We have noted that the Helmholtz free energy equated to work at constant temperature (*T*) and sometimes we also used constant volume (*V*) (see Equation (2.99)). Similarly, for constant temperature and pressure (*P*) corrosion processes, the change in the Gibbs free energy *G* bounds the ability to do useful work (see Section 2.11.3).

In detail, the change in the Gibbs free energy is given by

From the first law we have that the internal energy is the heat minus any type of work, which we describe as compressive and non-compressive:

Then substituting this equation into Δ*G* for quasistatic isothermal processes
we can write as in Equation (2.108)

Therefore, the Gibbs free energy bounds the work process. In actuality, for many corrosion processes we will be able to determine the corrosion damage from the thermodynamic work done as they are reasonably quasistatic, so that

The equality ...

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