ACTIVE AND PASSIVE CORROSION STATES IN HIGH-TEMPERATURE AQUEOUS CORROSION OF MILD STEEL

In static high-temperature (300°C) corrosion tests of mild steel in dilute NaOH solutions, both the corrosion rate and morphology of the generated oxide film were observed to vary in a reproducible and presumably cor rectable fashion as the NaOH concentration increased. A transitory corro sion rate minimum was found circa 2 g/1 NaOH; there was at this same point a change in the structure of the oxide film, with a compact, randomly oriented, apparently continuous film giving way to a crystallographic surface oxide as the critical concentration was exceeded. The activation energy for corrosion differed depending on which oxide type prevailed, being 5-8 kcal/ mol for the noncrystallographic oxide and 16 kcal/mol for the crystallographic. The transformation is thought to be basically a transition between passive and active corrosion states. © 1969, The Electrochemical Society, Inc. All rights reserved.