above, if it is possible to expect corrosion or immunity to corrosion (in other words, passivity). Existence of some corrosive ions such as, but not limited to, chlorides, can somehow change the domains and thus shift the potentials in which corrosion or passivation can be expected.
One very important aspect of Pourbaix diagrams in addition to enabling us to predict safe and unsafe values of potential and pH with regards to corrosion for a given environment is to put emphasis on what is almost forgotten by a majority of non‐corrosion expert professionals; it is normally assumed that as long as we know about the pH of the environment, it is safe to say if it is corrosive or not! The rule of thumb for these self‐acclaimed corrosionists is that if pH is below 7 the environment is acidic and thus corrosive, if pH is 7 it is neutral, and if it is larger than 7 the environment is basic. This is wrong! In order to interpret correctly, one has to know both corrosion potential and pH. As an example, take the Pourbaix diagram in Figure 2.4 when chloride is present; at an acidic pH = 6 and potential = −0.6 V (red dashed line intersection in Figure 2.4), there is immunity against corrosion, whereas in the same system, but this time for a neutral pH = 7 and potential = −0.4 V, corrosion is highly likely to happen (black dashed line intersection in Figure 2.4). This alone can serve to show how powerful Pourbaix diagrams are in dealing with corrosion and predicting it. However, as noticed by our readers, the restrictions due to temperature do still remain.
Figure 2.4 A typical Pourbaix diagram (simplified) for an Fe–water system at iron concentration of 10–6 mol/l and 25 °C. The region between the two dashed lines is the water stability zone. The two dashed lines, upper and lower, define the domains for oxygen and hydrogen stability; above the upper line water is oxidized to O2, so oxygen is evolved above the upper line. Below the lower line, water decomposes to H2 and thus, hydrogen will be liberated below the hydrogen line. The zones shown indicate the chemical species formed. Red and black dashed lines, show immunity under acidic and corrosion under neutral pH values, respectively, emphasizing the necessity for specifying and studying pH–voltage pair to decide about possible corrosivity scenarios. Hematite: Fe2O3; Magnetite: Fe3O4; Ferric ion: Fe+3; Ferrous ion: Fe+2.
2.2 Important Technical Treatment Strategies for Corrosion Treatment
Corrosion treatment strategies can be categorized into five categories:
1 Design modification‐change/materials 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.
