impressed current cathodic protection.

How to effectively solve the problem of oil storage tank anticorrosion? Comparison of sacrificial anode and impressed current cathodic protection.

Cathodic protection is installed on existing steel underground storage tanks and pipes to meet corrosion protection requirements. However, just installing a cathodic protection system does not mean that everything is done, cathodic protection systems are not always very effective, and technicians need to continuously monitor the effect of corrosion protection and repair the defects generated during the operation of the cathodic protection system. How to inspect and verify cathodic protection systems so that they can be used properly, we will discuss cathodic protection methods and related monitoring standards in this section.

Buried pipes and tanks often contain corrosion protection equipment, and corrosion of underground tanks and pipes used to be the main cause of underground tank leaks. In most cases, corrosion protection of steel structures includes the application of cathodic protection (CP). Cathodic protection medium coatings on new steel tanks and the installation of cathodic protection on existing steel tanks are cost-effective ways to control corrosion and prevent leaks. The key to an effective cathodic protection system is to ensure proper installation and maintenance. Maintenance includes monitoring and testing based.

Basic knowledge of cathodic protection

Cathodic protection allows the surface of the buried steel on tanks and pipes to collect current, thereby reversing the flow of the natural corrosion current on the unprotected steel. The collection of currents on the tank makes the tank the cathode of an electrochemical corrosion tank (i.e., a cathodic protection system). Corrosion does not stop completely, but simply transfers from the tank to corroding the external anode. When cathodic protection is used, corrosion always occurs at the anode (where the current leaves the metal surface and enters the soil) and does not occur at the cathode (where the metal surface receives the electrolyte current).

Therefore, the entire buried external metal surface can be forced to become a cathode, and it will not corrode, so this technique is called cathodic protection. Cathodic protection is defined as "reducing the rate of corrosion by applying an external electromotive force that shifts the corrosion potential of the electrode to a lower oxidation potential." Current must be continuously supplied to the tank system. If the CP system is interrupted, the tank will begin to corrode again.

To reduce the current required for corrosion protection, cathodic protection is often used with protective coatings. These high performance coatings (paints) keep most metal surfaces out of contact with the soil/water environment. Cathodic protection provides current only to those steel surfaces that have been exposed to the electrolyte due to coating failure or damage, which are often referred to as leak points.

There are two types of cathodic protection systems: sacrificial anodes and impressed current systems. Both types do exactly the same thing - deliver electricity to steel tanks and pipes that come into contact with soil and/or water.

Sacrificial anode advantage

1, no external power supply;

2, the installation is very simple;

3, lower installation cost; 

4, lower maintenance costs; 

5. Few interference problems;

6, effective protection of small;

7, electrical isolation structure.

Sacrificial anode defect

1, drive voltage and current output is low; 

2, not practical in soil with very high or low resistivity; 

3, no effect on large uncoated steel structure; 

4, when protecting a large area of buried steel, the anode life may be short; 

5. Tanks and pipes are required to be electrically isolated from related pipes and other underground metal structures.

Advantages and disadvantages of impressed current system 

The couple/sacrificial anode system generates a protective current through an electrochemical reaction similar to that of an everyday flashlight battery. Choose a metal that is less valuable or more active than the metal to be protected as a sacrificial anode. The anode is electrically connected to the tank/tube to be protected and buried in soil. The more active metal (anode) corrodes (sacrifices), while the metal tank/pipe (cathode) is protected. Protects the current from entering the structure to overcome and prevent the flow of corrosive current.

When protecting the underground storage tank, the anode material can choose magnesium or zinc. The number, size, type and location of the anodes shall be determined by a detailed survey of the facilities to be protected. Before deciding to use a sacrificial anode cathodic protection system, factors such as the electrical isolation/continuity of the tank system, soil resistivity, the response of the buried structure to applied current, and external coating must be considered. Sacrificial anode systems have a low capacity to protect tanks and are typically used in new, well-coated, electrically isolated tanks.

In general, sacrificial anodes are used in the following situations: 

1, low soil resistivity (rarely used when the resistivity is higher than 10,000 ω-cm); 

2. Serious interference with other metal underground structures; 

3, no power supply; 

4. Tanks and pipes are coated and electrically isolated.

In most cases, magnesium or zinc anodes are connected directly to the tank or pipe that needs protection. These special anodes do not require an external power source. The magnesium and zinc anodes are surrounded by a special backfill (gypsum/bentonite). This backpacking is required to ensure that the sacrificial anode is always in a humid, low-resistivity environment, improving anode efficiency and extending anode life.

The impressed current cathodic protection system is designed to protect the outside of the tank. Riser/piping electrically isolated from tank, not included. The tank has a good external coating and factory installed anode. The purpose of providing anodes is to protect the tank surface from coating gaps and exposed areas where corrosion occurs. The surface area is very small and does not require a large number of sacrificial anodes for protection.

When the external structure and the tank (such as pipes) are electrically short circuited, the protection will be exhausted and the anode of the tank will be consumed. Therefore, it is important to maintain effective electrical isolation of the storage tank. In addition, sacrificial anodes have a limited design life. That is, due to anodic corrosion or sacrifice, they will be depleted and no longer provide corrosion protection. Therefore, a cathodic protection survey is generally required every three years.

Impressed current system

Impressed current systems are more flexible in use and design than sacrificial anode systems. Impressed current cathodic protection systems work on the same principle as sacrificial anode protection systems. However, impressed current systems use an external power source to excite the anode at a higher voltage.

The external power supply is called a cathodic protection rectifier. It converts alternating current (AC) into direct current (DC), which is injected from the anode into the electrolyte in the tank (cathode). Protect the current from electrolyte injection into the external metal surface of the tank/pipe to overcome and prevent any corrosion current flowing from the metal surface of the tank/pipe. Cathodic protection rectifiers can be automatically controlled to provide a constant current to the device. In addition, some rectifiers are equipped with alarm lights to indicate proper operation. Other commonly used rectifiers have more basic voltage plugs for current output adjustment and one or two gauges for monitoring.

Many different types and sizes of impressed current anodes are available for impressed current systems. They include:

1. Graphite

2, high silicon cast iron

3. Platinum niobium

4. Titanium or tantalum

5. Mix metal oxides

Each anode has special design, installation and operation characteristics. Most anodes are prepackaged or backfilled on site with highly conductive coke particles, a conductive coal byproduct. Specialized backfills provide uniform low resistance to increase anode life and efficiency. The anodes are typically installed in indicating holes (4-6 inches in diameter) at the depth of the tank bottom and are distributed geometrically around the device to more evenly distribute the protective current to the tank system. These are the advantages and disadvantages of impressed current systems.

Before installing an impressed current system, many facts about the site must be considered. They include, but are not limited to:

1. Physical conditions and geometry;

2. Soil resistivity;

3, the existence of other underground metal structures;

4. Electrical isolation/continuity of facilities;

5, storage tank/pipeline coating efficiency;

6. Current demand;

7. Available power supply.

Impressed current advantage

1. It has great driving potential.

2, high current output, can protect other underground structures, low operating costs.

3, flexible output current.

4, suitable for any soil resistivity.

5, can protect large bare steel structure.

6, there can be alarms to prompt correct operation.

Impressed current defect

1, may cause interference to the external underground metal structure;

2, the current may be cut off, the protection temporarily disappeared;

3, must be regularly monitored and maintained;

4, maintenance and operating costs are relatively high.