Corrosion control techniques

Different examples will demonstrate the existing techniques to perform a due process of corrosion control.

by Juan Manuel Álvarez*

The structure to which you want to protect from the onslaught of the environment in order to control the formation of corrosion products, must be valued from the point of view of the variables that prevail for the corrosive attack.

In this way, the design or protection system must be selected thinking that both the technical and the economic are preferred for its installation or application.

The corrosion control techniques with which the protection system can be designed or selected are the following:

• Design of materials or components.
• Selection of materials.
• Passivators or inhibitors.
• Coatings or coatings.
• Cathodic protection.

From the very design of a structure that has a specific task, in which the loads and efforts are calculated from the detailed design and where additionally the environment where it will fulfill its work as a structure is taken into account, the service variables of the structure are taken into account, such as temperature, humidity, pressure, chemical information of the environment, vibrations, and other mechanical considerations concerning the operation of the structure.

The design of the materials involves the alloy system and even the treatments to which it will be given as a finish, as is the case of heat treatments. Grain sizes and compressibility are variables that must be taken into account for material design purposes. As for the design of the components that make up the structure, the geometry of the same is very important for the effects of points or areas of apozamientos, runoff, splashes or drips of electrolytes on the surface, which would make those components susceptible to the formation of corrosion by effects of the reaction time of the electrolyte on the surface of the component.

The selection of materials is directly related to their strength, in what has to do with the loads to which the structure will be exposed, and with respect to the stresses it will meet when it is in a dynamic or static state.

The protection system can be complemented in metal materials with the application of passivators, which have as their purpose the reduction of the corrosion rate due to the formation of a passive or little reactive film on the surface of the material. On the other hand, the application of inhibitors can be of importance in their application on surfaces with coatings or even present in fluids that can generate the formation of corrosion products.

As for the fourth corrosion control technique, the information in this article could be considerably expanded by the number of products that can be selected for the formation of coatings; but the most important thing is that the coatings are in themselves a continuous and uniform film that aims to protect the surfaces from the onslaught of the environment through the insulation method. 

The above is based on the compatibility of the coating material with the material on which it is to be applied, the adhesion achieved, and the continuity of the film that ensures the isolation of the environment with the surface material of the structure. Coatings and coatings cover a wide range of products ranging from paints, through plastics, to metal films and even ceramics.

Cathodic and anodic protection refers to the use of the electrochemical circuit, so that the material of the structure behaves like a cathode and thus is protected by sacrificial materials that behave like anodes or by the supply of current that allows the structure not to undergo oxidation reactions losing electrons.

The protection system is always constituted by more than one method or technique of corrosion control, in this way the following systems can be cited as examples:

First example:

• Design of the structure (geometry).
• Material design (alloy system).
• Selection of the material of the structure.
• Passivation of the metallic material.
• Coating resistant to the external environment and compatible with the surface material.

Second example:

• Design of the structure (geometry).
• Material design (heat treatment).
• Selection of various materials for different components of the structure.
• Application of paints compatible with surface materials.
• Application of inhibitors on paint coatings.

Third example:

• Design of the structure (geometry).
• Design of the material of the structure (specific properties).
• Application of metal coatings (cadmification).
• Passivation of the cadmium coating.

Fourth example:

• Design of the structure (geometry).
• Selection of metallic materials (by loads and stresses).
• Coatings compatible with fiberglass (paints).

Cathodic protection by sacrificial anodes (of material with anodic behavior with respect to the material of the structure). With use of the humidity of the ground where the structure is located, as an electrolyte of the electrochemical system.

There are other examples that allow to determine the most convenient for the project, taking into account the corrosion resistance of the structure in a specific environment or the durability of the protection system.

What can be seen in any protection system is the importance and dependence of the duration or reactivity for the formation of corrosion products from the geometry of the structure and its components, the rest of the protection system will be the control determined by the type of material (metallic or non-metallic), the treatment applied to the selected material, the specific properties chosen from the material, its reactivity, the environment to which the structure is to be exposed and the life time of the structure.

All of the above contains the good criteria that must be taken regarding the objectivity of the structure as a project.

The next article will deal with another topic related to the "valuation of corrosion" such as the "Service time of the affected structure or the affected component", a topic that will aim to estimate a period of time from the variables related to the corrosion of materials, components or structures from the concept of durability.

As always I wish you a good aging and many successes in your work.

BOX

A recognition of the work

I am proud to have obtained the International Certification as Trainair Plus Course Developer (Course Preparer for ICAO), not only personally but for the benefit of the country because within the ICAO (International Civil Aviation Organization) will appear from the present a course preparer for instruction at the aeronautical level of our beloved country, Colombia.

To achieve this certification I had the support of the Indo-American Educational Corporation, an institution that is endorsed by ICAO within the Trainair Plus program and is currently certified as a center of excellence in South America.

I obtained this certification after preparing the course for the training of "Aircraft Paint and Coating Inspectors" coded within icao as PAINTIN (CMDN No.: 300/101/AFT PNT QC), a course I taught in October in Brazil and Paraguay.

For the above I want to give my sincere thanks to the company Embraer of Brazil (the third largest aircraft factory in the world) for its support and logistics in the realization of the course, as well as to the ANAC (National Association of Civil Aeronautics) of Brazil, in the same way I want to thank the INAC (National Institute of Civil Aeronautics) of Paraguay.

* Juan Manuel Álvarez is a Consulting Member of the technical committees of NACE International, Member of A.S.T.M, of ACICOR (Colombian Association of Corrosion Engineers), of ASCOR (Colombian Association of Corrosion and Protection), of ACOSEND (Colombian Association of Welding and Non-Destructive Testing), of ICONTEC (Colombian Institute of Technical Standards), of the technical committees of END, Welding, Coatings, Fuels and others. He is also a university professor and lecturer. You can write to the e-mail: [email protected]