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Corrosion Policy Decision Making


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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).

      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).

      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).

Schematic illustration of some examples of active and passive metals in seawater at 25 °C for the specific environment seawater.

      2.1.2.3 Pourbaix Diagrams

      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.

Schematic illustration of a typical Pourbaix diagram (simplified) for an Fe–water system at iron concentration of 10–6 mol/l and 25 °C.

      Corrosion treatment strategies can be categorized into five categories:

      1 Design