where erfc is the complementary error function. This is the case when corrosive gases diffuse through the oxidative layer towards the metal surface (if we assume that concentration of gases in the environment is constant and the diffusion space – that is, the corrosion product layer – is semi-infinite, starting at 0 at the surface and spreading infinitely deep in the material). If, in its turn, the diffusion space is infinite (lasting both through the layer with n (x, 0) = 0, x > 0 and that with n (x, 0) = n 0, x ≤ 0), then the solution is amended only with coefficient ⁠ 1 / 2 ⁠ in front of n 0 (as the diffusion now occurs in both directions). This case is valid when some solution with concentration n 0 is put in contact with a layer of pure solvent. (Bokstein, 2005) The length 2 √ Dt is called the diffusion length and provides a measure of how far the concentration has propagated in the x -direction by diffusion in time t (Bird, 1976).