Question

Which of the following metal-acid combination will shift metal from active to passive state with an increase in oxidizer concentration?

(a) Monel in HCl+O2

(b) 18Cr-8Ni in H2SO4+Fe^+3

(c) Fe in H2O+O2

(d) 18Cr-8Ni in HNO3 + Cr2O3

This question was posed to me in class test.

My question comes from Corrosion Principles in division Corrosion Principles of Corrosion Engineering

Answer 1

Correct option is (b) 18Cr-8Ni in H2SO4+Fe^+3

Best explanation: Initially corrosion rate of Stainless steel (18Cr – 8Ni) in sulfuric acid increases with an increase in the concentration of ferric ion (Fe^+3). And then it is followed by a rapid decrease in corrosion rate due to the formation of a passive layer. Critical current density (ic) must be achieved to undergo the transition from active to passive.