Introduction:

In 1824, Humphry Davy in London first used cathodic protection to protect copper sheets used for metal plating of ship hulls.

Davy realized that he could preserve copper in seawater by combining small amounts of iron or zinc. The chemical reaction that took place between iron and copper slowed down the rate of copper corrosion, and in this way the copper metal was protected from corrosion. In order to prevent corrosion of ships, this scientist suggested that iron pieces be installed as sacrificial anodes on the copper hulls of ships. So that the surface ratio of iron to copper is 1:100.

As mentioned in the previous discussions, in the corrosion process, the metal loses electrons and becomes a metal ion, and thus corrodes.

In the cathodic protection method, by applying a direct electric current to the metal or by contacting it with a sacrificial anode, the corrosion rate is reduced and in this case, on the metal surface where both cathodic and anodic regions exist, the anodic regions become cathodes and as a result the desired system or equipment becomes completely cathodic. The cathodic protection method is related to the external current, as a result of which the cathode elements of the local cells are polarized to the anode open circuit potential, that is, in this case the entire metal surface is equipotentialized (the anode and cathode potentials are equal) and the corrosion currents are stopped.

Or in other words, cathodic protection means that the desired part is placed in the cathode position of an electrochemical cell and then the metal potential is shifted towards more negative values, in such a way that the applied potential is more negative than the corrosion potential of the metal in an aqueous environment. In this way, the dissolution of the metal is prevented.

One of the methods of preventing corrosion is to apply a coating to industrial parts and equipment. If the coatings (conversion, metallic, organic and inorganic) were permanent and not damaged during installation or operation, metal pipes would never corrode. The occurrence of defects in the protective layers with holes, even if accidental, requires us to consider the second type of protection for metals buried in the soil. The cathodic protection method can be used alone, but this method requires a large amount of current. Therefore, to protect a part from corrosion, it is better to first coat the part properly and then strengthen it with cathodic protection. A proper protection of buried structures is a combination of coating and cathodic protection as follows:

80% coatings + 20% cathodic protection = proper protection

Cathodic protection prevents both uniform corrosion and localized corrosion (such as pitting corrosion, intergranular corrosion and selective delamination corrosion). Cathodic protection can be used to reduce stress corrosion cracking (SCC). Of course, hydrogen intrusion cracking (HIC) cannot be stopped by this method. Fatigue corrosion can also be reduced by cathodic protection.

Corrosion in the upper parts of the tank that are not in contact with water cannot be prevented by applying cathodic protection. This is because the applied current cannot enter areas of the metal that are not in contact with the electrolyte (such as the inner surface of the pipes), in which case auxiliary anodes must be installed inside the pipes or corrosion inhibitors must be used.

The most important application of cathodic protection is the protection of steel structures buried in soil or immersed in water, where water or soil act as the electrolyte. Using this method, the external surfaces of pipelines, ship hulls, docks, platforms in seawater and steel embedded in concrete can be protected from corrosion.

Principles of Cathodic Protection

If we look at the reactions related to metal corrosion such as M in an acidic environment, it is seen that in the anodic reaction (metal dissolution), the dissolution reaction can be stopped by giving electrons to the system.

Anodic reaction          M2++2e→M

Cathodic reaction         H2→ 2H++2e

Corrosion in aqueous solutions occurs through an electrochemical process and the anodic and cathodic electrochemical reactions occur simultaneously. Given that the rates of the anodic and cathodic reactions are equal, no charge is accumulated on the metal lattice.

By taking electrons from the metal part, the rate of the anodic reaction increases and the dissolution of the metal will be greater, while the rate of the cathodic reaction decreases. Of course, the opposite is also true. That is, if we transfer excess electrons from an external source to the metal part, in this case, the rate of the anodic reaction decreases and the rate of the cathodic reaction increases.

To prevent corrosion, the continuous transfer of electrons from an external power source causes the corroding metal to become cathodic and the metal is protected. The above principle can be shown by plotting the metal potential in terms of the logarithm of the anodic and cathodic reaction rates.

The presence of anodic and cathodic regions is necessary for corrosion reactions to occur. If the electrons of the structure are supplied from an external source, the rate of movement of positive ions from the metal surface decreases and the rate of cathodic reaction increases. And if the metal potential is reduced from the corrosion potential of the metal in the natural state to the protective potential of the metal after applying cathodic protection by applying external electrons, as a result, the anodic current and the corrosion process stop and cathodic protection occurs. This source is a direct current voltage source or a metal anode (sacrificial anode).

Note: In hydrogen embrittlement, since the reaction itself is cathodic, corrosion is not reduced but increased if cathodic protection is applied, but in stress corrosion, since corrosion results from metal dissolution, we can apply cathodic protection.

Methods of applying cathodic protection

Cathodic protection can be done in two ways:

1. Applied current method
2. Sacrificial anode method

Applied current method

For example, to protect steel pipes buried in the soil, we connect these pipes to a direct current generator and then the electric current is transferred to the pipe through a neutral auxiliary electrode located under the soil. In this case, the entire main pipe, the cathode and the auxiliary electrode, become the anode.

In other words, sufficient electrons are delivered to the entire surface of the pipe to reduce Fe2+ ions, as a result of which no anodic zone will be formed on the pipe.

In the mentioned cathodic protection systems, materials such as scrap iron, magnetite, granite, persilicon iron, lead and platinum alloys are used as auxiliary electrodes.

The most commonly used anodes in the applied current method are as follows:

• DSA anode
• High-silica cast iron anode (silicon)
• Durikel alloy anode
• High-silica cast iron anode with chromium
• Platinum anode
• High-silica cast iron anode with molybdenum
• Graphite anode
• Tantalum oxides

Note 1: DSA anodes are very expensive due to their long life and high efficiency.

Note 2: High-silica anodes are not used today due to environmental pollution.

In the applied current cathodic protection system, all communication systems have electrical insulation and even their connection to the auxiliary electrode or the protected pipe is completely insulated.

Usually, the anode is covered with a backing, which includes coke, gypsum or bentonite powder, to make it easier to establish an electric current between the ground and the anode. The electric current from the ground (electrolyte) enters the pipe and reduces its corrosion.

Sacrificial Anode Method

To better understand how sacrificial anodes work for cathodic protection, we first recall the galvanic series of metals. The galvanic series for a limited number of selected metals in seawater are shown in the table below.

By using metals whose voltage is more negative than the voltage of the metal to be protected, it is possible to protect the structures in seawater without using a direct current generator.

Creating protective currents by forming a galvanic cell and using a metal that is more anodic than the equipment exposed to corrosion causes the said metal to become the anode and the equipment to become the cathode. In this method, the anode metal is consumed and for this reason this anode is called a sacrificial anode. Because by sacrificing itself, it protects the other metal (cathode).

Care must be taken in selecting the consumable anode, because this anode must be more active than the metal to be protected so that when it corrodes, it provides the current necessary to protect the other metal. Usually, three metals, zinc, aluminum and magnesium, are used as sacrificial anodes. Depending on the electrical resistance of the environment in which the protection is to be performed, each of these consumable anodes is used with different sizes and weights. Sometimes it is necessary to use multiple consumable anodes to protect a buried pipe.

Note: In applied current cathodic protection, the anode is the positive pole, but in sacrificial anode cathodic protection, the anode is the negative pole.