Question

Explain Passivity of iron

Answer 1

When a piece of iron is brought in contact with dilute sulphuric acid, a vigorous reaction begins and hydrogen gas starts coming out. The same piece of iron will displace copper from a solution of copper sulphate. It is noticed, however, that if the iron piece is kept for some time in conc. HNO3, and then washed free from nitric acid and allowed to react in the above manner, it can neither liberate hydrogen from H2SO4 nor precipitate copper from copper sulphate solution. In other words, by treatment with nitric acid iron has so changed that it would not behave in the same manner as any other untreated piece of iron would do. Iron in this form is said to be passive, i.e., it shows lesser reactivity than it would normally show.