Studies on the effect of sulphide ions on the inhibition efficiency of chromate on mild steel using radio‐tracer technique

The results of the corrosion studies on mild steel in aqueous solutions containing 0.5 to 100 ppm of S= either in inactive or radioactive Na2S for an immersion period of 24 hours, show that rusting of mild steel surface appeared at the S= concentrations of 5 and 10 ppm, staining of surface at 25 ppm and pitting at 50 and 100 ppm. Under similar conditions, the studies on mild steel in Na2CrO4 (0.0025 M, 0.005 M) solutions containing varying concentrations of inactive or radioactive S= (0.5 to 150 ppm) indicate that rusting/staining/pitting on mild steel surface did not occur even at higher concentrations of S=. In aqueous solutions containing 0.5 to 100 ppm of S= either in radioactive or inactive Na2S, the open circuit potential shifted towards less noble values, whereas in Na2CrO4 (0.0025 M; 0.005 M) solutions containing 0.5 to 150 ppm of inactive or radioactive S= the potential shift was comparatively towards the noble direction. In Na2CrO4 (0.0025 M; 0.005 M) solutions containing 0.5 to 150 ppm of radioactive S= in Na2S, S pick‐up increased linearly with the increase in concentration of S= and the effect of increase in concentration of Na2CrO4 from 0.0025 M to 0.005 M did not show any appreciable increase in S pick‐up. Autoradiographs of the specimens immersed in the above solutions showed the presence of adsorbed S= on the mild steel surface and the linear increase in S pick‐up with the increase in concentration of S= in radioactive Na2S was observed. Mode of Cr pick‐up on mild steel in Na2CrO4 (0.0025 M, 0.00125 M) solutions was also studied for comparison purposes.