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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34 NOVEMBER 2014 MATERIALS PERFORMANCE NACE INTERNATIONAL: VOL. 53, NO. 11 CATHODIC PROTECTION • Position anodes at the furthest point from the rebar. 2. Condition : Acid generation at the anode-concrete interface. Reversal: • Control current magnitude and di stri b ution to elimin at e hi g h current discharge areas. 3. Condition: Incompatibility of anode and cementitious grout. Reversal: • Use only cementitious grout with known properties. 4. Condition: Increase in the electrical circuit resistance of the anode system due to electrochemical osmosis. Reversal: • Distribute the CP current as uni- formly as possible. • Expand the active anode area to reduce the operating voltage. • Create a low-resistance anode in- FIGURE 3 (a) Installation of MMO titanium tape anodes. (b) Application of semi-conductive layer. (a) (b) FIGURE 4 Comparison of CP current distribution to the rebar. terface with reduced dependence on water or moisture. 5. Condition : Po o r g r o u t i n g i n si d e saw-cut slots. Reversal: • Simplify installation of the anode system. • Av o i d c u tt i n g o r d r i l l i n g t h e concrete for anode installation. • Minimi z e d et ai l ed in st al l ation work. By combining all of these changes, or reversal s, th e semi-conductive layered MMO titanium tape anode system was developed. The system consists of: • MMO titanium tape anodes (20-mm wide by 0.1-mm thick) • Titanium conductor tapes (20-mm wide by 0.1-mm thick) that connect the MMO titanium tape anodes • A thin (5-mm) carbon-rich semi- conductive cementitious material (Conducrete † ) that covers the tape system The MMO titanium tape anodes and the titanium conductor tape are directly adhered to the concrete surface using a sili- con adhesive (Figure 3[a]). The semi-con- ductive layer is sprayed (Figure 3[b]). The semi-conductive layer is formu- lated with a resistivity of 3,000 to 6,000 Ω-cm, which is approximately an order of magnitude lower than conventional con- crete resistivities. One side of the tape anode is coated with an iridium oxide ( Ir O 2 ) - b a s e d MM O c o a t i n g , w h i c h i s designed to last 50 years at a CD of 150 mA/ m 2 . The CP current discharging from the tape anodes distributes inside the semi- conductive layer before penetrating the concrete. The system features include: • The anodes can be readily spaced on the concrete surface according to the C P c u r re n t re q u i re m e n t f o r th e reinforcing steel. • C P an o d e sy st em in st al l ation i s simple and quick. • The installation cost is significantly reduced because the system does not demand the cutting or drilling of concrete. • In exp erienced contactors can b e easily trained to install the system w ith a minimum of en g in e erin g supervision. • Current from the MMO surface is di sp e r s e d t h r o u g h o u t t h e s e m i - conductive layer to the concrete and rebar. • The anode configuration prevents electrical short or near-short circuits in concrete structures where rebar has a thin concrete cover. • The stable resistivity of the semi- conductive layer reduces the inf lu- ence of local moisture on CP current distribution. Therefore, the system can be used for structures where the concrete is wet and exposed to water splash, or in dry situations such as in tunnels and in doors. † Trade name.

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