Effect of Potassium Dichromate on the Passivation of Stainless Steel (AISI 321) in Sulphuric Acid

Abstract

The effect of potassium dichromate on the passivation of stainless steel in sulphuric acid has been investigated. Tests with K₂Cr₂O₇ up to 4N showed that passivation could be achieved in 3·4 N H₂SO₄ containing 0·1 N K₂Cr₂O₇, and that the quality of the passivating film increased with the concentration of K₂Cr₂O₇ and with the duration of the passivation treatment. Although passivity could be induced in these conditions at 30°C, at higher passivation temperatures the film quality diminished as shown by long-term decay studies after transfer of the metal to sulphuric acid solutions containing no dichromate. The metal could be passivated in all concentrations of H₂SO₄. Passivation was also possible in 3·4 N H₂SO₄ containing 0·2 N K₂Cr₂O₇ and up to 5% NaCl. However, the breakdown of the film produced in the presence of NaCl occurred earlier the higher the concentration of NaCl present during passivation.