)nitrogen from a gaseous phase is to be diffused into pure iron at 675°c. if the surface concentration is maintained at 0.2 wt% n, what will be the concentration 2 mm from the surface after 25 h? the diffusion coefficient for nitrogen in iron at 675°c is 2.8 × 10–11 m2/

0.0895 wt%

Explanation:

To treat a diffusive process in function of time and distance we need to solve  2nd Ficks Law. This a partial differential equation, with certain condition the solution looks like this:

Where Cs is the concentration in the surface of the solid

Cx is the concentration at certain deep X

Co is the initial concentration of solute in the solid

and erf is the error function

First we need to solve the x/2sqrt(D*t) on the right to search the corresponding value later on a table.

We look on a table and we see for z=0.42 erf(z)=0.5525

Then we solve for Cx

Assuming Co=0