Corrosion Policy Decision Making. Группа авторов
selection (change and/or modification of design including change/upgrading of materials).
2 Chemical treatment; use of various kinds of corrosion inhibitors (anodic inhibitors, cathodic inhibitors and mixed effect inhibitors, biocides either as oxidizing biocides or non‐oxidizing biocides).
3 Electrical treatment (the most noticeable example in this context is cathodic protection).
4 Mechanical treatment (pigging).
5 Physical treatment (paints/coatings).
As we can see, the corrosion treatment strategies and techniques are indeed very limited in number. However, depending on the corrosion process in focus, there can exist some alterations. For example, if the corrosion case happens to be MIC, then yet another option can be added; the biological treatment of MIC. These alterations do not mean that the essential five strategies cannot be applied. For instance, in the case of MIC, physical treatment as well as electrical and mechanical treatment may also be applicable. The alterations are only applicable where, due to the very nature of the corrosion process, more treatment options may be made available.
Below we will briefly review these five treatment strategies. Once again, we should remind our readers that neither this chapter nor the book itself is about teaching corrosion science and the electrochemistry behind it in the way that many famous handbooks and textbooks do. Our main focus here is on knowing as much necessary for an engineer who happens to become responsible for developing a CM strategy and who may need to know some basic, important elements of the science of corrosion when trying to differentiate between CM various models. As we have described in full in Chapter 3 under the title of “Smart Corrosion Management Elements,” having an efficient, realistic, smart model for CM is an integral part of any strategy that seriously considers management of corrosion.
2.2.1 Design Modification‐change/Materials Selection
Contrary to what many engineers and operators may think, the as‐is design of assets is not the last word when it comes to CM of the asset. In fact, many factors during fabrication, welding, coating, testing, and operation may become the points at which corrosion can be invited. For instance, it is usually recommended to use continuous welding instead of spot welding to avoid problems such as moisture entrance, or having as few branches as possible in a pipeline to avoid stagnation points. In addition, it is highly likely that MIC problems can occur in post‐hydrostatic testing of pipelines, which is mainly applied to test the welding strength (contrary to pneumatic test which is essentially a leak test, hydrotest is both a strength and leak test).
Yet another issue that may arise is to have formed galvanic cells by joining two metals that each can have different electrochemical properties (such as welding together the same metals with and without coating, or coupling dissimilar metals to each other so that in addition to creating a potential difference, anode/cathode area ratios less than one will be created). Operation conditions may lead into initiation and development of MIC in the asset, an example can be seen for example, in underground firewater rings or within reverse osmosis membranes systems. Examples are numerous.
All that calls for being constantly alert to consider the option for a design change‐modification. In this context, design modification means that the existing design is altered with minimum costs, whereas change may mean complete make‐over, which would of course bring many costs.
The policy is that before opting for design change‐modification/materials selection, do a thorough feasibility study to compare this option with the economy and possible outcome with the other four options (when applicable), and then decide if you want to continue with it or leave it, either temporarily or permanently.
Yet another strategy in this respect could be increasing the thickness of the metal. This technique is technically referred to as creating “Corrosion Allowance.” As the very name implies, the method is essentially based on increasing the thickness of the metallic asset. This way we actually “allow” corrosion to work its way through a certain thickness of the asset before too much corrosion of the thickness occurs. It is important to note that corrosion is essentially effective on the thickness, as it is this dimension that on a larger scale carries the stress imposed on the material. Corrosion allowance is applied not to treat corrosion but to delay it. In addition, this method can be taken as a practically useful measure should the form of corrosion be uniform. However, as it may occur with many corrosion process types, localized corrosion in the formation of pits leads to deepening of these pits and results in through‐wall leak, and failure of coalescence of the pits leads to cracks and thus lowers the threshold of tolerating applied stress that can occur. Corrosion allowance is not widely used except in occasions such as submarine (deep and ultra‐deep) pipelines and pressure vessels.3 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]).