Abstract

Thermal-sprayed zinc anodes are used in impressed current cathodic protection systems for some of Oregon's coastal reinforced concrete bridges. Electrochemical aging of zinc anodes results in physical and chemical changes at the zinc-concrete interface. Concrete surfaces heated prior to thermal-spraying had initial adhesion strengths 80% higher than unheated surfaces. For electrochemical aging greater than 200 kC/m2 (5.2 A-h/ft2), there was no difference in adhesion strengths for zinc on preheated and unheated concrete. Adhesion strengths decreased monotonically after about 400 to 600 kC/m2 (10.4 to 15.6 A-h/ft2) as a result of the reaction zones at the zinc-concrete interface. A zone adjacent to the metallic zinc (and originally part of the zinc coating) was primarily zincite (ZnO), with minor constituents of wulfingite (Zn(OH)2), simonkolleite (Zn5(OH)8Cl2H20), and hydrated zinc hydroxide sulfates (Zn4S04(OH)5xH20). This zone is the locus for cohesive fracture when the zinc coating separates from the concrete during adhesion tests. Zinc ions substitute for calcium in the cement paste adjacent to the coating as the result of secondary mineralization. The initial estimate of the coating service life based on adhesion strength measurements in accelerated impressed current cathodic protection tests is about 27 years.

This content is only available as a PDF.
You do not currently have access to this content.