Page 74 - Corrosion Engineering Principles and Practice

54 C h a p t e r 4 C o r r o s i o n T h e r m o d y n a m i c s 55

such as ions are the product of the molar concentration and the
activity coefficient of each chemical species (i):
a i = γ i [ ] ≈ [ ] (4.14)
i
i
i
The activity coefficient (γ ) in Eq. (4.14) can be a complex function
i
highly dependent on a multitude of variables often difficult to even
estimate. For this reason it is usually convenient to ignore (γ ) and use
i
the concentration term [i] as an approximation of a .
i
4.4 Thermodynamic Calculations
The present section illustrates how calculations from basic thermody-
namic data can lead to open-circuit cell potential in any condition
of temperature and pressure. Chemical power sources, with the excep-
tion of fuel cells, are all based on the corrosion of a metal connected to
the negative terminal. The aluminum-air power source, that owes its
energy to the corrosion of aluminum in caustic, was chosen for this
example because of the relative simple chemistry.

4.4.1 The Aluminum-Air Power Source
The high electrochemical potential and low equivalent weight of alu-
minum combine to produce a theoretical energy density* of 2.6 kWh/
kg and make it an attractive candidate as an anode material in metal/
air electrochemical cells. The development of aluminum-based cells
dates back to 1855 when M. Hulot described a voltaic cell containing
aluminum with an acid electrolyte. Since then, many attempts to sub-
stitute aluminum for zinc in zinc/carbon and zinc/manganese diox-
ide cells have been reported.
Figure 4.2 shows a general schematic of a typical aluminum-air
system. Tables 4.3 and 4.4, respectively, contain thermodynamic data
FIGURE 4.2 Load
Schematic of
an aluminum-air
power source. e –

Aluminum Air
anode cathode

KOH
electrolyte
* Characteristic parameter of a battery indicating the amount of electrical energy
stored per unit weight or volume.