Materials Performance

NOV 2014

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.

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33 NACE INTERNATIONAL: VOL. 53, NO. 11 MATERIALS PERFORMANCE NOVEMBER 2014 • El e ctro ch emical o smo si s can in- crease the electrical circuit resis- tance of anode systems. • Complete grouting inside saw-cut slots can be difficult. Slotted Ribbon Mesh Anode Systems: A Review Mixed metal oxide (MMO)-coated tita- nium ribbon mesh is most commonly used as the CP anode on existing concrete struc- tures and is typically installed in saw-cut slot s w ith c em entitiou s g rout th at i s applied by hand, or injected to embed the anodes. While this method overcomes the problem of bonding the concrete overlay with the titanium mesh anode, other tech- nical problems have evolved. 1 Poor CP Current Distribution (Near-Short Circuit) Many concrete structures experiencing rebar corrosion have a concrete cover that is too shallow to permit installation of a slotted system. When an anode is too close to the rebar, an uneven CP current distribu- tion occurs by current dumping (near - short circuit) (Figure 1), which renders the CP ineffective. Acid Generation at the Anode-Concrete Interface Acidification at the anode-concrete interface occurs when the anode CD is excessive (typically >110 mA/m 2 for MMO titanium anodes) in chloride-contami- nated concrete. This can happen when: • Cementitious grout is not sealed and permits salt water penetration. • Local portions of the anode system are wet. • Improper system design requires a high CD to achieve adequate CP. • A ruthenium-based MMO coating on the anode causes acidification to occur even though the CD does not exceed the limit. 2 Anode-Grout Incompatibility Low-resistivity grout is generally used to embed the anodes into their slots; how- ever, some types of grout that are in use are FIGURE 1 Uniform CP current distribution (left) and uneven current distribution by near-short circuit (right). FIGURE 2 Demonstration of electrochemical osmosis. n ot c omp ati bl e w ith a ctivat ed MMO- coated titanium ribbon mesh. To deter- mine the compatibility, a simulation test needs to be conducted for a long period of time. Anode manufacturers can provide a list of suitable grout; however, it was found th at ch emi cal c omp o sition s can var y within the same brand of grout, depending on the manu facturing locations. Therefore, it is difficult to find a compatible grout in different countries. Electrochemical Osmosis Increases the Circuit Resistance of the Anode System When a slotted ribbon mesh system is used for concrete structures that are not exposed to direct moisture (rain, splash, etc.), the anode-to-grout resistance can increase with time due to electrochemical o smo si s. Bip ol ar wat er mol e cul e s are attracted by negative charges (Figure 2). As a re s u l t , t h e a n o d e s y s t e m re q u i re s increased voltage over time to drive suffi- cient CP current. Th e electroch emical osmosis also increases with increasing voltage. As the CP rectifier approaches its rated voltage output, the current output decreases and eventually is inadequate for providing ef fective CP. Slotted systems installed indoors, in tunnels, or on deck soffits can receive insufficient moisture. In 1809, F.F. Reuss originally described this concept of electroosmosis. 3 Poor Grouting in Saw-Cut Slots In many countries, there is a lack of experienced contractors capable of prop- erly injecting or hand-applying grout to completely embed ribbon mesh anodes. Incompletely embedded anodes cannot discharge CP current efficiently. Development of a Semi-Conductive Layer for the Tape Anode System The new anode system was developed by "the idea of reversals." As described pre- viously, there are many problems associ- ated with slotted ribbon mesh anode sys- tems. The new approach can result in better CP systems by reversing these condi- tions. 1. Condition : Poor CP current distribu- tion by near-short circuit. Reversal: • Use semi-conductive material that can eliminate near-shorts.

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