Estudio por SEM de los productos de corrosión de armaduras galvanizadas sumergidas en disoluciones de pH comprendido entre 12,6 y 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 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 detected from the data of the polarisation technique agreed with the results of an SEM study. They show that below the threshold pH value the surface is covered with a continuous layer of calcium hydroxyzincate, Ca[Zn(OH)₃]₂.2H₂O, and above this pH, the layer increase its proportion of non-passivating ZnO, to the detriment of the calcium hydroxyzincate contents. They have also established that it is the (Ca²⁺) ions that control the amount of calcium hydroxizincate formed, and therefore the likelihood of the formation of a continuous passivating layer. Finally, they report that if the calcium hydroxyzincate is perfectly developed, a later increase of the pH does not affect the stability.