(b) To protect an iron (Fe) rod of radius of 20 cm from being oxidized, a layer of chromium (Cr) is coated. Suppose the oxidation rate constant for Cr and Fe is respectively 2.510-8 cm/sec and 2x10-6 cm/sec. It is assumed that the oxidation rate constant (K) of iron under chromium oxide coating remains the same as it is in direct contact with air. Determine what is the minimum thickness of Cr coating is needed to protect Fe for 5 years (365 days per year) from oxidation up to a maximum thickness of 2 cm (i.e., the maximum thickness of Fe2O3 allowed is 2 cm)? Assume negligible atomic density change during the oxidation process. (8 points)