Cathodic protection continued surface area

In order to install cathodic protection, continuity can be established by welding in extra rebars. However, at Florida DOT one approach has been to expose bars in damaged areas, grit blast them clean and apply arc sprayed zinc directly onto the steel and then across the steel surface. This provides galvanizing directly on the steel and SACP to the steel embedded in the concrete. Multiple continuity connections are established by the sprayed zinc. 

The anode is fixed to the concrete using non-metallic fixings and may be supplied as a prefabricated mesh or more often as a continuous anode strand which is laid over the surface of the structure to be protected. The spacing between the anode strands may be adjusted to give the required current distribution and current density per unit area of concrete necessary to provide cathodic protection to a particular structure. 

In sodium chloride solutions, on the other hand, the conductivity is greater hence, additional anodes and cathodes can operate much further removed one from the other. At such cathodes, NaOH does not react immediately with FeCl2 formed at anodes instead, these substances diffuse into the solution and react to form Fe(OH)2 away from the metal surface. Any Fe(OH)2 so formed does not provide a protective barrier layer on the metal surface. Hence, iron corrodes more rapidly in dilute sodium chloride solution because more dissolved oxygen can reach cathodic areas. Above 3% NaCl, the continuing decreased solubility of oxygen becomes more important than any change in the diffusion-barrier layer hence, the corrosion rate decreases. 

SCC starts by an electrochemical mechanism. A pit, scratch, or rupture in a protective film can act as the starting point for corrosion. Anodic and cathodic areas form on the metal surface, with the weakly film-covered region and the tip of the crack acting as an anode and the oxide-covered region acting as a cathode. Once corrosion starts, the stresses tend to concentrate at the tip of the crack, which remains active. At some critical stress value, deformation results in the formation of a fresh surface (at tips where all the stresses are relieved). The electrochemical mechanism takes over on the fresh surface, building up stress at the tip of the crack. This sequence of events repeats continuously. 

Theoretically, it would be possible to hold the potential at a value where the metal is thermodynamically immune from corrosion. In practice, however, this Is likely to be too expensive. The continuous passage of current requires the consumption of power (j cell 0 tid, particulary if the medium between the protected surface and the anode is not highly conducting, the use of anything but a very low current will entail an unacceptably high energy consumption, The voltage required of the power supply also depends upon the active area of Che structure, the nature of the electrode reactions and resistance of the anode. The cathode potential is kept within the desired limits by ... 

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