Samuel Cubero wrote to our email [email protected] sending us the following concern:
"I work in an oil industry in Costa Rica and we are improving the corrosion protection system through paints and coatings. The consultation is to know about the recommended combinations of primary, intermediate, finish for storage tanks (external and internal) of crude oil, gasoline, diesel, etc. and anhydrous alcohol tanks to produce gasoline with ethanol. The conditions would be in harsh environments and near the sea."
We consulted the opinion of the engineer Jaime Baldeón Garibaldi who works in the Research and Development Laboratory of the Peruvian company Corporación Peruana de Productos Químicos SA (CPPQ).
Here's his response:
Storage tanks are used all over the world to store all kinds of chemicals in refineries, petrochemical facilities, terminals, dispatch warehouses, power stations, etc. Due to their location, tanks can be buried and aerial. This article will deal mainly with aerial tanks.
Due to their shape these are: conical roof (the vast majority), floating roof and internal floating roof. And its storage capacity can vary from a few cubic meters, to capacities of more than 160,000 cubic meters.
The construction of these storage tanks generally follows international standards such as The British Standards Institution (BS 2645) and The American Petroleum Institutes (API 650). These standards do not consider the corrosive nature of the product to be contained and the degree of corrosivity of the environment.
Therefore, the thickness of the plate designed under these standards cannot have the expected durability if the steel is not adequately protected. The selection of the inner and outer coating is a very important task for specifiers. For the correct design of protective systems, specifiers must consider different parameters such as: service conditions (product to be stored, pH, temperature, etc.), the type of coatings used (type of polymer, number of layers, chemical resistance, etc.).
Coating Selection
1.By its position in the system
1.1 Primary Layer: It is the first layer applied to the substrate and must have the main characteristics:
Adhesion: The product adheres by primary valences or chemical bonds (e.g. Inorganic Zinc ZIO), on the other hand it can also originate secondary or polar valence bonds (e.g. Epoxy, modified epoxies). And finally mechanical adhesion, related to the roughness profile of the substrate.
Anticorrosive Protection: The primary layers must have pigments that give anticorrosive properties to the paint through the cathodic protection mechanism (Zinc), or through the anodic protection mechanism (Phosphates, molybdates). It is worth mentioning that due to environmental regulations lead and chromate-based pigments are being discontinued and are not currently being specified.
1.2 Intermediate: The main protection mechanism of these layers is the "barrier layer effect" given by the thickness of the paint and the type of polymer used. The cross-linking produced in the curing of the paint will produce a more or less impermeable layer. Normally the intermediate layers can be Acrylic, Epoxy, Epoxy Novolacas.
1.3 Finishing: Many times the product used as an intermediate layer can be used as a finishing layer. But the main features of this layer are:
Barrier Layer Effect- Like the middle layer these products must offer high impermeability.
Medium Resistance- Depending on whether the inside or outside of the storage tanks is protected. The product must offer the resistance to the medium that will be in contact (the atmosphere for the outer tanks) and (the type of fluid that the tank will contain).
For outdoor tanks, stability to gloss and UV rays is a very important factor, so products based on Polyurethane or Polysiloxanes (PSX) are usually used as finishes.
2.By the type of Resin
The most important classification is given by the type of polymer that is used to protect the storage tanks. The intrinsic characteristics of each of them will offer us practically the characteristics of the paint and therefore its operation or not under certain service conditions.
2.1 Acrylics: They are resins that have an evaporative curing mechanism forming a stable and semi-rigid film. They now replace alkyd systems more efficiently.
There are solvent-based acrylics on the market, but the most used today are those based on water, due to the requirements of environmental protection. Manufacturers can offer as primary (DTM direct to metal) and as intermediate and finishes. They are characterized by their good stability to light and UV rays. They are recommended for not very aggressive environments and only for outdoors.
2.2 Epoxy: They are the most used resins for the protection of storage tanks, interior and exterior respectively. In recent years the chemical technology of Epoxicos has varied tremendously, existing today a very important variety.
Bisphenol A- They are the ones that have traditionally been used and have been used to develop protective paints for tanks. According to their molecular weight, these can originate products with low solids and what is used today is Epoxicos of high solids. They have a characteristic of good chemical and physical resistance.
They have a disadvantage that they are not stable to UV light producing a degradation and loss of color and brightness.
2.3 Polyurethanes: It is the product resulting from the reaction of a diisocyanate, usually aliphatic with a difunctional polyol (acrylic or polyester), producing a polyurethane product of good physical and chemical resistance and with a high stability to brightness and color. Normally used as an outdoor finishing layer of storage tanks on an Epoxy intermediate layer. Outdoors, the ZIO/Epoxy/PUR system is the best performer.
2.4 Polysiloxanes: The application of this type of technology is relatively new in the field of protective coatings. The chemistry of these products is based on silicon (inorganic) unlike Epoxicos and PUR products which are based on carbon. This property makes PSXs have high waterproofing, therefore greater corrosion protection similar to epoxy; and excellent brightness stability and color similar to PUR. So they usually replace the Epoxy/PUR system. With a good primary layer and a PSX layer it will offer high performance on the outside of tanks.
Conclusion
An incorrect selection of the protective coating for interior and exterior of storage tanks can cause premature failure and usually in very high maintenance and repair costs for any company.
To avoid and anticipate these problems, specifiers must know all the most important parameters to specify the best protection system. For outdoors it is very important to know the atmospheric environment to which the tanks will be exposed: Rural, Industrial, Marine or Industrial Marine.
For Interiors it is of vital importance to know the type of product that the tank will contain, its chemical aggressiveness, pH, temperature, etc. By type of performance, interior coatings are designed and specified following special selection procedures. {jcomments off}
Gracias.