will appear.
A thin, pale green outer layer variously forms on the surface of copper by oxidation or other chemical processes, and is called a patina [9]. Although the patina can give a protective covering to the surface, it should be removed before painting.
Due to the high density and smoothness of copper surface, adhesion of the coatings to copper is weak. Copper has very good resistance in atmospheric conditions but is very sensitive in sulfur and acidic environments. It is recommended that the surface is prepared with a soft blasting and applying a very resistant coating for these environments.
Cast iron has from 1.8 to 6% carbon, and it provides a rather rough, porous surface, but it is much harder and brittle than the steel. Adhesion of coatings is weak, and the phenomenon of graphite corrosion is a major problem in immersed or buried conditions [7]. Sand blasting and using a suitable paint and coating system is essential.
Some examples of coating degradation on various substrates shown in the Figure 2.8.
Figure 2.8 Paint degradation on various substrates. (a) Rust on stainless steel in an oil platform in the Persian Gulf. (b) Rust on water‐line copper pipes in a gas processing unit. (c) Paint defect on cast iron.
2.2.5.3 Characteristics of the Environment and Local Features
The first cause of premature paint degradation is improper choice of paint for the intended application. Lack of exact knowledge on the type of paints, environmental properties, and operating conditions of the equipment leads to improper choice of paint. Figure 2.9 shows that the manufacturer of the device as well as the manufacturer of the paint did not predict the possibility of chemical leakage on the equipment. Obviously, the used paint system rapidly degraded by the spillage of this material.
Figure 2.9 Paint defects because of Amin leakage on the equipment.
Industrial paints are needed in a variety of environmental and climatic conditions. Different corrosion mechanisms of each region have led to the choice of specific paint systems. The atmosphere of offshore oilrigs and inlands is full of chloride ions. The inland petroleum and chemical industries are associated with factors such as NOx and sulfur compounds. Carbon dioxide compounds are more common in large industrial cities. Large oil complexes often have sour oil‐related mercaptans (thiol) and H2S vapor. Various acid vapors such as sulfuric acid, phosphoric acid, urea, and ammonia are abundant in petrochemical plants. The passage of oil and gas transmission lines under the seabed is of special importance due to the high concentration of marine salts and the relatively high temperature of the substrate.
Environmental contaminants, especially chloride and sulfur, affect the atmospheric corrosion rate. Variations of the conditions in the area of oil and chemical facilities makes it difficult to choose a suitable paint system for each piece of equipment. Drawing an atmospheric corrosion map is very efficient for this propose at industrial complexes [10].
There is a lot of variety of liquids inside the tanks in a petrochemical complex. In addition, there is a huge difference in the internal ambient temperature of containers, vessels, tanks, etc. in industrial facilities. All of these environmental parameters must be considered when choosing paint and coatings for each application.
Recognition and classification of atmospheric and/or immersion environments, determination of their degree of corrosion, and choice of proper paint systems for each class is briefly explained in some standards [11, 12].
By measuring environmental characteristics and operating conditions such as substrate temperature, it is possible to extract the proper type and thickness of a suitable paint system and the number and thickness of layers based on standards [11].
Users must have knowledge of the standards for choosing the suitable paint systems for various industrial applications. They must be familiar with the environmental factors affecting the early degradation of paints.
Some of the factors needed to consider for choosing paint systems are as follows.
1 The substrate surface conditions;
2 environmental classification and determination of the environmental corrosion class;
3 the operating temperature of the equipment and the allowable service temperature;
4 the effect of UV rays;
5 the presence or absence of CP of substrate;
6 optimal life of the paint system;
7 possibility of workshop and/or field application;
8 the need for a special color of paint for luxury and/or identification;
9 health, safety, and environmental restrictions;
10 surface preparation and application conditions;
11 previous experiences;
12 possibility and cost of repairs; and
13 paint properties.
Lack of knowledge on the properties of paints and improper use of them is another major cause of paint degradation. If the inspectors and consultants of the industrial production of equipment do not have accurate information of the properties of ordered paints, unsuitable choices will cause premature destruction of the paints. For example, epoxy paint that is used in the open industrial environment of an oil refinery and exposed to direct sun radiation is rapidly damaged due to the weak resistance of the epoxy to the ultraviolet rays of sunlight (Figure 2.10).
In addition, due to the lack of knowledge about the method of curing ethyl silicate paint (requires high humidity for curing), its use in an industrial complex located in a dry environment has led to lack of primer curing, and ultimately the destruction of the paint system. This mistake was repeated for three times (Figure 2.11).
Therefore, in order to reduce or avoid paint problems, it is necessary for inspectors and users of industrial paints to have enough information about the properties of paints. Many documents have written with the knowledge of the type, classification, and use of paint [11, 13, 14].
Figure 2.10 Paint checking due to weak resistance of the epoxy to the sunlight in an oil refinery.
