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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 + Cr2O3This question was posed to me in class test.My question comes from Corrosion Principles in division Corrosion Principles of Corrosion Engineering

Answer»

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.



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