Effect of temperature on oxide film morphology for steel exposed to 3.5 M sodium hydroxide

Mild steel in contact with sodium hydroxide solutions develops surface films (including a passive oxide layer) that offer good corrosion protection. This is particularly important in the alumina industry where mild steel heat exchanger tubes are in contact with hot Bayer liquor (a mixture of dissolved alumina and sodium hydroxide).There is evidence in the literature that temperature affects the film morphology and that this, in turn, affects the corrosion of the mild steel in contact with the Bayer liquor (Lu, Q., et al, Materials and Corrosion, 51 (2000) p705-711). Such evidence has been limited to studies at only a few discrete temperatures however heat exchanger tubes in refineries are often subjected to a range of temperatures for process reasons. In this paper we report results of a systematic study on the qualitative effect of temperature on heater tube steel morphology for samples exposed to free caustic soda and Bayer liquor. Scanning electron microscopy (SEM) was used to observe changes to the film surface.We have found that the surface of A179 mild steel in 3.5 M NaOH is consistent with duplex oxide layer formation over the temperature range of 100 oC to 220 oC. The surface appears unstable at approximately 140 oC. At 220 oC a small amount of pitting was observed even though the oxide coating appeared compact and uniform. Pitting was not observed for samples exposed to spent Bayer liquor at about the same temperature.