Corrosion Policy Decision Making. Группа авторов
polymers are used as binders in paints. Their properties, such as the molecular weight of the polymers, the mechanical strength of the bonds formed in film, the permeability of the film, thermal resistance, etc., are effective in the paint and coating capabilities. The paint curing mechanism is the most important method for classification of binders.
There is a group of binders in which the polymers react with air humidity or oxygen of the air for curing; they are known as air‐dry binders. Drying oils and phenolic varnish castor oil (polished oil) are natural air‐dry and alkyd binders; vinyl alkyd, epoxy ester, silicon alkyd, etc. are synthetic air‐dry binders [7]. This group of binders is used in single‐component paints. They begin to form a skin during shelf time, especially when the ambient temperature rises or the can or container lid is left open or the container damaged. It is also important to adjust the ratio of surface drying and deep drying additives in these paint formulations.
There is another group of binders where the polymerization reaction of the binder is completed during fabrication at the factory. When they apply to the substrate, the paint sticks to the substrate and forms a dry film only by the evaporation of solvent. Nitrocellulose binders, vinyl resins, and chlorinated rubbers are of this type. These paints are a single part similar to the previous group. Paint solvents play an essential role in the drying speed and only the use of suitable solvents made by the same factory is recommended for them to reduce the paint defects.
Another group of paints is known as co‐reactive, and is packaged in two or three separate containers during storage. In order to start the polymerization reaction, they have to be mixed in right amounts and the mixture needs to rest for some minutes before application. The main binder is a pre‐polymer and includes some other paint constituents packaged separately. Hardeners are other chemical compounds that are needed for initiation and to continue polymerization reactions and which are stored in separate container. The contents of the containers are added to each other before application. In this way, complementary polymerization reactions of the paint take place after mixing and application. Epoxy, polyurethane, polyester, and coal tar epoxy paints are of this type. To cut the defects in this group of binders, it is necessary to obey the exact ratio of mixing of the paint and hardener. It is often suggested to wait half an hour after mixing to start the pre‐reactions and then start diluting and applying. It often takes more than a few days for full curing of these binders. Therefore, in the primer and middle layers, we have to wait at least 24 hours before the next layer and for the topcoat paint, and at least one week before service operation.
The polymerization of another group of binders can be completed by increasing the temperature to a certain extent; paints containing these binders are known as thermal curing. Pure phenolic paints and phenolic epoxy are two well‐known categories of thermal curing [7]. It is possible to meet their last curing at the temperature of 230 °C.
Silicon binders are another heat‐curing polymer. It is a mineral‐based heat‐bonding binder with the main constituents of silicon and oxygen. Polysiloxane‐type silicone binders are also used in paints; this binder has a very good resistance to chemical agents due to having silica in the formula. Silicone binders are suitable for use at high temperatures (540–640 °C) and may be used to make paints resistant to the same temperature. For example, when using ceramic pigment particles, using them at up to 760 °C is recommended. This binder may also modify with alkyd for use at lower temperatures.
This group of paints has a very low mechanical strength before the final curing. Temperature of curing is mentioned in the manufacturer's catalog temperature. Therefore, special considerations are required when they are applied on large chemical equipment, especially before providing the curing temperature, which is to be provided at the start of activity of the factory in terms of the required chemical process (es).
Solvents may be used in paint formulation for two main purposes: first, to dilute the binders that allow the pigments to mix properly; and second, to balance the evaporation rate, which allows the wet film to dry over a desired time. They also play an essential role in promoting the rheological properties (fluidity) of paint.
Two categories of solvents can usually be used for each paint formulation, these are known as main solvents and auxiliary solvents. In terms of chemical structure, the main solvent has a special chemical group or band in its molecules that can better dissolve the binder. Auxiliary solvents can be used to optimize other properties, such as drying rate. Therefore, specific solvents for dilution of each paint may be used by the manufacturer, and using other solvents may be associated with complications.
Additives may also be added to formulation for optimizing some quality properties of paints. Complete information on specific applications for additives are discussed in scientific and commercial documents [16].
Sometimes there are some problems with the paint during the storage period, and they can only be recovered by returning it to the factory. The factory will improve the properties of the paint again by using additives, solvents, and homogenizaton.
In order to meet the proper performance of paints in industrial facilities, they apply a paint system in several layers. In general, the role of layers can summarized as follows.
2.2.5.3.1 Primer
The main role of the primer layer is to prevent corrosion reaction or to change reaction path on the metal surface to form a stable corrosion‐resistant surface. For example, zinc content is very important for the CP of the steel substrates in zinc‐rich primers. It is also essential that the primers have good adhesion to the metallic substrate and to the middle layer.
2.2.5.3.2 Intermediate Layers
Thick, continuous, and cross‐linked intermediate layers will prevent the penetration of corrosive ions from the top surface to the substrate. This layer will also have proper adhesion to the primer and top layers.
2.2.5.3.3 Topcoat
Resistance to atmospheric factors, such as ultraviolet rays, is the most important role of the topcoat layer. Special properties such as abrasion, scratches, resistance to the growth of microbial agents, etc. are also dependent on this layer. The top layer should have good adhesion to the middle layer and have good decorative properties.
One of the causes of paint system degradation is the incomplete use or displacement of layers. In addition, in local repairs and touch‐up paints, it is often observed that a new primer is applied on the middle layers or the old paint surface. This mistake can lead to premature damage to the paint. Removing the old topcoat layer, neglecting the new primer, and starting the implementation paint system from the new intermediate layer and then the new topcoat is a more proper method.
It is essential that paint users and paint inspectors heed the comprehensive scientific information of paint types and their potential capabilities on corrosion prevention.
2.2.5.4 Paints Quality Control
Sometimes, for economic reasons, the paint manufacturer substitutes the use of standard raw materials for some inexpensive ones. In addition, other reasons such as lack of proper formulation, exhaustion of equipment, use of raw materials in long storing periods, etc. lead to poor quality paint production.
Absent a valid, reliable QC laboratory, the risk of using weak paints is high. Therefore, sampling and sending for quality control tests is necessary by inspectors. In Figure 2.14 some typical complications of the paint system arising from the poor quality of the paints are seen.
It is recommended that you send samples of the paints to a reputable laboratory center for quality control to make sure their quality is in accordance to the standards at least three months before application.
In addition, before beginning the surface preparation in the field, check