Right choice is (b) 77
Easy explanation: We know MDD = W/At = (I*am) / (d*n*F). We should convert the given unit values in milli inches per year.
Given, I = 1.69*10^-4 amp/cm^2
Atomic weight of iron = am = 55.86 g/mol
n = number of electrons = 2 [Fe ==> Fe^+2 +2e^–]
F = faraday’s constant = 96500 coulombs
d=density of iron = 7.875 g/cm^3
= [(1.96×10^-4 coulombs/sec*cm^2) × (55.86)] ÷ [(7.874 g/cm^3×2×96500 coulombs)]
= [(1.96×10^-4 coulombs/sec*cm^2) × (55.86)] ÷ [(7.874 g/cm^2*cm×2×96500 coulombs)]
= [(1.96×10^-4 1/sec) × (55.86)] ÷ [(7.874 g/cm ×2×96500)]
= [1.69×10^-4 × (60*60*24*365) × (55.86) × 10^3] / [2 × 96500 × 7.875 × 2.54]
= 77.117 MPY.