SEM Study of the Corrosion Products of Galvanized Reinforcements Immersed in Solutions in the pH Range 12·6 to 13·6

Abstract

In previous work on galvanized reinforcements in contact with Ca-containing highly alkaline media the authors have reported the existence of a threshold pH below which the Zn in contact with such a medium is passivated, but at pH values above this it corrodes continuously until it totally disappears. In this paper, after a brief literature review concerning the identification of the corrosion products in these media, they report that the passivation detectedfrom the data of the polarisation technique agreed with the results of an SEM study. This showed that below the threshold pH value the surface is covered with a continuous layer of calcium hydr.oxyzinccite, Ca (Zn(OH)₃)₂.2H₂O, and above this pH this layer is increasingly composed ofnon-passivating ZnO, to the detriment of the calcium hydroxyzincate content. They have also established that it is the (Ca²⁺) ions that control the amount of calcium hydroxyzincate formed, and therefore the likelihood of the formation of a continuous passivating layer. Finally, they report that if the calcium hydroxyzincate isperfectly developed, a later increase of thepH does not affect the stability.