Magnetite scale

Reaction of iron containing a discontinuous magnetite scale with oxygenated water, crevice corrosion, water-line attack, long-line corrosion of buried iron pipes, etc. — separable A/C type. 

Even the presence of magnetite scale that often covers the surface of the bars, which can cause dangerous localized attack on steel in contact with neutral solutions (such as fresh water or seawater), is not dangerous in concrete. In fact, non-carbonated and chloride-free concrete passivates all the surface of the steel. If adherent oxide films are present, they do not create problems. If the oxide layer contains chlorides, because for example it is formed in a marine environment, it must be removed completely because it can hinder passivation. 

These terms are evidently ambiguous for example, it is not always clear whether wet is confined to aqueous solutions—the wetting of solids by mercury indicates that liquid-metal corrosion should be classified as wet . Even if the term is restricted to aqueous solutions, the difficulty arises that the mechanism of growth of magnetite scale during the reaction of the interior of a boiler drum with dilute caustic soda at high temperatures and pressures is best interpreted in terms of a dry corrosion process. Similar considerations apply to the reactions of aluminium and zirconium with high-temperature water. 

Stresses in boiler walls can cause the magnetite scale to spall. In the process the small exposed areas of the steel surface become anodic with respect to the large cath-odically acting undamaged surfaces so that rapid corrosion of the unprotected surface is likely to occur [53]. 

Opinions vary concerning the observed pH value (set at room temperature) in boiler water. There is consensus that the pH value should never lie below 7 since then the magnetite scale will be attacked. In practice and research a pH value of 9.6 is seen as optimum. However, from other sources a pH value of 10.5 is considered as a prerequisite that a protective, adherent magnetite scale exists and stress corrosion cracking does not occur [55]. 

To apply the finite-difference approach mentioned above to oxidation processes of Cr-containing steels, it has to be taken into account that, generally, the oxide scale consists of three or more separate layers, e.g., in the case of low-Cr steels an outer magnetite scale (below an outermost hematite layer) on top of an inward-growing magnetite/spinel-phase scale and an intercrystalline oxidation zone below the scale/substrate interface. 

The oxidation behavior of steels depends strongly on the Cr concentration. While the magnetite scale formed on low-alloy steels provides only moderate... 

M. Ueda, K. Kawamura and T. Maruyama, Void formation in magnetite scale formed on iron at 823 K - elucidation by chemical potential distribution . Mater. Sci. Forum 522-523, 37 4 (2006). 

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