As a supplement for the former experiment on the influence of added salt on the passivity of chromium steel in sulphuric and hydrochloric acid solutions, the writer carried out the similar experiment for nitric acid solution.
It was ascertained that 10-14 percent chromium steels can be passivified in dilute (say, 5% HNO3) nitric acid solutions containing metallic cation of higher valency, for example, Cu++, Fe+++, but not passivified in solutions containing metallic cation of lower valency or that of non-variable. These relations are quite similar to that in the case of sulphuric and hydrochloric acid solutions.
Next, 10-12 percent chromium steels are spontaneously passivified in dilute nitric acid solution containing no metallic cation of higher valency, or initial rate of attack of the acid gradually falls off, as the duration of the test increases. The passivity thus produced are explained as follows.
(1) In the case of 12 percent chromium steel, the passivity is produced by the oxidizing power of the nitric acid solution, after being etched away the “shattered” metal which is difficult to render passive.
(2) In the case of 10 percent chromium steel, the passivity is due to the oxidizing power of Fe+++ ions which is produced by attack and accumulated around the specimen (if specimen is rotated and Fe+++ ions are scattered, the passivity can not be obtained).