Growth Characteristics of Iron Oxide Films Generated in Dilute Lithium Hydroxide Solution at 300°C

Abstract

In the early stages of iron oxide growth on mild steel in pH 11 normalLiOH solution at 300°C, the oxide is largely oriented and grows at a rate and with a morphology dependent on the substrate grain orientation. After a certain film thickness is attained, however, the epitaxial oxide growing in the (001) Fe3O4//false(001false)α‐normalFe , false[1true1¯0false]Fe3O4//false[100false]α‐normalFe orientation degenerates, leaving in its place a much thinner, fine‐grained, randomly oriented “base film” partially covered by large solution‐grown Fe3O4 crystals. Experimental evidence indicates that the transformation is induced by stress generated within the epitaxial oxide as the film thickens. The two‐phased oxide structure which exists after completion of the epitaxial oxide degeneration persists through oxidation periods of up to at least 300 hr. Observations of its growth behavior lead to the postulation of a growth mechanism in which iron passes through the base film into solution, subsequently to precipitate forming the upper surface crystals. It is shown possible, using this hypothesis, to derive a theoretical rate law which predicts that, in agreement with the experimentally determined corrosion rate data, the corrosion rate should decrease in a logarithmic fashion.

The Electrochemistry of Iron in lM Lithium Hydroxide Solution at 22° and 200°C

“…The film is similar to that reported previously by others (13,14) and consists of two layers: a base layer with little apparent structure at a magnification of 1200X and an upper layer of octahedral crystals. The highly developed crystallographic faces suggest growth from solution as suggested by Castle and M a n n (15) and Moore and Jones (16). Similar films were formed on iron electrodes exposed to the same oxidizing conditions and at the stable corrosion potential of ~1250 mV.…”

“…It is interesting to note that Bignold (6) could find no evidence for similar processes on iron in 0.1-10M NaOH at temperatures between 200 ~ and 250~ Cyclic voltammetry studies were carried out at 22 ~ and 200~ using sweep rates of 20, 50, 60, and 150 V/hr. At both temperatures the peak currents were found to increase markedly with sweep rate as has been observed for other systems at 25~ (9,16). Unfortunately, the peak currents were not sufficiently reproducible at room temperature to allow a quantitative determination of the functional dependence of peak current on sweep rate.…”

Using Microscopy to Help with the Understanding of Degradation Mechanisms Observed in Materials of Pressurized Water Reactors

“…Many factors affecting corrosion rate and cracks growth were studied by several authors [74][75][76][77][78][79][80][81][82][83]. Nevertheless, the contribution of the oxide layers themselves remains crucial and is still suffering from a lack of understanding.…”

The Products of the Anodic Oxidation of an Iron Electrode in Alkaline Solution

“…No changes were observed in the x-ray diffraction patterns of the fully discharged electrode surface, after storing the electrode in a covered container of the KOH/LiOH mixed electrolyte for about 20 days. Moreover, no lithium compounds were detected on any of the iron electrodes cycled in the KOH/LiOH electrolyte (21,22).…”