Hyperfine Interactions 139/140: 239–244, 2002. © 2002 Kluwer Academic Publishers. Printed in the Netherlands. 239 Corrosion Products Formed on Mild Steel Samples Submerged in Various Aqueous Solutions F. B. WAANDERS ⋆ , S. W. VORSTER and G. J. OLIVIER School of Chemical and Minerals Engineering, PU for CHE, Potchefstroom, 2531, South Africa; e-mail: chifbw@puknet.puk.ac.za Abstract. Corroded samples, from the steel shell of an industrial evaporator system were inves- tigated. A protective magnetite layer had formed, which subsequently dissolved in localised areas, resulting in failure of the shell. To clarify the mechanisms involved, mild steel samples of similar composition to the steel shell were submerged in the condensate and experiments were performed at room temperature and at 90 ◦ C under both static and dynamic conditions for exposure times up to 30 days. Control samples were submerged in deionised water under similar conditions. The dynamic corrosion rates in the industrial condensate were a factor of 2 higher than the rates for the deionised water, whilst static corrosion rates, measured in both media, were lower by a factor of 3 to 4 found for the dynamic experiments. The corrosion products were identified by means of CEMS analyses. The main components were magnetite and oxyhydroxides of iron. Additional to the species mentioned, γ -Fe 2 O 3 , goethite and hematite formed. Key words: corrosion, corrosion rates, mild steel, oxyhydroxides of iron, iron oxides. 1. Introduction The resistance to the corrosion of metals is strongly affected by the composition of passive layers, which may form on the metal surface. Pourbaix diagrams are useful idealised representations of corrosion and passivity domains in pure metals [1]. In practice, however, corrosion reactions can lead to unexpected compounds on alloys exposed to industrial media. Characterisation of the passive film composition is therefore frequently required to clarify corrosion and protection mechanisms. The oxides Fe 3 O 4 and Fe 2 O 3 , the hydrated oxides and the oxyhydroxides of iron are the typical corrosion layers that form on pure iron [2, 3]. In static conditions corrosion products consist mainly of Fe 2 O 3 and FeOOH [3]. These products stay in suspension and dissolved oxygen can easily reach the pure metal surface. In dynamic conditions however, the main product of corrosion is Fe 3 O 4 . The corro- sion products sticking tightly to the inside and around pits will suppress dissolved oxygen from diffusion, resulting in the formation of Fe 3 O 4 [3]. Adhesion loss and a decrease in density of the passivating layer makes diffusion of metallic ions through this layer easier, thus frustrating the passivation process. ⋆ Corresponding author.