The Corrosion

Corrosion is the deterioration of a material, usually a metal, by chemical or electrochemical action of the environment, in conjunction or not with mechanical action. Corrosion acts on the surface of a material (e.g. a metal or a concrete structure) or inside it, when it canít be seen and the material failure develops suddenly. Without control it results in the destruction o f structures and equipments, yielding high maintenance and operational costs, as well as safety risks. The corrosion of a metal may contaminate products that become unsuitable for consumption, such as the reduction of soapís shelf life caused by cupric ions contamination due to the corrosion of a copper line. A corroded structure, metallic or not, is always undervaluated and this happen to equipments too, as storage tanks, processing vessels, chemical reactors, heat exchangers, pumps, ducts, columns and towers, boilers, etc.

Corrosion costs so much that it is very hard to evaluate and quantify its cost, but it is estimated today in up to 5% of a country Gross National Product. More than 15% of the steel made each year is lost as corrosion products.

There are two kinds of corrosion costs: the direct, that encompass the deterioration of materials, equipments, structures and industrial plants, ships, and so on, and the indirect one that embraces the consequences yielded by the former, such as loss of efficiency, product contamination, environmental pollution, plant shutdown, profit losses, etc. The greatest cost is unaccountable: the loss of one's life. There is no money that can account for this.

Thermodynamically and electrochemically the corrosion processes are spontaneous and, if not controlled, the material will be inevitably destroyed because the chemical or the electrochemical reactions that do occur on the materials surface or inside it, around grain boundaries, these reactions attain to well established principles.

There is no material that is 100% corrosion-proof.

Stainless steel, although very resistant, is corroded by chloride in aqueous solution (like sea water) and is subjected to intergranular corrosion under suitable conditions.
Aluminum is resistant to sulphuric and nitric acids due to the aluminum oxide layer that covers its surface, but canít be exposed to sodium hydroxide; mercury salts rapidly corrode aluminum. Copper and its alloys are attacked by the combined action of ammonia, water and oxygen.
Hot dip galvanized steel suffers severe corrosion under the action of humid atmospheres polluted by chlorides, SO2, SO3 or other acid gases, needing specific protective coatings.

When a material is going to be protected against corrosion, first of all it is necessary to do a technical survey aiming to know:

• the environment chemical composition and physical data, temperature, humidity, wind speed and direction, dust, solar radiation, etc.

The nature of the substrate to be protected end the technical conditions to do the work;
The life expectancy of the protective systems that may be used;
The viability and selection of surface preparation;
The viability and selection of the paint both the material and its application;
The aesthetic of the finished job.

A painting specification shall describe in detail the surface preparation, the painting and the paints and thinners to be used. It must state:

The suitable painting techniques to do the painting job;
The quality control procedures and instruments;
Surface preparation - materials to be used, its specifications and approved manufactures;
Number of coats and thickness of each coat and of the coating system;
Pot life and shelf life of all paints;
Time interval between coats and for the complete cure of the painting system.