Experimental Research on Cathodic Protection of Embedded Sacrificial Zinc Anode for Reinforced Concrete

Cathodic protection with sacrificial anode is an economical and effective auxiliary measure for enhancing durability of concrete structures. However, the resistivity of currently available anode available on market is too high to ensure a high protection current and thus it can only achieve an insufficient protection effect. At the same time, the surface of anode is easy to crust which will affect anode's activity, and even cause expansion cracking of the concrete structure layer. For this reason, the work aims to develop a novel embedded sacrificial anode with high-purity zinc anode as the core material, coated with mortar (including cement, special pore agent and active substance) and conductive steel wire. In order to determine the characteristic performance and protection effect of embedded sacrificial anode, the resistivity of such coating mortar was tested by Double Electrode Method. The protective potential, attenuation potential and ampere density of the sacrificial anode embedded rebar were measured by auto-transformer and experiments. The working mechanism of the embedded sacrificial anode was analyzed by accelerated test and SEM-EDS. The resistivity of active mortar was 18.48 Ω·m. The resistivity was low enough to facilitate cathodic protection reaction. During the 30 day curing period, the anode output current rapidly dropped to 0.72 mA, and after 60 days, the current output was stable at 0.40-0.50 mA. An effective basis was provided for the design and application of embedded sacrificial anode in offshore concrete structures. The closed circuit potential and instantaneous off potential tests showed that the protection potential of the rebar was stabled at −400-−440 mV, and the instant off potential was −218 mV, meeting the requirements of NACE standard for the minimum attenuation potential of 200 mV. The protective current density provided by the embedded anode was 6.1-7.7 mA/m2, meeting the current density requirements in EN12696. The maximum effective protection distance of the developed embedded sacrificial anode could reach 700 mm under the highly corrosive environment when the reinforcement density ratio was 0.20. The effective protection ranges under other different reinforcement density ratios and corrosive environment conditions were also measured and analyzed. SEM-EDS analysis showed that the zinc anode reaction generated zincate (ZnO22−), which would migrate from the zinc anode surface to the interior of the mortar as the reaction proceeded, and eventually gradually dispersed into the pores of the mortar, preventing the lowering of anode activity due to the aggregation of corrosion products on the zinc anode surface, and therefore eliminating the expansion stress caused by the volume increasing of corrosion products. Finally, such design was applied and tested in a real project, i.e. the concrete foundation structure of the cable tower of Rushankou Bridge in Shandong China. The measured protective potential at all test points were all lower than −400 mV, meeting the protection requirements. As a result, it is safe to conclude that the developed embedded sacrificial anode has a good protection effect on rebar as the testing results prove that the protection potential and current output are stable, which will not affect the anode activity or cause expansion stress on the concrete structure. © 2023 Chongqing Wujiu Periodicals Press. All rights reserved.