Corrosion Fatigue in Binary Gas Mixtures

It is assumed that (i) both gases are strongly adsorbed on the clean metal surfaces produced by cracking, (ii) chemical adsorption of either gas at a given surface site would preclude further adsorption at that site, (hi) the ratio of partial pressures of the gases at the crack tip is essentially the same as that of the surrounding (external) environment, (iv) no capillary condensation of either gas occurs at the crack tip. 

The subscripts a and i denote the deleterious and inhibitor gases, respectively. The quantities ka,Pa, h, and pi are, respectively, the reaction rate constants and partial pressures of the gases at the crack tip. The coverages 9a and 9i denote the fraction of crack-tip surface that has reacted with the deleterious and inhibitor gases, respectively, with the total coverage 9 = 9a + 9t and 0 6 1. 

Equation (9.12) may be solved straightforwardly to obtain the extent of reaction, or surface coverage, with each gas as follows  

The surface coverage by the deleterious gas (9a) relative to the total surface coverage is given by solving Eqn. (9.13), and is given by Eqn. (9.14) [3]  

If the combination of total pressure of the gas mixture at the crack tip (pm = Pa + Pi) and the cyclic loading frequency (f), namely, Pm/f, is such that the reactions with the newly exposed crack surface are completed, then 0 = 1 and 0 achieves its maximum value 9am, namely, 

---