Atmospheric Penetration and Condensation

Two phenomena distinguish corrosive gas diffusion into refractories from liquid penetration. One is that the driving force for the former at steady state is the pressure gradient due to consumption of the gas by reaction. On that account, it is reasonable to characterize gas penetration broadly as less rapid in mass terms than that of wetting liquids. The other distinction is that the reaction does not necessarily commence at the hot face. On both accounts, the depth of penetration by gases can be much greater in some circumstances than that by liquids [79-83]. 

A simple isothermal equation for kinetic order of unity is adapted for use in the case of penetration by gas  

h2 hi is the height at which gas consumption begins p i is initial po2 prevails at height h2 and k is a constant. Corrosion of the refractory heat-exchanger packing or checkerwork is intensely concentrated close to height hi, and then falls off in quasi-exponential fashion with increasing h. 

In condensation-corrosion by gases, there are three possible modes of interaction of a gas with the external and internal surfaces of a refractory. First, if the gas is condensable (referred to as a vapor), in the course of penetration of the porosity and moving down the temperature gradient in a wall, the gas may literally condense or liquefy progressively. Second, the gas may condense by virtue of dissolving in the refractory and third, it may condense 

Once condensation has occurred, however, the basic criteria for liquid corrosion apply. Liquid interaction products are the hallmark of corrosive gases. Rapid slabbing is absent in gas-exposed refractories. 

---