1993 Volume 42 Issue 6 Pages 377-383
In order to verify the mechanism of the formation process of hemimorphite (Zn4Si2O7(OH)2· H2O) in galvanized steel pipe for water service, the formation of hemimorphite in the corrosion process of zinc was confirmed through a procedure by which anodic oxidation was united with aging, which was named the anodic oxidation method. Zinc coupons were anodically oxidized in sodium metasilicate (Na2SiO3·9H2O) solution using a galvanostat and then were aged in aqueous solution. The corrosion products on the surface of the zinc coupons aged for the prescribed periods were analyzed using Fourier-transform infrared micro spectroscopy and energy dispersive X-ray analysis. The surface morphologies of the aged coupons were observed using a scanning electron microscope and an optical microscope. Samples of the corrosion products and the solution in which the coupons were aged were analyzed using inductively coupled plasma emission spectroscopy. In combining the results of the co-precipitation method with the results of the present study, it was proved that hemimorphite formation in the zinc-silicate-water corrosion system proceeded in the following sequence:
1) precipitation of Zn(OH)2 as a corrosion product of zinc
2) adsorption of dissolved silicate onto the precipitate of Zn(OH)2
3) breaking of the structure of Zn(OH)2 by the adsorbed silicate
4) formation of an amorphous compound as a precursor of hemimorphite
5) rearrangement of atoms into the framework of the hemimorphite crystal
In addition, two types of morphology of the hemimorphite crystal were found.
