Page 84 - Corrosion Engineering Principles and Practice

62 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 63

By considering the global expression of the cell chemistry
expressed in Eq. (4.19), one can relate the weight of the active materials
to a certain energy and power. In the present case 12 mol of e are
–
produced by using

4 mol of Al 4 × 26.98 g mol , or 107.92 g
–1
4 mol of OH as KOH 4 × 56.11 g mol , or 224.44 92 g
–1
–
3 mol of O (as air) 0 g
2
Total 332.4 g or 0.3324 kg for 12 mol e –
The theoretical specific capacity is thus 26.80 × 12/0.3324 = 967.5
Ah kg .
–1
Step Four: Calculate the Energy Density (Wh kg )
—1
The energy density can then be obtained by multiplying the specific
capacity obtained from calculating the specific capacity with the
thermodynamic voltage calculated when translating AG into
potentials: 2.74 × 967.5 = 2651 Wh kg , or 2.651 kWh kg if running
–1
–1
on air and, because the voltage for running on pure oxygen is slightly
higher (i.e., 2.78 V), 2.78 × 750.7 = 2087 Wh kg , or 2.087 kWh kg if
–1
–1
running on compressed or cryogenic oxygen.
4.4.3 Reference Electrodes
The thermodynamic equilibrium of any other chemical or
electrochemical reaction can be calculated in the same manner,
provided the basic information is found. Table 4.7 contains the
chemical description of most reference electrodes used in laboratories
and field units, and Tables 4.8 and 4.9, respectively, contain the
thermodynamic data associated with the solid and soluble chemical
species making these electrodes. Table 4.10 presents the results of
the calculations performed to obtain the potential of each electrode
at 60°C.

4.5 Reference Half-Cells (Electrodes)
As mentioned earlier, the standard hydrogen half-cell is rather
awkward to use under most circumstances. The other half-cells most
frequently used in corrosion studies, along with their potentials
relative to the standard hydrogen half-cell, are listed in Table 4.7.
Reference electrodes are commonly used with a saturated solution
and an excess of salt crystals. The extra salt dissolves into the half-cell
solution as some of the ions diffuse out of the reference cell body
through the liquid junction during normal use. This extra buffer of
salt extends the time before the reference cell starts to drift due to the
depletion of ions as predicted by Nernst equation [Eq. (4.12)].