Corrosion Policy Decision Making. Группа авторов

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СКАЧАТЬ 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 that the type and amount of paints, hardener, and solvents of each layer is proper and under the manufacturers recommendations.

      Checking the color shade, density, and weight of the paint for each layer and reviewing its conformity according to the catalog and data sheets is also recommended. Other features to be careful about are appearance of the wet and the dry film (WFT and DFT) as well as possibility of complications for each layer preferred.

Figure 2.14 Some typical complications of the paints resulting from poor quality paint features. Weak adhesion of top coat by one manufacturer on the primer (left). Weak adhesion of topcoat to intermediate (right). Weak adhesion of intermediate to primer (down).

      2.2.5.5 Paint Warehousing and Storage

      One of the problems for paint application projects is the long‐time duration between the paint production and its use in the field. Most industrial users buy paint a few months before using it due to the long QC test time, and storage of paints in most projects can be up to a year. In some countries, due to political and economic reasons early forecasting and purchasing are also needed.

      In the executive projects, a separate warehouse is not provided for the paints and they may be stored with other equipment in a public warehouse. The standard conditions for paint storage, such as setting the right temperature, are not indicated most of the time; this is another problem that reduces the paint quality. The method of storing and using paints during storage is very important and stated in manufacturer catalog.

      2.2.5.6 Role of Executors and Contractors

One of the main problems leading to the reduction of a paints lifetime is the lack of knowledge of executors and contractors and the lack of a precise working plan. The planning and scheduling of work in many cases is based on the weather, but sometimes it is very difficult to predict the field conditions and climate change can disrupt the work schedule, accuracy, and order of work. The selection of skilled and capable contractors and providing proper facilities for implementing the standards of surface preparation and paint application play an essential role in the life of the paint. In addition to being fully aware of operation standards, the contractor must have the proper equipment to assure the health and safety of the workers and comply with the local environment regulations. Examples of unqualified workers and contractors are seen at Figure 2.15.

      2.2.5.7 Surface Preparation

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