Corrosion of galvanized steel reinforcements in alkaline solutions: Part 2: SEM study and identification of corrosion products

Abstract

A report is presented of the morphology and composition of the corrosion products formed on galvanized corrugated steel reinforcing bars immersed in $0\cdot 05-1\cdot 5M~~KOH$ solutions designed to simulate concrete pore environments. The morphology was studied by SEM of the in situ corrosion products and optical examination of transverse sections of the bar; phase identification was performed by X-ray diffraction and IR absorption analyses. Passivation events on the corrosion potential and corrosion intensity curves can be related to precipitation of prismatic $Zn~~(OH)_{2}$ crystals which form a compact layer on the surface. Active corrosion is associated with formation of porous $ZnO$ layers or precipitation of $Zn~~(OH)_{2}$ in the solution rather than on the galvanized surface. A solution-precipitation mechanism involving $ZnO$ and $Zn~~(OH)_{4}^{2-}$ intermediate species is proposed for formation of the passivating $Zn~~(OH)^{2}$ coating, although this scheme must be complemented to take account of the localized corrosion observed at $pH<12$ in the first part of this study.