For the continuity of the cycle of articles related to the assessment of corrosion, in this edition we will deal with the topic regarding the "Material affected by the corrosion process".
by Juan Manuel Álvarez*
For the continuity of the cycle of articles related to the assessment of corrosion, in this edition we will deal with the topic regarding the "Material affected by the corrosion process".
This topic is directly related to the selection of materials used in the structures and the tendency of these to the way they react with the environment to which they are exposed, focused on corrosivity, which finally is what will determine the useful life of the material and what is required for the control of corrosion of those materials in terms of mechanisms, procedures and products.
We must start that the selection of materials involves the mechanical resistances that the material must have to properly comply with the loads demanded by the component or structure in its useful or planned working life, and the low reactivity that the selected material presents with the environment to which it will be exposed in working conditions. Therefore, and based on the objective of this article, we will focus on the second criterion of material selection: what is the trend in the formation of corrosion products or the reactivity of the material to the external environment. I will start from the term "Corrosion resistance", which is eventually awarded as property in the materials, either in technical sheets or in commercial information.
In practice, "Corrosion Resistance" should not be considered a property of materials; technically it is not, nor is it a specific property of materials, nor intrinsic. The formation of corrosion products comes as a consequence of the reactivity of the material with the environment that is in contact with the surface.
We must consider the "Corrosion Resistance" as the result of the resistance of the material to reactivity with the surrounding environment versus the aggressiveness of this environment that is in contact with the surface of the material; this reactivity occurs at the interface by chemical or electrochemical reactions that create an electronic imbalance that by oxidation initiates the formation of corroded products.
In order to create a suitable term for this process, we will call it "Corrosivity". In this way the "Corrosivity is therefore the assessment of the reactivity of the selected material when exposing it to any of the different environmental variables, and the analysis of the aggressiveness of the environment that is in contact with the surface of the material, which is measured through each of the variables that make up this environment.
Without mentioning specific materials we will give an example: there are materials that behave very well in humid environments, but not in environments where chemical information or pH makes them quite reactive, and corroded products will be generated at unnatural speeds.
In the same way we could cite examples in which the materials are little reactive in acidic pH but in the presence of high temperatures they are quite reactive, quickly promoting the formation of corrosion products. We could cite multiple examples, all taking us to the same criterion, which is that materials do not react the same in all environments, which allows us to order the materials for the same environment according to their reactivity.
The order of materials based on their reactivity in a specific environment will change the moment the environment is modified. For example, the behavior of different materials will not be the same in a marine environment, it will change when exposed in an industrial environment.
For the management of these material selection criteria from the point of view of "Corrosivity" the so-called galvanic series are used, which establish an order of materials according to their reactivity in specific environments.
This first step, for the purposes of material selection, must be complemented with the different mechanisms that generate the existing types of corrosion, since the reactivity ordering only indicates its tendency or the ease with which a product corrodes with respect to other materials; but it is not determining that it can be susceptible to any type of corrosion, since each type of corrosion has a direct relationship with some or some variables of the environment, as highlighted in the previous article.
Based on the above, it is important to note that the selected material will be affected by the corrosion process once the reactivity of the surface material is initiated with an environment-specific variable, with which this material has greater reactivity or is more susceptible to reacting to generate specific types of corrosion.
This means that the inspections and the number of frequencies that are agreed can be established in advance as a preventive mode through monitoring or predictive actions of failure in the materials.
Therefore, the result of the material affected by the corrosion process should not necessarily be taken as a poor selection of the material, but as a poor selection of inspection techniques; errors in the determination of inspection frequencies, poor selection of the products applied for the control of corrosion of these materials in a certain environment, bad practices in the application, and other reasons that will lead us to consider that the levels and maintenance procedures have not been properly carried out following the preventive policies, which generates that the materials, components or structures present affectation by the presence of corrosion products, being affected their useful life, their reliability and their cost through depreciation.
Consequently, the material affected by the corrosion process should not be seen as a natural fact in the industry, but as a technical failure that denotes a lack of knowledge, awareness or due budget, which may not be commensurate with production or costs, which is why corrosion costs must be separated for control, which make up everything concerning what is necessary to achieve the purpose of controlling the corrosion process.
The following article will deal with another topic related to the assessment of corrosion. As always I wish you a good aging and many successes in your work.
* 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]
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