Thermodynamic reasons cause metals obtained from minerals in nature in their normal use to have a tendency to return to the combined state. The phenomenon leading to the progressive deterioration of metallic properties is known as corrosion. 

Corrosion is nearly always of an electrochemical nature, that is, an electrical current that circulates between determined zones of the metal surface, known as anodes and cathodes, through a solution called electrolyte, able to conduct such current. This conjunction consists of micro and macro piles in which the anodic zones suffer the effects of corrosion.

When atoms of the anode dissolve to form ions, the electrons that are freed make the anode negative with respect to the solution. It's electrons pass to the cathode through the metallic mass and neutralize the positive ions. Therefore, corrosion is supported by simultaneous anodic and cathodic processes.
This corrosion of an electrochemical type, characteristic of submerged or buried structures, is highly dangerous, not for the metal loss in itself, but because it deals with localized corrosion, which could be the origins of deep punctures. 

For corrosion to exist we have seen the need of the simultaneous existence of anodes, cathodes and an electrolyte.
These anodes and cathodes are micro and macro piles with a potential differential between its two semi-elements.
The micro piles can have their origin in the metal or in the electrolyte, being in each case caused by several reasons. 

For micro piles to exist in the metal the presence of a heterogeneousness, that can be of several types, is needed. i.e.
• construction: metals or poliphysic alloys
• structure: fine, thick, deformed, etc.
• mechanical: created by external or internal tensions
• due to different superficial states: degree of polish, streaks, coupling, oxides.

The micro piles due to electrolyte or corrosive medium, can be the result of different temperatures, PH, concentration and in particular, from differences in the oxygen content, forming the piles of differential aeration which are a very important source of the corrosion phenomenon.
The non uniform distribution of oxygen is an important factor of corrosion, independently of the metal nature; the most ventilated parts function as cathodes and the less ventilated (streaks, acute inlets, couplings with insufficient radius of curvature, etc.) as anodes, and are, consequently attacked.

The macro piles have their origin, for example, in couplings of different metals, or in differences of soil resistivity or in the effects of stray currents.
We have described the electrochemical mechanism of corrosion that is always produced, except in oxidation at high temperatures.
As such, to fight corrosion we must eliminate or separate from the other two one of the previously mentioned elements: anode, cathode and electrolyte.
If we distinguish between atmospheric corrosion and corrosion of submerged or buried structures, we can also distinguish the protection method.

In the case of metals situated in an aggressive atmosphere, the electrolyte is the humid zone near the metal which we can not modify, except in certain cases, through inhibitors in phase of vapour. Neither can we employ the classic cathodic protection.
Thus only remaining the design and selection of metals and the use of protective coatings that separate the electrolyte from the anodes and cathodes and that in some cases, should also have an active character, such as galvanization.
When dealing with buried structures, in addition to the stated above, we can also apply cathodic protection and rarely modify the electrolyte.
Finally, submerged structures can be acted upon as in buried structures, and in many cases, modify the electrolyte. For example, chemical treatment of the water in a boiler.

We can subdivide the methods to use as follows:

• Design methods
In designing any metal structure, the materials to be used must be selected, taking precautions, in the cases of working with materials with different electrochemical potentials, to insulating them electrically.
he design must also avoid situations that propitiate corrosion piles.

• Protective coatings
Material which used in avoiding contact between the electrolyte and the metal are considered as protective coating.
This coating can be metallic by immersion, projection, electro-deposition or chemical-deposition, seeking to cover the base metal with an unassailable metal in the medium in question, or that forms with it products of passive corrosion.
Another method consists of changing the composition of the metallic surface with an anodized, phosphated, chrome plated, or bronzed application of an oxide stabilizer, etc.
In addition, inorganic coatings can be used, such as vitrified, zinc silicates, concrete or organic coatings applied hot or cold, these being the most used in buried structures.
The organic coverings should fit the following conditions to be an ideal solution:
a. Perfect adherence to the metallic surface as much in the application as in the whole life of the installation.
b. Be compact and not porous.
c. High dielectric power.
d. Not absorb humidity.
e. Hardness to avoid breaks (which cause the metallic structure to be exposed)
f. Elasticity to absorb the expansion of the metal base without fissuring.
g. Inalterable by chemical agents.
h. Inert against fungus and bacteria.

• Electrochemical methods
We can cite: anodic protection, cathodic protection and the canalization of stray currents, which is no more than a particular case of cathodic protection.
Of these, cathodic protection is the most extensive method, and having a great field of application in buried and submerged metals and in recipients containing liquids.