Corrosion Policy Decision Making. Группа авторов
contractors, inspectors, executors, and technicians, are under double pressure to complete the job.
Being in hurry leads to neglect of many standard considerations, and the results shorten the life of the paints and lead to their failure. The author and colleagues monitored seven petrochemical centers, two oil refineries, two oil offshore docks, and four gas refineries periodically during four years with emphasis on studying paint damages. Paint problems and solutions are reviewed in this chapter.
Type and size of the abrasive particle are important due to speed of work. Less attention to them may result low surface roughness, sand dirt, and surface contamination due to performing several operations simultaneously. In addition, many other factors contribute to the weakness of the surface preparation and that is one of the main causes of weakness in paints. Collecting atmospheric corrosion data and choosing the right coating system based on corrosion parameters is very important. It also helps in deciding the right time for the painting program.
Proper application of multi‐layered paint systems, consideration the curing times and interlayer times, and observance of the weight balances of paint components and the skill of workers and technicians have led to successive results of long life paints and coating in many industrial projects.
Paint quality control tests almost always need a long duration time. However, managers and executives are in a hurry to start the operation of the units, and this sometimes leads to ignoring some necessary tests, and ultimately leads to using weak paints.
Defects of the paints often appear after the beginning of service. Several field variables play a role in coating defects, and these factors can work separately or intensify each other.
2.2.5.2 Features of Substrate
The material of substrate and shape of a surface are effective limits to useful life of coating and paint systems [7]. In industrial facilities, some base metals and sub‐surfaces include; steel, stainless steel, cast iron, galvanized steel, aluminum, zinc, copper, concrete, etc. The corrosion mechanisms of these substrates are various and each requires a different method of surface preparation before application of the coatings [7]. Stainless steel, zinc, and aluminum corrode slowly under the atmospheric condition [8].
Steel has a dense and smooth surface. It also has a high chemical reactivity with the atmosphere and pollutants. Therefore, complete de‐rusting along with enough roughness are necessary in surface preparation.
Stainless steel 304 has 18–20% chromium by weight [8]. It has a much harder surface under normal atmospheric conditions than carbon steel, is smoother and denser, and is more resistant to corrosion under normal atmospheric conditions. It has many applications in atmospheric environments. Stainless steel strikes the rust point in an atmosphere containing chlorine and sulfur, however, it is more difficult for the coating to adhere to it. It is therefore recommended that you perform a weak and suitable acid wash on the surface of stainless steel before sand blasting, for proper adhesion of the primer [7].
Concrete is not dense and has a rough surface; its surface is porous and is vulnerable to the attack of undesirable chemicals such as acid, alkaline, and hydration of cement due to its surface pores. Therefore, it is necessary to create surface uniformity with the help of sealers which add consistency to the coatings. Concrete is also very sensitive to the application of acid base coatings.
Aluminum is a metal that has a dense and smooth surface covered with a layer of aluminum oxide. Alkaline materials degrade aluminum surfaces, and surface coatings on aluminum are less durable in alkaline environments. On the other hand, sulfur has many corrosive effects on iron, copper, silver, and lead substrates, but it is almost ineffective on aluminum; and coated aluminum is less problematic in a sulfur environment. One of the problems with aluminum substrate is the difficulty of paint adhesion on it. Anodizing is suitable for workshop treatments, but to overcoming this problem in the field, some chemical treatments and soft blasting are recommended.
Galvanized steel is prepared by hot or electrogalvanized zinc spray and provides better corrosion resistance in atmospheric environments. In the acidic environment, zinc reacts rapidly with acidic agents, and forms a white layer of zinc chloride on the surface in a chlorine‐containing environment. If ions pass through the paint system layers on galvanized steel, whitening and surface porosity 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