Corrosion Policy Decision Making. Группа авторов
href="#ulink_9a057050-6ce0-50aa-8991-5fb1525608fc">Figure 2.6 Carbon steel (hot rolled, cold rolled, and galvanized coils) price fluctuations with an average price of $0.5027/Ib (top). Stainless steel (sheet) price fluctuations with an average price of $1.219/Ib (bottom) [1] (all average prices based on prices given on 1 January 2021).
Change or modification of design can be performed without a need for materials selection if applied smartly. For instance, where possibility of galvanic corrosion exists, it is possible that by either changing the anode/cathode ratio, applying CP (to reverse the polarities), or coating either anode or cathode (or sometimes both) solve the corrosion treatment issue as a corrosion prevention/control approach.
Last but not least, the option for design modification‐change/materials selection could be an option which is paradoxically both inexpensive and expensive; cost of design per se may not become too unbearable, whereas costs imposed by materials selection can become so exceedingly high that the project management will be pushed to consider other measures. Therefore, in dealing with corrosion treatment via this option, any proposal for doing so should come with a smartly planned economic analysis for whole process and not just relying on decreasing corrosion rates and severity alone.
2.2.2 Chemical Treatment
One the ways by which corrosion can be brought under control is application of corrosion inhibitors, and in case of MIC, biocides. The reason we emphasize mentioning biocides along with corrosion inhibitors is the incorrect belief by some corrosionists that by applying inhibitors, MIC can also be treated, or rather, MIC is less important than non‐MIC corrosion problems. While MIC is also an electrochemical corrosion in essence, corrosion inhibitors (or nearly all of them) cannot treat MIC, as the contributing factor to corrosion is micro‐organisms.
Corrosion inhibitors can be grouped by selecting various criteria, but we prefer to categorize them into main three groups; anodic, cathodic, and mixed effect inhibitors.
Anodic inhibitors form a protective oxide film on the surface of the metal. This film will be instrumental in causing a large anodic shift of the corrosion potential. This shift forces the metallic surface into the passivation region where the material can be assumed to become immune to corrosion. Due to this effect, anodic corrosion inhibitors are also sometimes referred to as passivators. Chromates, nitrates, tungstate, and molybdates are some examples of anodic inhibitors. On the other hand, cathodic inhibitors act mainly via two mechanisms; either by slowing the cathodic reaction itself, or selectively precipitating on cathodic areas to limit the diffusion of reducing species to the surface. The mechanism of mixed effect corrosion inhibitors can be summarized as the three points below:
Mixed inhibitors work by reducing both the cathodic and anodic reactions. They are typically film‐forming compounds that cause the formation of precipitates on the surface, blocking both anodic and cathodic sites indirectly.
The most common inhibitors of this category are silicates and phosphates.
Silicates and phosphates do not afford the degree of protection provided by anodic inhibitors such as chromates and nitrites, however, they are very useful in situations where non‐toxic additives are required.
Biocides are chemicals that can serve to kill bacteria and this terminology is used particularly to address the bacteria contributing to MIC reactions. Biocides can be divided into oxidizing and non‐oxidizing, depending on the detrimental mechanism and effects on the targeted bacteria. Important features of biocides [2] as well as pros and cons of frequently used biocides [3] have been reviewed elsewhere.
The main point about both corrosion inhibitors and biocides is that as they are both classified as chemicals, their entrance into the environment must be carefully assessed and monitored on a systematic basis [4].
2.2.3 Electrical Treatment
CP is based on reversing the anodic reaction(s) by returning the electrons liberated via anodic reactions back to the metal. This feature of CP can be schematically shown in Figure 2.7. As the figure shows, essential elements of CP are to control electron release via anodic reactions, that is via corrosion, are to be controlled by supplying electrons by either sacrificial anode CP technology or impressed current CP. It is possible to also mix the two methods to make the electron pool available to the object to be protected even more powerful. In addition, based on Pourbaix diagrams, CP “pulls down” the corrosion potential toward more negative potentials, thus bringing the whole reaction to where the atom of the metal remains an atom and not an ion. This way, CP also serves to protect the metal against corrosion.
CP, like any other technology, has its own advantages and disadvantages. The list of these advantages and disadvantages can be too lengthy and we see no need to mention them here. However it is important to note that installing CP systems near metallic objects (above ground or underground) without applying required insulation as well not considering telluric effects on CP systems may actually decrease the workability of such systems. In addition, based on the locations these systems are installed, vandalization may also prove to be a serious matter.
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