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, under the given conditions for the three parameters mentioned 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