A sheet of steel 1.5 mm thick has nitrogen atmospheres

A sheet of steel 1.5 mm thick has nitrogen atmospheres on both sides at 1200(C and is permitted to achieve a steady-state diffusion condition. The diffusion coefficient for nitrogen in steel at this temperature is 6 ( 10-11 m2/s, and the diffusion flux is found to be 1.2 ( 10-7 kg/m2-s. Also, it is known that the concentration of nitrogen in the steel at the high-pressure surface is 4 kg/m3. How far into the sheet from this high-pressure side will the concentration be 2.0 kg/m3? Assume a linear concentration profile.