Nitridized steel is hard yet brittle, and this means that the bulk of a steel object must remain free of nitrogen. Assuming D = 10⁻⁹ cm²/s for interstitial diffusion of nitrogen in steel at 700 °C, estimate the minimum thickness of the above steel blade such that the nitrogen concentration at its center would increase by no more than 0.005 of the surface change under 1 (alternatively 25) hour(s) of nitridization. Assume for simplicity that the concentration at the center is twice the concentration that would be caused by nitridization from one side only (in reality it will be less than twice due to decreasing gradient after the two fluxes penetrate each other).