Before starting the corrosion assessment process, an in-depth analysis must be made on the different types of corrosion that exist, which will allow a more efficient and concrete work to be carried out.
by Juan Manuel Álvarez*
In compliance with the corrosion assessment procedure that is used to determine repairability and perform the treatment for the return to the service of a structure, normally the inspector does not take the time to stop to classify the type of corrosion that is generating the failure of the material, component or structure.
Well, taking into account the type of corrosion that is consuming the material and is spreading is an important fact to analyze the causes that originated this type of corrosion. It is feasible that the corrosion failure will recur when the treatment of the same does not contemplate the solution to the origin of the process that has been presented.
It is important to know about corrosion, not only the mechanism, but the types that by classification can be presented, since each of these corresponds to a specific origin or conditions for a type of corrosion to occur.
The type of corrosion, which is the basic form of recognition and classification is what we call morphology, which leads us to determine 27 types of corrosion based on specific considerations or conditions for each of them to occur.
Corrosion classification
For example, galvanic corrosion requires the presence and contact of dissimilar materials in their potential difference and connected, surrounded, submerged or immersed in an electrolyte, which by the reaction or contact time promotes an electrochemical system that generates an oxidation process in the most electronegative material or with the greatest capacity to yield electrons to become the anode that is known as the area, area or material to be corroded.
Exfoliation corrosion dispenses with the presence of moisture under certain specific conditions such as reaction time or contact with water, the type of material based on the process of making it, the quality and the alloy ratio it has.
Other types of corrosion such as SCC, whether of intergranular or transgranular class, have as their main condition the fulfillment of a type of mechanical work that induces this kind of phenomenon to happen. The mechanical requirement of the materials starts from the configuration and machining processes to the work carried out as a component or structure in fulfillment of the function for which it was duly selected. The loads and stresses made by the material in a thermodynamically specific environment determine the conditions that generate this morphology.
The uniform corrosion morphologically is recognized because its presentation obeys a lateralized propagation rather than depth or reaction located on the surface of the material, this type of propagation is generated by a uniform environment, for example, of gaseous type that surrounds the surface of the material and reacts simultaneously at different points of its surface. For the example of a gaseous environment, it may well be by flue gases or by air in certain relative humidity regimes.
The pitting, whose morphology is very typical and graphic, is a corrosion that due to its reaction with the surface where the consumption of material is of greater advance in depth than in lateral propagation, is classified as a localized corrosion.
Its morphology suggests the reaction or contact with a product or solution that allows this local reaction, so that it is closely related to the presence of chemicals that react locally, as is the case of splashes or apozamientos by geometric effects of design or deformation, to mention an example.
Other types of corrosion occur due to the requirement of fluid transport, such as cavitation and abrasion corrosion, which differ mainly because the first demands that the fluid be in a turbulence condition, while the second is necessarily related to solids present in the fluid.
Based on the above, identifying the morphology of the corrosion that is intended to be treated based on the acceptance of reparability, and knowing the origin or conditions by which this type of corrosion was presented helps to foresee to a large extent that this same phenomenon will be repeated, at least in a wide range of time depending on the solution and the origin of the problem.
We could cite each of the types of corrosion but in addition to being extensive it would be more an article aimed at the knowledge of the types of corrosion through morphology than the assessment of corrosion for the purposes of durability, repairability and structural reliability; subject of this article as part of the corrosion assessment cycle.
If there is a particular interest on the part of the reader towards the knowledge of the morphology of corrosion, contact the publisher so that a specific morphology article or even a course on the subject is scheduled.
In the next edition we will continue with our cycle of articles related to the assessment of corrosion, so I remind you of the topics to be developed:
1 Type of corrosion.
2 Material affected by the corrosion process.
3 Location of corrosion.
4 Structure affected by corrosion.
5 Environment in which the structure is located.
6 Working parameters (Humidity, temperature, pH, pressure, mechanical work, vectors).
7 Corrosion control product, process or technique applied to the structure originally.
8 Service time of the affected structure or affected component.
9 Service time of the product, process or corrosion control technique applied to the structure or component.
10 Remaining service life of the structure or component.
In compliance with the topics to be developed, the competencies that a corrosion inspector must have will be determined with a view to monitoring the state of a structure or its components, estimating the degree of aging, and even participating in the design or assembly of structures.
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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