Question

Iron is corroding at a current density of 1.69*10^-4 amp/cm^2. What would be the corrosion rate in MPY?

(a) 423

(b) 77

(c) 473

(d) 97

This question was addressed to me in an internship interview.

This is a very interesting question from Corrosion Rate Expression topic in portion Corrosion Principles of Corrosion Engineering

Answer 1

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.