Materials Performance

MAR 2018

Materials Performance is the world's most widely circulated magazine dedicated to corrosion prevention and control. MP provides information about the latest corrosion control technologies and practical applications for every industry and environment.

Issue link:

Contents of this Issue


Page 21 of 88

19 MATERIALS PERFORMANCE: VOL. 57, NO. 3 MARCH 2018 Information on corrosion control and prevention prised of expanded mesh of nearly pure zinc alloy with small additions of ele- ments intended to improve its formability and anodic performance. The anode mesh is mounted in a glass f iber jacket that pro- vides an annular space between the mesh and the protected structure. The space is f illed with a mortar mix of Portland cement, f ine aggregate, and sand. The jacket is placed around the pile, starting at the lowest tide point and extending up to ~1.8 m above low tide. Because there are no activating admixtures in the jacket f iller, any activa- tion of the anode must rely on the absorp- tion of seawater by the f iller after the jacket is placed in service. Studies have shown that zinc readily passivates in nor- mal Portland cement concrete, and that a high concentration of chloride ions in the concrete is needed for zinc to be consis- tently active and able to protect the embedded rebar. Since the f ill mortar is chloride free, the zinc alloy in the jacket tended to passivate in the splash and atmospheric zones and protection there is limited to physical barrier effects. This system has become a standard method of pile repairing for the FDOT, which has repaired hundreds of piles in numerous bridges with good results. In applications such as patch repairs and other general sacrif icial CP installa- tions, localized "point" anodes are used. According to the researchers, it is well established that repairs of chloride-con- taminated or carbonated concrete can create electrochemical incompatibilities between the "new" and "old " concrete that may lead to the accelerated corro- sion of the steel reinforcement in the con- crete near the patch repair. This is known as the "halo" effect. The resulting corro- sion can induce cracking that may require extending the patching repair after a short time period (e.g., three to f ive years). The idea of the localized CP anode application is that small galvanic point anodes installed in the patch repair will sacrif icially corrode and reduce the pos- sibility of a new active corrosion zone on the surrounding rebar, the authors com- ment. There are various versions of these anodes. Some, with an overall diameter of ~60 mm and a height of ~30 mm, may uti- lize a single zinc alloy core (with a mass of several tens of grams) surrounded by a cylindrical active matrix of cementitious components. Others, with larger dimen- sions, may incorporate multiple zinc alloy cores embedded in a common activating matrix. The zinc alloy cores are usually connected to rebar tie wires that can be easily tied to the reinforcing steel in the area to be patched before casting the repair concrete. The overall result, according to the f indings of some investi- gators, is an appreciable extension of ser- vice life in patch repairs. The authors cau- tion, however, that the service range of those anodes may be limited and it should be carefully evaluated against other options. For more information on these sacrificial anode systems, see the CORROSION article available at References 1 "Materials and Methods for Corrosion Con- trol of Reinforced and Prestressed Concrete Structures in New Construction," U.S. De- partment of Transportation, Federal High- way Administration, Publication no. 00-081, August 2000. 2 O. Troconis de Rincón, A. Torres-Acosta, A. Sagüés, M. Martinez-Madrid, "Galvanic Anodes for Reinforced Concrete Structures: A Review," Corrosion 74, in press (2018): 3 "Long-Term Effectiveness of Cathodic Pro- tection Systems on Highway Structures," U.S. D e p ar tm ent of Transp or t ation , Fed eral Highway Administration, Publication no. FHWA-RD-01-096, April 2001. 4 O. Troconis de Rincón, A. Torres-Acosta, A. Sagüés, "Sacrificial Anodes for Reinforced Concrete Structures: A Review," CORROSION 2017, paper no. 9078 (Houston, TX: NACE International, 2017). —K.R. Larsen

Articles in this issue

Links on this page

Archives of this issue

view archives of Materials Performance - MAR 2018