Corrosion Policy Decision Making. Группа авторов

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Figure 2.7 Conceptualization of the elements of cathodic protection (CP); 1: Sacrificial anode‐galvanic CP, 2: Impressed current CP.

      2.2.4 Mechanical Treatment

      PIG stands for Pipeline Inspection Gauge. Pigging is the act of running a PIG through a pipeline. The main aim of pigging from a CM point of view is not treating corrosion but somehow decreasing the likelihood of corrosion. Debris and scales formed within a pipeline are capable of facilitating the establishment of spots with differences in local partial oxygen pressure or concentration of chemical species. The former forms differential aeration electrochemical cells and the latter serve to form differential concentration electrochemical cells. In both cases, electrochemistry under the deposits will be highly likely to be leading to corrosion. Also, if temenos [5] (see footnote 1 in this chapter) forms, the corrosion products along with the temenos pieces can form deposits that would enhance the corrosive effects [6] of the deposits formed in this regards. This is when use of under‐deposit corrosion will make sense. Pigging will damage and destroy these deposits and scales and thus will decrease the likelihood of under‐deposit corrosion, by which these corrosion mechanisms may do detrimental effects on the mechanical integrity of the pipe.

      Any action that will intervene with formation of deposits and establishing electrochemical cells by mechanical action can be grouped in this category. The deposits can be formed on the exterior of the asset, thus leading to external corrosion, or within the asset resulting in internal corrosion. Borrowing the terminology we have used in Chapter 3 to distinguish between corrosion control and corrosion prevention, mechanical measures can mostly be referred to as technologies to prevent corrosion by lowering the possibility of under‐deposit corrosion in the way we previously addressed.

      2.2.5 Physical Treatment

      2.2.5.1 Paints, Coating Systems, and Premature Destruction in Industrial Facilities

      Paints are very important in CM and its economy due to their high cost. Defects of paint and coating are inevitable. The users are trying to get optimal, long service time for paint systems.

During construction of refineries and industrial complexes, painting of structures and equipment programs is one of the last stages before getting started and commissioning. Managers and workers have a strong wish to get the job done, so painting teams, including 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 СКАЧАТЬ