Thermodynamic Degradation Science

8Diffusion

8.1 The Diffusion Process

Diffusion is one of the four main types of aging described in Chapter 1. Diffusion often occurs in mass-transport processes, which occur essentially from three processes: (1) convection and stirring; (2) electrical migration due to an electric field; and (3) diffusion from a concentration gradient. The first two are categorized as being under the control of an external force. This includes subtle process such as corrosion and where mass is transported in the electrolyte solution to and from the electrodes. Of these three processes, diffusion is more of a spontaneous process where external work is not involved. Diffusion can also occur in corrosion and can be a rate-controlling step. This is often the case in “hot corrosion” or “aqueous corrosion” due to oxidation. Furthermore, many aging processes due to diffusion do not involve electrochemical transitions.

For a system that is diffusing into an environment due to a concentration gradient, its energy is related to the chemical potential change. The following equilibrium thermodynamic example clarifies the important role of the chemical potential.

8.2 Example 8.1: Describing Diffusion Using Equilibrium Thermodynamics

Consider a system that can exchange energy with its environment via the diffusion process. This is shown in Figure 8.1 which as n system particles of any type in contact with an environment consisting of n_env particles of another or the same type. The particles could be atoms, ...