The Albany Research Center, in collaboration with the Oregon Department of Transportation, is studying changes in the bond strength of thermal-sprayed zinc anodes on reinforced concrete during the early stages of electrochemical aging in impressed current cathodic protection (CP) systems where the zinc surface was not wetted. The bond strength of the zinc to the concrete decreased more rapidly with electrochemical aging when the zinc surface was not wetted than when wetted. The zinc-concrete interfacial chemistry for samples not wetted showed a greater buildup of chlorides and only weak evidence of secondary mineralization. pH at the zinc-concrete interface was around 7, which was similar to that measured for wetted surfaces. pH at the steel-concrete interface did not change on aging, remained strongly basic, and was similar to that for wetted samples.
Zinc anode cathodic protection systems are increasingly being used to protect deteriorating steel-reinforced concrete structures. While application of zinc to concrete has taken the form of perforated sheets1 and ribbons2, it is the thermal-sprayed zinc coating that is primarily used today. In addition to bridges3-4, thermal-sprayed zinc coatings have been used to protect parking garages5, fuel oil terminals6, and concrete overpasses7.
Improvements in the procedures used to apply thermal-sprayed zinc anodes to concrete has been an area of continuing interest. This was addressed in a recent laboratory study8. Another area of interest has been the effects of application procedures, operating and environmental conditions on the service life of the zinc anode. The impressed current cathodic protection (ICCP) system using thermal-sprayed zinc anodes with the longest operating history is installed on the Richmond-San Raphael Bridge9. Coating loss and loss of bond strength has been observed on the Richmond-San Raphael Bridge in the 12 years it has been in service. Because there is no other field experience of a sufficient longevity (the ICCP system on the Cape Creek Bridge in Oregon has operated less than 5 years), a joint Albany Research Center (ALRC)/Oregon Department of Transportation (ODOT) accelerated laboratory study was conducted to determine effects of coating application procedures and environmental effects on the service life of the coatings10-15. That study showed that the approximate lifetime for thermal-sprayed zinc anodes on chloride-contaminated concrete with frequent wetting and drying was about 27 years. The primary reason zinc anode service terminated in that time was not anode consumption but loss of anode bond to the concrete, the result of low strength zinc dissolution products and minerals forming at the zinc-concrete interface during anode operation.
The laboratory studies10-15 showed that less than 3 mils ( 7.62x 105 m) of coating would be consumed in 20 years during anode operation at current densities typical of those used by ODOT in coastal ICCP systems (0.0022 Aim2). The dissolution products from that 3 mils (7.62 x 105m) of zinc react over time with concrete paste and water at the zinc-concrete interface to form zinc minerals. After an initial decline in zinc bond strength due to loss of mechanical bond between coating and concrete.