Practical nobility

Factors Involved in Galvanic Corrosion. Emf series and practical nobility of metals and metalloids. The emf. series is a list of half-cell potentials proportional to the free energy changes of the corresponding reversible half-cell reactions for standard state of unit activity with respect to the standard hydrogen electrode (SHE). This is also known as Nernst scale of solution potentials since it allows to classification of the metals in order of nobility according to the value of the equilibrium potential of their reaction of dissolution in the standard state (1 g ion/1). This thermodynamic nobility can differ from practical nobility due to the formation of a passive layer and electrochemical kinetics. 

To avoid the galvanic coupling between different materials, especially when the electrical conductivity of the solutions is high. A higher conductivity corresponds to a higher extent of the cathodic area related to the same anodic area or vice versa. Sometimes, because of differences in degradability, it may be appropriate to use bolts and/or weld material more noble in comparison to the materials that are connected, but we must keep in mind that the electrochemical scale of nobility is not always that of seawater but depends on the environment and the pollutants. Copper is normally more noble than iron, but in the presence of sulfides, the practical nobility of the two materials can be virtually coincident, while in the presence of ammonia, the nobility of copper may be lower than that of iron. Stainless steel is usually more noble than carbon steel, but in aerated alkaline enviromnents, the order of nobility can be inverted. 

It is interesting to compare the order of nobility of magnesium with some other current metals such as Al, Ti and Fe. Figure 2.2 gives AN, i.e. expresses the difference between the practical nobility of 9 metals and the... 

Comparison of AN (practical nobility - thermodynamic nobility) of Mg and that of eight other elements Au, Cu, Fe, Al, Zn, Mn, Zr and Ti as deduced from Pourbaix -pH diagrams (Ghali, 2010). (Positive AN indicates better possible projected performance due to passivity.)... 

The wide range of corrosion-resistant nickel alloys that are produced commercially is capable in practice of handling most types of acid. Since the nickel-alloy range includes some that are corrosion resistant by virtue of their relative nobility and others that owe their resistance to passivity, alloys suitable both for hydrogen-evolving acids and for more oxidising acids are available. Table 4.27 contains a summary of data mainly derived from laboratory corrosion tests to illustrate the behaviour of individual alloys in some common mineral and organic acids. 

Each metal or metal area will develop an electrode with a measurable potential. This potential can be referenced to that of a standard hydrogen electrode, which, by convention, is set at zero. Thus all metals have either a higher or lower potential compared with hydrogen, and a comparative list of metals can be produced indicating their relative nobility (galvanic or electrochemical series). The real potential of various metals and their alloys can, under practical operating conditions, be considerably different from their standard potential under ideal conditions. 

The order of nobility observed in actual practice may differ from that predicted thermodynamically. The reasons are that some metals become covered with a passivating film of reaction products which protects the metal from further attack. The dissolution reaction may be strongly irreversible so that a potential barrier must be overcome. In this case, corrosion may be inhibited even though it remains energetically favorable. The kinetics of corrosion reactions are not determined by the thermodynamics alone. 

---