Characterisation of coating on rebar surface using Hot-dip Zn and Zn-4.9Al-0.1 misch metal bath

By Manna, M.; Naidu, G.; Rani, N. & Bandyopadhyay, N.
Published in Surface and Coatings Technology NULL 2008

Abstract

The corrosion of rebar is one of the primary causes of premature deterioration of the concrete structure. The ideal option to overcome this situation would be to provide corrosion protection right at the time of manufacturing of the rebar before it is encased in the concrete and hence, warrants the use of corrosion resistance rebar. The present paper outlines characterisation of coating obtained on rebar surface from pure Zn and Zn-4.9Al-0.1 misch metal bath. The coating was characterised by SEM, EDS, Galvanostatic and XRD techniques. In case of pure Zn bath, distinct phases such as eta, zeta, delta and gamma 1 and gamma were identified in coating where as in case of Zn-4.9Al-0.1 misch metal bath no such distinctive phases were found. The coating obtained from Zn-4.9Al-0.1 misch metal bath was thinner and consisted of outer Al2O3 phase followed Zn–Al phase resulting in better ductility compare to the coating obtained from pure Zn bath. Comparative corrosion resistance performances of both types of coating respect to uncoated rebar were evaluated by salt spray and tafel test. were conducted in simulated aggressive chloride and concrete pore solution of coated and The coating obtained form Zn-4.9Al-0.1 misch metal bath was found to be more anodic and showed 1.5-3 times better corrosion resistance in concrete pore solution and 2.5 times better resistance against aggressive chloride attack compare to the coating obtained from pure Zn bath. Both the coatings dissolved in faster rate in highly alkaline environment (pH = 13.6) where as dissolution rate decreased with decrease of pH in pore solution. The sacrificial as well as barrier protection of Zn–Al alloy coating was found to be more effective than pure Zn coating. Both types of coated bars showed reduction in bond strength in concrete structure. It is attributed by the faster dissolution of the coating, leading to hydrogen gas evolution thereby creating a gap between the rebar surface and concrete structure.

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