2010 is a year of new challenges and recovery of good things, so in INPRA LATINA we decided to resume one of the sections that was most successful among our readers: the Office.
We started then with a concern that Carolina Villalobos, from Venezuela, left on our website:
What product could you recommend for the inner part of a boiler feed water tank, where the water is at an operating temperature of approximately 120 °C, filtered, smoothed and has approximately 10% dissolved oxygen?
We then consulted Jordi Calvo Carbonell, a retired paint and coating technician who for more than forty years worked in various companies in the formulation and application development department. He has written and read lectures in various associations of technicians in France and the Nordic countries and has given seminars in various companies in the sector. He is also the author of the book "Paints and Coatings: Introduction to Their Technology". Here's his response:
First of all I must say that the conditions of this tank are certainly very hard, since if it is at 120ºC this indicates that it is a closed and pressure tank which possibly hinders the operation of preparation and painting.
We must assume, however, that it is possible to access it to carry out all the precise operations. The tank must be made of iron, since if it were made of stainless steel it would not be necessary to make any protection.
Nor do you indicate if the tank is currently new or already coated in some way. In any case, the first thing that must be done is to carry out a thorough cleaning of it, if it already has any coating it must be eliminated. Then make a suitable surface treatment since the result of the painting system will be as much better as the previous treatment, you should not forget that "a painting of medium characteristics on a well-prepared surface gives better results than an excellent paint on a poorly prepared surface".
Surface preparation
If the tank is painted, the old paint should be removed. If it is not painted it will have rust that can be red, rust, if the deposit has a certain time; or it will be lamination if the deposit is new.
Lamination oxide is as dangerous or more dangerous than red oxide, rust, since due to the potential differences between this rolling oxide and iron there is a stack effect with iron oxidation.
The preparation, once the remains of previous coatings have been removed, is to perform a sp-10 sandblasting (SSPC Standards) with which the metal will be perfectly clean and free of calamine, rolling oxide, and from this point it must be protected with a paint system.
Note: It is very important that the preparation is done very scrupulously in the welding areas where the same stack effect that I mentioned before is where oxidations can begin.
Painting of the tank
We must protect the tank in two directions, the first to prevent oxidation and the second to prevent water, whether in liquid or vapor form, from reaching the support. On the other hand, it should be borne in mind that if the working temperature is 120ºC, there will be expansion and contraction movements that can damage the coating.
Primer: The most suitable primer is of the "Rich in Zinc" type (minimum zinc content 85%), here you could discuss whether it should be inorganic or epoxy. The most suitable is the "Epoxy Zinc Rich Primer" since its ability to absorb structural movements is superior to those of inorganic type, much more rigid. With reference to the type of catalyst, an amine adduct is the most advisable from the point of view of application and an amine would be the most advisable in order to obtain maximum resistance.
Finishing: The finish with the greatest guarantees is an epoxy bituminous paint based on a liquid epoxy resin and a bitumen or solid asphalt, so that no softening occurs during the work process. These paints have an additional advantage over those formulated with solid epoxy resins, which is the ease of applying thick layers with fewer hanging problems.
It would be convenient for you to discuss the characteristics of the proposed paints with the manufacturer you choose.
Painting system
Immediately after the surface preparation should proceed to the application of the primer of two layers of Zinc-Rich Epoxy Primer, with a thickness of 3-4 mils (75-100 microns) each. The time between the application of the first and second layer is recommended to be 24 hours and never more than 48 hours, as this could cause adhesion problems between layers. It is important to ensure good protection in the welding areas due to the aforementioned stack effect.
The application of the finish should be done in a similar way and 24 hours after the application of the last layer of primer. Two coats of epoxy bituminous paint with a thickness per layer of 3-4 mils (75-100 microns).
It is very important that the tank is not put into operation before seven days if the ambient temperature is 20-25ºC or 15 days if the ambient temperature is below 20ºC since the reaction between the epoxy resin and its catalyst lasts for several days depending on the temperature.
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