Potter-Mann and Bloom Iron Oxide Films Grown on Mild Steel in 280 C 5N NaOH: Their Significance in Pitting

The formation of iron oxide films on polished mild steel heated to 280 C (536 F) in 5N NaOH has been studied using a small high temperature cell with a working electrode of thin walled tubing to allow hydrogen diffusion measurements. Potter-Mann oxide films formed when the working electrode was connected with the cell wall (heavily oxidized mild steel) during heat up; Bloom films were formed when it was not. Characteristic potential and hydrogen flux behaviors were observed as the two different films grew. Coupled Potter-Mann films gave high flux rates which fell little with time. Permeation rates through the Bloom films were high initially, but decreased rapidly. Potter-Mann films persisted, once initiated, even after the electrodes were uncoupled, but with diminished flux rates. A discussion is given of the ways in which the cell behavior may be equivalent to corrosion pits on metals.