A 10 m thick metal sheet of size 10 cm 10 cm is annealed in a pure nitrogen atmosphere with pN2 = 10 atm,

A 10 μm thick metal sheet of size 10 cm × 10 cm is annealed in a pure nitrogen atmosphere with pN2 = 10 atm, until a uniform concentration of N atoms is established within the sheet. Then the sheet is quenched and subsequently annealed in a nitrogen-free stream of argon for different times until equilibrium is reached. Which equation would you use to describe the concentration profile as a function of time?