Criteria for cathodic protection

The information in Sections 2.2,2.4 and 3.3 is relevant for protection criteria. Investigations [43] with steel-concrete test bodies have shown that even in unfavorable conditions with aerated large-area cathodes and small-area damp anodes in Cl -rich alkaline environments, or in decalcified (neutral) surroundings with additions of CU at test potentials of f/cn.cuso4 = -0.75 and -0.85 V, cell formation is suppressed. After the experiments had proceeded for 6 months, the demounted specimens showed no recognizable corrosive attack. 

At the beginning of the experiment, the measured free corrosion potential of the reinforcing steel in the Cl -rich environment was Ucu-Cusoj=-0.58 to -0.63 V in the neutral environment it was -0.46 to -0.55 V, and in straight concrete it was -0.16 V. The demounted test pieces at the end of the experiment are shown in Fig. 19-2. After 6 months, no corrosive attack on the cathodically protected test pieces was detected. 

Test potential Medium at the anode Protection current density (mA m ) Anode 1 Anode 2  

Anode 1 = plastic cable anode anode 2 = mixed metal oxide-titanium. 

With unprotected comparison test pieces, the corrosion rate was 4 mm a , which from cell current measurements indicated that the self-corrosion was 50%. 

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When cathodic polarization is a result of negative total current densities 7., the potential becomes more negative and the corrosion rate lower. Finally, at the equilibrium potential it becomes zero. In neutral water equilibrium potentials are undefined or not attainable. Instead, protective potentials are quoted at which the corrosion rate is negligibly low. This is the case when = 1 flA cm (w = lOjUm a ) which is described by the following criteria for cathodic protection ... 

The criteria for cathodic protection are not free from criticism. It is beheved that all the listed criteria are deficient to some extent and therefore qualitative in practical appKcation. However, one should be optimistic that any level of cathodic polarization is beneficial, and a broad range of ca-thodically applied potentials will yield adequate protection. As a result, the use of any criterion listed in Table 4 [24] will produce adequate cathodic protection if applied judiciously. The amount of cathodic protection should be sufficient to reduce the corrosion rate to an acceptable range. Caution should be exercised to avoid overprotection. Overprotection results in the premature consumption of sacrificial anodes or excessive amounts of impressed current demands. Moreover, the application of too much cathodic protection can result in damage to the structure to be protected as a result of hydrogen embrittlement. 

D.A. Hausmann, Criteria for cathodic protection of steel in concrete structures. Mater. Prot. 8 (1969) 23-25. 

Kuhn [19] ilrst postulated in 1933 that the potential needed to stop corrosion is probably in the neighborhood of —0.85 V vs. CU/CUSO4. The results obtained from extensive studies on cathodic protection [20-27] helped National Association of Corrosion Engineers (NACE) to establish criteria for cathodic protection [28]. NACE RP-01-69 specifies A negative (cathodic) potential of at least 850 mV vs. Cu/CUSO4 should be appfied to protect the structure [28,29], However, in the presence of sulfides, bacteria, elevated temperatures, acid environments, and dissimilar metals, the criteria of—850 mV may not be sufficient [5,30—33]. According to NACE, one should also account for the IR drop at the metal-soil interface, which is included in most practical measurements and is an uncertain value depending on the electrolyte (soil) resistance. 

At the beginning of Section 7.3 we saw that the T/R is typically designed to deliver 10 to 20 niA-m of steel surface area. We have also seen that we must minimize the anodic reaction to reduce acid attack at the anode/concrete interface. This can reduce bond and thus increase the resistance of the circuit. It is therefore essential that we apply enough current to stop corrosion but only just enough. The control criteria for cathodic protection of atmospherically exposed concrete has been the subject of considerable debate. 

Bennett, J.E. and Turk, T. (1994). Technical Alert Criteria for Cathodic Protection of Reinforced Concrete Bridge Elements, Strategic Highway Research Program, SHRP-S-359, National Research Council, Washington, DC. 

Control Criteria and Materials Performance Studies for Cathodic Protection of Reinforced Concrete. Demonstrates through mathematical models the feasibility of improved and simplified control criteria for cathodic protection of concrete structures. The models were developed to establish concentration profiles which develop as a result of cathodic protection current, and to study current distributions which result from geometric factors. 260 pages. SHRP-S-670... 

Table 3-3 Practical criteria for cathodic protection of plain carbon and low-alloy steels in soil...

Measurement Techniques Related to Criteria for Cathodic Protection on Underground or Submerged Metallic Piping Systems... 

Barlo, T. J. (1994), Field Testing the Criteria for Cathodic Protection of Buried Pipelines. Arlington, VA American Gas Association, Catalog No. L 51715. 

Summing up, experience is the best guide in selecting the criteria for cathodic protection as it is strongly affected by quality of concrete, water saturation level, degree of contamination and water-cement ratio. 

Chaudhary, Z., "Design and Protection Criteria for Cathodic Protection of Seawater Intake Structures in Petrochemical Plants," Marine Corrosion in Tropical Environments, ASTM STP 1399, Dean, S.W. Hernandez-Duque Delgadillo, G. Bushman, J.B. (Eds.) , West Conshohocken, PA, American Society for Testing and Materials, 2000. 

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