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 is 25 °C.
4 Hydrogen pressure in the reference electrode is one atmosphere.Figure 2.2 Schematic presentation of electrochemical series with some reactions as cathodic and anodic reactions. Standard potentials (Eo) are in volts vs. SHE (standard hydrogen electrode).
Figure 2.2 schematically shows an example of an electrochemical series:
However, it is evident that items ii–iv can only be achieved under strict laboratory conditions and under real life, industrial conditions it is not possible to main temperatures and pressures as required by the electrochemical series. It can be said that it is mainly due to these restrictions as dictated by laboratory‐controlled conditions that industrial application of electrochemical series must be replaced with a more application‐friendly option. Although, as we see later, standard hydrogen potential is a necessary element in constructing Pourbaix diagrams, which are very useful in applications.
2.1.2.2 Galvanic Series
Due to limitations of hydrogen electrode measurement for constructing electrochemical series, another option has been applied which is called “electrochemical series.” The potentials used in a galvanic series are only valid in:
1 a given environment (electrolyte), and
2 at a given temperature (25 °C).
Temperatures can be corrected for any given environment, but the importance is to know what environment we are talking about. An example of a galvanic series for seawater at 25 °C can be seen in Figure 2.3.
As it can be seen from the figure, some reactions are still considered as cathodic and anodic; mainly those sitting near to the top of the galvanic series are considered noble (cathodic) and those ranked below are considered active (anodic). While the range of reactions for constructing a galvanic series for seawater at 25 °C is not limited to the few examples given in Figure 2.3, it is obvious that due to environment–specific property of galvanic series, these reactions hold true only for the electrolyte, that is to say, the environment for which they have been constructed.
It is also evident that we still have the restrictions imposed by the specific temperature that must be maintained in ordering anodic and cathodic reactions for the given environment. Furthermore, what is to be noticed with regards to the galvanic series given in Figure 2.3 the same as electrochemical series ranking, any reaction which is placed above another reaction is regarded cathodic to that reaction. For instance, while in Figure 2.2, aluminum reaction was more cathodic with respects to magnesium, in Figure 2.3, bronzes are regarded more anodic with regards to copper–nickel (70–30). It follows that while electrochemical and galvanic series may differ in many respects, it is still the ranking of a particular reaction with regards to the other one that determines if it is noble (cathode) or active (anode).
Figure 2.3 Some examples of active and passive metals in seawater at 25 °C for the specific environment seawater.
As we mentioned earlier, although galvanic series seem to be more practical than electrochemical series, there are still some limitations. Being environment‐ and temperature‐specific is an inescapable limitation on all chemical reactions as they do happen under certain conditions, the most important of which are the electrolyte (that is, the environment), the temperature (as chemical reactions are kinetics highly dependent on temperature), and of course, pressure. However, there is yet another measure that can assist us in predicting corrosion behavior of metallic alloys and it was invented by a PhD student some decades ago, named Marcel Pourbaix.2
2.1.2.3 Pourbaix Diagrams
An example of a Pourbaix diagram is shown in Figure 2.4.
Pourbaix diagrams have two axes, one for corrosion potential as measured in hydrogen potential, and one for measuring acidity of the environment shown as pH. For a given set of potential‐pH‐environment, some “domains” will be created. These domains can be used to predict,
