Thermodynamic 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. 

The Nemst scale of solution potentials permits the metals to be classified in order of thermodynamic nobility according to the value of the equilibrium... 

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.)... 

Deposition of the first ML of CdTe was straightforward the problem with development of an electrochemical ALE cycle for CdTe was forming the second ML of CdTe. UPD of a less noble element on the UPD of a more noble element appears to work well. However, in the second cycle, use of the same conditions for UPD of the more noble element is a thermodynamically untenable arrangement, as it must form on the deposit of the less noble element (Fig. 9). Figures 9A and B are Pourbaix diagrams for Cd and Te, respectively, and the nobility of Te relative to Cd is evident. 

Only the + 2 state of cobalt has thermodynamic stability in acid solution. The instability of Co3+ is referred to in Section 5.3. Only the + 3 states of Rh and Ir are stable in acid solution their +3/0 standard reduction potentials are quite positive, consistent with their nobility . In alkaline solutions the + 2 and + 3 states of the elements exist as insoluble hydroxides. 

The electrochemical series (Table 8.3) gives thermodynamic information on the so-called nobility of various metals the higher the standard electrode potential, the more noble is the metal, silver being more noble than Cu and Cu being more noble than Zn. 

The Pourbaix diagram for the copper/water system is shown in Fig. 2.15. The more positive standard electrode potential of copper (+337 mV (SHE)) as compared to iron (-440 mV (SHE)) is evident. This greater nobility results in copper being thermodynamically stable in water that is, line 14 (-6) representing aCu2+ = 10-6 lies above line a. 

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. 

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