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.
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