ABSTRACT
It is normally expected that materials will corrode, and thus it is important to know the kinetics of the reaction so that predictions of service life can be made. Thus the most important parameter of corrosion from the engineering viewpoint is the reaction rate. Systems can often exist for extended periods of time in a state that is not the equilibrium state or the state of lowest free energy. These states are called metastable states and may occur for many reasons. One case is where a surface reaction forms a diffusion barrier that blocks or drastically diminishes further reaction. In another more important case, for the reaction to proceed to the lowest free energy state it must first pass through an intermediate state where the energy is higher than either the initial or final states. The energy required to overcome this barrier is called the activation energy (Q) and the net energy released is the heat of reaction (H). This is depicted in Figure 7.1 where the movement of an atom from an initial metastable state (a) to the final stable state (c) requires passage through the higher energy unstable state (b). The reaction is exothermic in going from (a) to (c) and endothermic in the reverse direction. The activation energy for the reverse direction obviously must be greater than for the forward direction. The speed of the reaction is dependent upon the total number of atoms in the metastable state, the vibration frequency of the atoms, and the probability that an atom during vibration will have the necessary energy to overcome the barrier. If sufficient energy is not acquired to overcome the activation energy barrier, the system will remain indefinitely in the metastable state. The number of atoms that pass over the barrier is then the rate of the reaction and is given by:
Reaction rate = Ae-Q/RT (7.1)
where A is a constant containing the frequency term and Q is the activation energy. Expressing this equation in logarithmic form one obtains:
ln (rate) = ln A – (Q/R)/T (7.2)
A plot of ln (rate) versus reciprocal temperature yields Q/R as the slope and the intercept at 1/T = 0 yields A.
