Corrosion Policy Decision Making. Группа авторов
We would not allow our readers to be, literally, buried under formulas, diagrams, and the like. We want to be straight‐forward and write frankly!
2.1.1 Essentials of Electrochemical Corrosion
If corrosion is a natural process by which energy of the system is lowered, then any approach toward corrosion must be the same when facing any natural process. The main thing here, however, is that as long as corrosion has not been allowed to make a hole in an asset—the process we call through‐wall pitting—it is safe. This approach may sound quite different from what we have been taught so far; any manual on CM starts with the unwritten assumption that the job for a corrosion engineer is to prevent corrosion. In fact, the job of a corrosion engineer—and for that matter an integrity management expert—is to prevent through‐wall pitting occurrence as manifested by severe, localized corrosion.
Figure 2.1 shows an example of two assets that have been severely corroded through localized corrosion. In both of the figures there are three elements of corrosion that can be recognized. The anode is the spot at which anodic reaction occurs, and in cases similar to those shown in Figure 2.1, it is where sever pitting in the form of severe through‐wall pitting happens. At the anode, electrons are being released and travel via the metallic material body toward the cathode, the cathode could be adjacent to the anode. However, the point is that in localized corrosion, contrary to uniform corrosion, anode and cathode spots do not move relative to each other, so that the anode is fixed and from the very spot the anode becomes fixed, pitting starts. It is also due to this feature that localized corrosion happens in one particular spot on the metallic structure. This may also be the main characteristic of localized corrosion that makes its prediction almost impossible; one cannot know where that attack will occur and with what severity, normally measured in terms of corrosion rate and expressed by pitting factor, it can occur. The moisture on the metallic surface that is basically the water vapor already available in the air will be the main electrolyte through which ion exchange can happen.
It is quite understandable if these very concepts that form electrochemical corrosion—or more generally referred to as aqueous corrosion—leave a lot of questions for the readers. For example, from a spatial point of view, while the position of anode is visible how can we decide where exactly the cathode adjacent to it is? Is it on the left side of the anode? Its right side? Where is it located? Another matter is how do we know if the anode and cathode “creep” relative to each other on a metallic surface? And, for that matter, how do we know if one particular spot and not another spot on the metal will become the anode?
Figure 2.1 Two examples of severely corroded equipment leading into a through‐wall localized failure. The anode is the point at which perforation has occurred (white arrow) and the cathode is immediately adjacent to it so that electron flow happens via the metallic surface (orange arrow), while ionic transfer happens through the moisture layer already formed on the metal.
(Source: Taken from the collection of Dr. R. Javaherdashti.)
The above questions are actually some of fundamental importance should we be writing this book as a textbook. There are numerous books and papers that may become handy in answering these questions and even more puzzling ones. For us, however, the above questions carry no value at all. From a practical point of view, all we need to care about are the following five basic concepts:
1 At anode, anodic reactions happen. These reactions involve emitting the extra electron given to the metal through extractive metallurgy processes.
2 These electrons will be taken by the cathode. In other words, electrons emitted by the anode will be gained at the cathode, leading to the establishment of an electron flow (electricity) between anode and cathode.
3 The electron flow constitutes what we refer to as “current.” The current thus produced is in fact the way by which voltage difference can be leveled between anode and cathode.
4 The moisture already established on the metal's surface acts as a carrier medium for the exchange of ions thus produced as a result of anodic and cathodic reactions. In other words, the electron emittance and electron gaining will serve to produce anion and cation ions which can be carried through the moisture layer (the electrolyte).
5 It follows, then, contrary to what textbooks on corrosion try to impose on our understanding saying that there need to be four elements (the anode, the cathode, the electrolyte, and the metallic path) for a electrochemical corrosion scenario to exist, there are actually only three elements necessary; these are the anode, the cathode, and the electrolyte. As corrosion is already happening on a metallic asset, we really do not require to make more confusion that that is already existing in the electrochemistry of corrosion. As some may already know, we may name a few of these confusions; one can name the concept of using the wrong term of oxidation instead of anodic reaction, the polarity signs of anode and cathode, the arguable term of under deposit corrosion, and even the term biofilm to explain the venue at which electrochemical cells are established.1
The above fundamental concepts explain both the science required to explain how corrosion occurs as well as the science needed to explain its prediction and treatment, as we will see in the next sections.
2.1.2 Prediction of Corrosion
We obviously need to know when corrosion happens, as this will enable us to prepare ourselves with the upcoming event and try to manage it as much as possible, in the best way achievable. As we have mentioned earlier, this is not a textbook about corrosion, and therefore many of the concepts that would have been covered in length in those books will be treated in the most practical approach possible to make these concepts as understandable for a corrosion specialist as possible.
There are mainly three tools that can be used to help us predict corrosion, in other words, to let us determine anodic and cathodic reactions and therefore find out about the possibility of corrosion.
These tools are:
1 standard hydrogen electrode scale (SHE)
2 galvanic series
3 Roubaix diagrams
By using these tools, it is possible to find out what chemical reactions can be considered as anodic and therefore giving off their extra electrons, and which reactions can be cathodic reactions to receive and gain those electrons. Obviously, the substrate on which these reactions will take place is a metallic surface (to allow electron transfer), and the environment will be water or water containing (the necessary electrolyte for ionic transfer).
2.1.2.1 Standard Hydrogen Electrode/Electrochemical Series
The main assumptions that need to be made are as follows.
1 It is assumed that the reduction voltage for hydrogen reaction is zero. In other words, it is assumed that the conversion of hydrogen ion (H+) to hydrogen gas is zero. Needless to say that the actual voltage is not zero at all, but this assumption is accepted so that some chemical reactions can be placed above hydrogen evolution (due to their positive reduction voltage), and some reactions will be placed below that due to their negative potential. Those with positive potential will be noble, meaning that they are very highly likely to act as a cathode and those with negative potentials will be branded as active, meaning that they are very highly likely to act as an anode.
2 All of the substances taking part in the electrode reaction have unit activity.
3 Temperature