On such occasions, the other four corrosion treatment strategies may not applicable all the time (such as installing cathodic protection [CP]). However limited the number of industries that use corrosion allowance, it is still an especially important option for a better corrosion treatment approach.
Materials selection is a very expensive option to control or prevent corrosion (see Chapter 3 to understand the difference between corrosion control and corrosion prevention). In this option, the existing material of an asset is replaced with a more corrosion‐resistant material so that the overall service life of the asset is increased. Figure 2.5 shows an example of materials selection for upstream oil and gas industry:
Figure 2.5 Materials selection chart for upstream exploration oil and gas industry.
While materials selection sounds like a viable option in treating corrosion by replacing less corrosion resistant materials with more resistant ones, one incredibly significant point to remember is its feasibility. In other words, while project managers may accept the superiority of the selected material over the existing material from a corrosion treatment strategy point of view, an important drawback could be the extra cost imposed by this option to the overall cost of the project. One has to bear in mind that the cost is not limited to the cost of the selected material per se, it is the person‐hours needed to replace and operate, and the downtime between these steps. Say that we have decided to replace a carbon steel part with a stainless steel part. The price difference for January 2021 has been shown in Figure 2.6, where the overall difference is about 40% per mass of the steel selected. This can be translated to huge extra costs for shifting from carbon steel to stainless steel.
The issue will even become more considerable for projects that are being carried out in countries whose currency is not US dollars. When translated into their national currency, the imposed cost may become more eye catching, rendering it more likely to get negative responses to finance a materials selection option. Therefore, it is of vital importance to convince the management not only in terms of corrosion treatment‐related terms but also in terms of the service life extension via an economic analysis such as life cycle cost analysis (we have briefly explained this and other models in Chapter 3 [Smart Corrosion Management Elements] and in more details related to economy in Chapter 4 [A Short Review of Some Fundamental Principles of Economics]).
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
