The significance of corrosion protection

Corrosion is one of the major problems facing today’s metallurgy. Aside from the impact on materials, corrosion is also a major economic problem. Through the development of science and technology, new methods of protection have been found, which are further refined every year. However, among all the methods of protection, one is considered to be the best, and that is electrochemical protection.

There are two basic methods of electrochemical protection:

• Cathodic protection
• Anodic protection

However, a few more words about electrochemical protection before the detailed explanation of these methods. The metal that is corrosion-protected through an electrochemical process, is held in a passive state. The metals that are being corrosion-protected in such a way are most often used in burial and immersion constructions.

Cathodic electrochemical protection

Cathodic protection is one of the most widely used methods of electrochemical protection. There are two ways of developing this kind of protection today:

• Protection through an external current source – the metal that is to be protected is connected to the negative pole of the DC current source
• Protection with the sacrificial anode – the metal is bonded to the material that is negatively charged, after which the anodic dissolution, ie cathodic protection happens

Some of the main advantages of this type of electrochemical metal protection are:

• Simplicity
• No need for frequent controls
• No power source dependency
• It has almost no effect on structures with a structure that is protected in such a way

However, besides the positive sides, there are several negative ones as well. They are primarily related to environmental pollution, ie application in a microenvironment that has characteristic properties (eg resistance, …).

Anodic metal protection

The principle of anodic metal protection is very similar to the principle of cathodic protection. There are two basic ways of protection:

• DC Protection – The DC current has a passivating effect on the metal
• Protector protection – protection is achieved in a way that the metal which is to be protected is connected to the protector that is generally positively charged

Anodic electrochemical protection has a much smaller application in practice compared to cathodic protection. For example, anodic protection is most often used for the passivation of steel, inox, aluminum, chromium, and titanium or their alloys. One of the most common applications of anodic corrosion protection is the protection of steel in an aggressive environment in which acids are prevalent.

Electrochemical metal protection in practice

Electrochemical metal protection is most commonly encountered, as mentioned above, in constructions that are immersed in water. It is worth mentioning the practice of cathodic protection of some parts of the ship that are constantly immersed in water. Thus, for example on the parts of the rudder or on the hull of the ship, the protectors that together with the DC current protect the hull from possible corrosion, are placed.

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Corrosion and corrosion protection of the metals

Corrosion is an unwanted process we encounter on a daily basis, which is due to the metal reaction and the environment the metal is in.

It is dangerous for several reasons, and one of the major is; damaging the metal to such extent that it changes some of its mechanical properties.

One of the curiosities related to corrosion and the importance of anti-corrosion protection; is the fact that the corrosion causes a loss of 3.5% of world GDP annually.

However, not all metals are subject to corrosion. Metals such as zinc, nickel, tin, aluminum, gold, platinum and alloy steel; where the percentage of chromium is greater than 13% are corrosion resistant.

Moreover, some of the precious metals such as gold and platinum are corrosion-resistant as well.

Corrosion types

If corrosion would be divided into types, then there would be:

• General corrosion which affects the entire surface of the metal. Such corrosion damages the entire surface of the material equally. The general corrosion is also known as “rust”.

• Pitting corrosion is such a corrosion which occurs in small wells on the surface of the material, which grow into larger holes over time.

• Intercrystalline corrosion penetrates the material and is, therefore, almost invisible on the surface.

• Selective corrosion which attacks only individual components of the complex material. This type of corrosion begins on the surface of the material and then penetrates its interior and attacks at different speeds.

• Stress corrosion cracking is such a corrosion which occurs when the material is subject to the ultimate tensile strength; as well as the action of an aggressive agent

Corrosion protection

Numerous methods have been developed for corrosion protection, which could be divided into:

• Corrosion protection by coating– For this type of protection, the first thing that is needed is to remove all impurities such as fat and corrosion. Surface preparation is usually carried out by grinding, brushing, polishing, blasting or blotting. As for the coatings applied to the material, they can be metal, enameled, ie the most known and most widespread, paints and varnishes

• Electrochemical corrosion protection – Electrochemical corrosion protection is carried out for metals that are not easily accessible and can’t be easily maintained. Electrochemical corrosion protection is applied to the protection of oil pipelines, ships, reservoirs and other appliances used in the industry

• Structural measures – corrosion protection related to the selection of those materials which have certain properties that reduce the likelihood of corrosion

• Protection by changing circumstances – This type of corrosion protection is carried out by changing the surrounding factors participating in the damage process. In this type of protection, mention should be made of corrosion inhibitors which, in small doses, prevent the corrosion process

Galvanization as a form of corrosion protection

One of the most known methods of protecting metal from corrosion is galvanization. Galvanizing is nothing more than applying a thin layer of zinc to a material that is to be protected from corrosive action.

Galvanization is the most common method of metal protection. The zinc coating process is carried out by immersing metals in molten zinc, by electrochemical process, zinc spraying and zinc diffusion.

However, the most commonly used galvanization method is the hot-dip galvanizing process whereby the metal is plunged into a zinc of 450 ° C temperature. This is also one of the oldest metal protection methods that is very cost effective and simple.

After galvanization, the metal is protected for almost its entire life span.

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