1. Basic principle of a heterogeneous gas-solid reaction

1.1 Physico-chemical mechanisms of carbon transfer by a heterogeneous gas-solid reaction

A heterogeneous gas-solid reaction is one involving the transfer of matter between these two phases. Generally speaking, the overall mechanism can be broken down into three main stages:

• transport of gas-phase reactive species to the steel surface;

• the physico-chemical reaction on the steel surface ;

• transport in the solid state by diffusion of atoms produced by the chemical reaction.

To illustrate our point, the reactive species will be a gaseous molecule containing carbon atoms, carried in a gaseous mixture, i.e. a molecule of the type C _...