Cathodic protection continued surface coating

The cost and economics of cathodic protection depend on a variety of parameters so that general statements on costs are not really possible. In particular, the protection current requirement and the specific electrical resistance of the electrolyte in the surroundings of the object to be protected and the anodes can vary considerably and thus affect the costs. Usually electrochemical protection is particularly economical if the structure can be ensured a long service life, maintained in continuous operation, and if repair costs are very high. As a rough estimate, the installation costs of cathodic protection of uncoated metal structures are about 1 to 2% of the construction costs of the structure, and are 0.1 to 0.2% for coated surfaces. 

Metallic coatings are either more noble than the substrate, as in chromium plate on mild steel, or are base metals which corrode more easily than the substrate. In the former case, the underlying metal is protected by a continuous impervious film of the noble metal which is itself resistant to attack. This method is adequate provided the surface coating contains no holes or flaws and remains intact. Penetration of this layer by the corrosive agent leads to galvanic corrosion at the interface of the two metals. If the object is coated with a more base metal, then protection is by both the physical barrier of the metal film and by cathodic protection at any subsequent defects (Fig. 19). 

Corrosion inhibitors in paints protect metallic surfaces from oxidation. Coating primers are used when there is continuous exposure to corrosive elements, e.g., in marine applications. Examples are coal-tar derivatives, epoxy resins and coal-tar modified epoxies. Primers that inhibit corrosion by anodic or cathodic polarization contain inorganic metallic pigments such as chromates or leads or both. Composite pigments containing calcium oxide, zinc, silica, and oxides of phosphorus and boron can also be used (Mathias 1984). Nowadays, powder paints such as polyester and epoxy powder paints can also be used for corrosion inhibition (Rose and Vance 1997). 

In the process droplets of semimolten zinc are sprayed from a special gun that is fed with either wire or powder onto a grit-blasted surface. The semimolten droplets coalesce with some zinc oxide present at each interface between droplets. Electrical continuity is maintained both throughout the coating and with the iron substrate so that full cathodic protection can be obtained since the zinc oxide forms only a small percentage of the coating. 

Adequate anode lifetime is obviously also an important factor related to the magnitude and imiformity of current flow. A variety of anode systems have evolved for cathodic protection of reinforcing steel, each with certain advantages and limitations. Continuous surface anodes have been based on conductive bituminous overlays and conductive surface coatings. The former are suited only to horizontal surfaces. In general, good current distribution is achievable with such systems. Discrete anodes have been used without overlays and with cementitious overlays. For horizontal surfaces, anodes without overlays can be recessed in the concrete surface. Nommiform current distribution is a... 

The anodic reaction corresponds to the oxidation of zinc particles (loss of electrons) while the cathodic one usually involves oxygen reduction (gain of electrons) on the surface of iron or steel the "pressure" of electrons released by zinc prevents or controls the oxidation of the metal substrate. Theoretically, the protective mechanism is similar to a continuous layer of zinc applied by galvanizing with some differences because the coating film initially presents in general a considerable porosity (Jegannathan et al., 2006). 

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