Protection

The first organic type protection type consists in a surface coating (enamel paint, which is based on resins and plastics). However, this antirust method ensures only passive resistance with the help of isolation from the environment.

Thus, there is a risk of destroying (corrosion) a protected product because of different factors: coating layer permeability, its abruption breaks because of impacts, abrasion and scratches, etc. These defects can be usually detected during the movement, storage, transportation and installation. The risk increases even more in the case of objects with the big size and weight. It’s enough a small initial passage in order to start the process of oxidation and destruction. Thus there is revelation of subsurface corrosion, blisters and swelling, which open the way to the process of destruction. Gradually, the corrosion products grow in size. Then there begin an irreversible oxidation process with the protective layer separation and the subsequent opening of new sections for the product process of destruction.

A typical phenomenon is the "explosion of rust," which is revealed in concrete. Rust attacks steel bar reinforcement and greatly increase the thickness. It causes damage and continued firing of cement.

Inorganic protection
The second method of protection is inorganic nature. It based on metals, showing all the properties for perfect protection. If we exclude the most expensive or difficult inflicted metals, cadmium, aluminum, magnesium are suitable for steel protection. And above all zinc is more suitable because of cost, practicality and reliability of the process of applying the results.

Fe + Zn reaction

In the case of hot-dip galvanizing protection application relates to the metallurgical type processes. Indeed, the reaction is caused between the zinc and iron, which forms a continuous metal fusion between the protective layer and the metal base. Really it is an alloy of Fe + Zn: zinc diffusion in iron and vice versa.

Ideal protection. Main characteristics

During the process of protecting the inorganic type (except for the treatments made with inorganic varnishes) base and covering get in one body (object). Thus it realizes the following ideal protection conditions: - Adhesion to the substrate (base); - Tightness; - Resistance to abrasion; - Malleability and flexibility of the protective layer.

Electrochemical corrosion and cathodic protection
The protection guaranteed by the "hot-dip galvanizing" is the most effective, because it connects the effect of "barrier" to the effect of cathodic protection. In practice, there is an electrolytic process, which is the basis of corrosion, but with a positive result.

The corrosion process is in the flow of electrons between two metals (or between two parts of one metal), from the metal with low electric potential to the metal with high electric potential. And this process is accompanied by destruction of the first by the second.

It happens when two different metals (or layers), partially or completely surrounded by an electrolyte, connected to each other. This condition gives a galvanic cell where ionic potentials of the metals determine which of them will be the anode and the cathode.

Therefore, if the surface of the protected iron or steel will be covered with a low electric potential metal (which functions as an anode) with respect to the protected material (which becomes the cathode), then during the electrolytic process the electron anode-cathode flow shows action in favor of the basics, harming the cover. Hence there is the concept and the term «consumable anodes".

This is what happens during a hot dip galvanizing process.

It became possible to create a table of metals and alloys arranged according to increasing values of their normal ionic potentials regarding to the hydrogen electrode.

Electrochemical series, built on NORMAL ionic potentials, is not absolute reference point, because many factors influence on the elements’ behavior concerning to corrosion. In fact, series, refers not to the elements, but to the electrodes (electrode processes) which are found in some circumstances.

If the processes or the conditions in which they occur change, the order of the metals may be different from order that shown in the table.

Theoretically, each of the metals more electronegative than steel, could be used for this purpose, but for different reasons, cathode protection is carried out mainly by the zinc.

Some of these metals are not used because of price, difficulties of application or other reasons. In practice, galvanic cell is created when the coating of zinc on steel (galvanized steel) damaged because of the presence of moisture in the air (which is like an electrolyte). There the zinc is a pole which is sacrificed (or dissolve) and corrode. The steel is the cathode there and remains protected.

Often on the initial phase there is visible rust at the point steel damage. After a short period, there are whitish-gray color formed special zones which gradually spread beyond the point of damage. A circus cover corroded and insoluble zinc compounds move into the galvanic cell to the cathode surface where they settle and protect the steel.

Self-covering
This phenomenon has the name "self-covering" but it is an inexact definition because the zinc coating is not restored.

For example, with the influence of water some zinc corrosion products often are not fall out on the point of damage. It happens because they are washed, but if the damage is not too large, the protective effect persists. The steel surface in the damaged point is protected from current arising in the galvanic cell, where the zinc coating corrodes. Thanks to the cathode protection, a rust don’t leak on the zinc coating (as it possible with a varnish-and-paint coating).

The special conditions of cathode protection can be achieved also by using different systems. For example, it’s possible by means of electrical connection between the structure and zinc elements (cathode). Imagine a ship's hull: the structure is immersed in salt water is a great conductor: for cathode protection will be enough to connect certain size zinc plates with the body. The whole body becomes the cathode and don’t corrode and the zinc which became the anode will corrode.

The same can be said about the structures in the ground.

Another alternative is the cathode polarization of steel structure with an electricity source compound which can provide cathode potential.

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