The early assessment of a structure that suffers the effects of corrosion will allow to act with greater time and prevent dangerous affectations by the corrosion factor.
by Juan Manuel Álvarez*
In the development of corrosion assessment, the structure affected by this phenomenon must be taken into account, since it is precisely this that suffers the onslaught of the environment for the formation of corrosion products.
Now, the way in which the theme of this article should be approached has to do with the geometry and mobility of the structure, an example of this are the vibrations that the structure presents.
It is known the incidence that the concept of fatigue in any of its forms has on the useful life of the materials that form the structure; however, the concept of the ageing of structures corresponds to the synergy of fatigue and corrosion, which is why both the geometry of a structure and its mobility are important in assessing the level of corrosion that occurs in a given physical structure.
In the case of spherical or circular structures such as gas storage vessels (tanks), or horizontal or vertical structures for liquid storage, aircraft fuselages, among many other examples, the cyclic behavior of the materials in these structures is part of the analysis that must be given at the time of corrosion assessment.
Thus, in the formation and propagation of corrosion, especially when it occurs at the grain edges of materials, the mobility and geometry of the structure contribute to the determination of the corrosion rate.
For example, in the "corrosion assessment" the formation or presence of an intergranular corrosion is considered serious when the affected material is part of a component or structure that in its performance presents great mobility or an important frequency of cyclic work, if the fatigue behavior is of an alternating type it will present a failure in time comparatively fast with respect to if this type of corrosion was not exposed to that large mobility or frequency of exertion or creep.
Another example can be the presence of a pitting in a material that at a point of confluence or "change of section" shows a speed of growth or propagation due to the effect of the geometry of the component or structure that directs or concentrates the energy of effort to that point where the pitting corrosion is located.
We can make from a certain perspective and to a certain extent a similarity that we have with a variable behavior in the operating temperature of a component or structure in which that fluctuation affects the corrosion rate, either for reasons of expansion during the "temporary separation" of the grains that make up the material of the component or structure.
If we consider the different types of corrosion, we could then order each of them according to the severity it could present in the corrosion rate, based on the location of each type of corrosion according to the geometry of the structure. In the same way, the exercise of ordering the different types of corrosion could be done according to the severity that could present in the corrosion rate, based on the location of each type of corrosion according to the mobility of the structure.
The "Structure affected by Corrosion" title or objective of this article is in itself a factor to be taken into account from the design of the structure, which is why it can be foreseen as far as possible or within the maintenance program or inspection manual to determine the frequency of monitoring in those "critical" areas of the structure, so that it can be controlled as best as possible through the Corrosion Control mechanism or product selected for the protection of the structure.
By the reasoning and mechanisms exposed is that the concept of design is contemplated as a method of "Corrosion Control", in such a way the structural engineer (design) and whoever acts as Quality Control (predictive maintenance) must have the sufficiency in the knowledge of corrosion and its control for the purposes of directing their efforts especially in this direction.
The following article will deal with another topic related to the "valuation of corrosion" more specifically touching on the topic related to "the environment in which the structure is located", a vital content in the selection of materials and monitoring for the determination of their useful life or durability.
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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