6. (a) State the difference(s) between surface oxidation and corrosion of metals. (b) Describe the Pilling-Bedworth ratio. (c) Calculate the Pilling-Bedworth ratio for tin (Sn) having a density of 7.30 g/cm. The oxide film formed on the surface of this metal is SnO2 having a density of 6.95 g/cm. The atomic mass of tin is 118.7 g/mol and the molecular mass of SnO2 150.7 g/mol. (d) Discuss whether the oxide scale formed will be protective or not. Provide justification to your answer.