by Abel De la Cruz Pérez* The magnitude of corrosion reaches real alarming effects when the atmospheres or the environment are chemically contaminated (vapors, gases, acids, salts, etc.) that parallel to a long period of wetting causes the accelerated deterioration of metal structures. This pollution has been progressive and is a consequence of the expansion and development of the chemical, metallurgical industry, etc., as well as the continuous advance of civilization.
In this way, technological advances impose increasingly severe conditions and corrosive fumes in industrial plants are greater, which translates into greater demands on the behavior of the materials used, including paints.
The corrosion intensity associated with a particular corrosive medium or with a certain category of corrosivity, represents a fundamental parameter that governs the choice of a protective scheme or painting system.
The specific environmental aggressiveness of the place of exposure decisively influences the useful life of the painting system, so we must study and know the resistance to the different types of coatings to the atmospheric action of the environment, of the pollutants present, such as: sea salts (sodium chloride), sulphur dioxide (SO2), carbon dioxide (CO2); other specific ones such as: hydrochloric acid, etc. generated in the industrial facilities themselves.
In general, the specification of a protective coating requires knowing the physical-chemical resistance to the action of chemical products or substances in the event that the paints act in direct contact or by immersion.
Remember that like bare metals, the behavior of paints depends on wetting times, oxidants and / or contaminants, including other factors such as surface preparation and application.
Atmospheric corrosion is a process that takes place in a thin layer of moisture deposited on the metal surface. If the rhh relative humidity is very high, the moisture layer is visible, but it can also come in the form of a very thin layer, not visible to the naked eye. The corrosion process can be accelerated by:
An increase in relative humidity.
Surface condensation, which occurs when the structural surface is at a temperature corresponding to the dew point or lower.
An increase in air pollution. In this case the contaminants can be deposited and reacted with the surface, forming reaction products.
Experience shows that significant corrosion occurs when the humidity is above 80% and the temperature is around 0°C. However, if contaminants are found, corrosion can start at lower humidity levels. The humidity and air temperature in a particular region of the world will depend on the type of climate that prevails in that place. For example, while in a certain atmosphere it is possible to carry out the maintenance repainting after eight years, in another atmosphere of great aggressiveness, this treatment must be applied, after only three years, with the consequent costs for application and replacement of the maintenance paint.
In Table 1 we can see the different degrees of corrosivity of the medium (C1, C2, C3, C4, C5) depending on the wetting time (TDH) expressed in hours that the surface remains moistened per year and the presence of typical contaminants such as sulfuric anhydride and chlorides expressed in milligrams present on a square meter of surface per day.
This classification of level or degree of corrosivity of the environment allows us to define in which medium our infrastructure is or will be operating, it is only necessary to evaluate the levels of moisture and / or relative humidity, as well as the levels of contaminants such as SO3 and Chlorides.
If the structure is semi-covered, the influence of rain (humidity) and sunlight decreases and, indoors, the influence of contaminants is reduced, although localized corrosion may appear caused by poor ventilation in a high humidity environment to which condensation phenomena can be added.
Table 1 shows the different levels of atmospheric aggressiveness, with data based on determinations of mass loss (or weight loss) assessed using specimens or patterns and describes the typical natural environments in which steel may be exposed, reporting on corrosivity estimates.
The materials referred to in the corrosive action of the medium are Iron and Zinc, two of the most common materials that constitute 95% of the productive and service infrastructure in the country.
ISO 12944 deals with the classification of the main aggressive media that structures may be exposed to and their corrosivity. It does not include the classification of those mediums of special atmospheres, such as those existing in the vicinity of chemical or metallurgical plants.
Note the corrosion rate of Zinc (Galvanized), which although it has a lower corrosion rate in dry rural environments or with low presence of contaminants, is multiplied by 80 to 100, when this material (galvanized) is exposed to environments with the presence of chlorides or contaminants. This is explained since the salts that are formed as products of the corrosive process are of high solubility whose appearance are like cotton flakes, very permeable and are easily detached from the surface, losing metallic material. On the other hand, the oxidation products that are formed in environments with the absence of contaminants are zinc oxides, hard, adherent and non-permeable, which even provide a protective layer to galvanizing, hence the custom of not giving the galvanized additional protection, remaining naked during its life in service.
However, when galvanizing works in C4 and C5 environments, or in a combination of them it is recommended to provide additional protection that allows extending the useful life of the surface, a system called "Duplex", when paints are applied. telecommunications and electrical energy new as old, whose construction material is made up of galvanized steel.
Figure 1 shows the differences in the levels of corrosive aggressiveness in Peru based on geographical characteristics and the concentration of pollutants, mainly along the Peruvian coast where the largest number of industrial plants generating these aggressive pollutants are concentrated.
For specialists in corrosion control and especially for specifiers of industrial painting systems it is essential to know the effect of the aggressive severity of the medium on the behavior of coatings, since the recommendation of the appropriate paint depends on it. In Peru, for many years there have been attempts to build corrosivity maps that reflect the real characteristic of the different areas of the country. However, to date, this task has not been carried out with the exception of some isolated evaluations of private companies consuming paints, who have exhibited different painting systems and have evaluated their behavior over time as a way of selecting the most appropriate type of paint and homologating the different alternatives of trademarks, to be qualified as authorized suppliers.
Paint manufacturers should also not be oblivious to the task of evaluating the different levels of corrosivity in our environment, as well as the behavior of their coatings in real atmospheric environments if they want to mark competitive advantages and if it is taken into account that 85% of the productive and service infrastructure of the country uses paints as a means of protection.
*AmericanConsult Peru. [email protected]
http://www.americanconsultperu.com
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