Cathodic Protection of Docks

Since docks are usually situated in brackish water, the anodes must have large surface areas to keep the grounding resistance and the driving voltage low. Cathodic protection of the interior is not necessary because the dock is only flooded for a short time and can be otherwise maintained. 

Bahlmann, E. Hargarter, H. Sass, D. Schwarz, Schiff u. Hafen 55, H.7, 50 (1981) W. Bahlmann, E. Hargarter, Schiff u.-Hafen, Sonderheft Meerwasser-Korrosion, Februar 1983, S.98. 

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Cathodic protection is used widely for the protection of submerged steel in waterfront structures. It also can provide considerable benefit in the intertidal zone and can even reduce the usually high corrosion rate experienced at the boundary between the intertidal zone and the splash and spray zone. Cathodic protection also is used to prevent corrosion of the soil side of steel in marine structures such as sheet steel bulkheads. Cathodic protection also is effective in the control of the corrosion of reinforcing steel in concrete in all exposure zones in waterfiont structures. Particularly for impressed current systems, it is important to select materials for the cathodic protection system components such as rectifiers and junction boxes with consideration of the environment to which they will be exposed. When considering cathodic protection, periodic inspection and maintenance is required for proper system operation. The costs for inspection and maintenance must be considered in the overall cost of cathodic protection. While there are no specific standards for cathodic protection of piers and docks, information in NACE RP0176 (Corrosion Control of Fixed Offshore Platforms Associated with Petroleum Production) and NACE RP-0187 (Design Considerations for Corrosion Control of Reinforcing Steel in Concrete) contain information that is applicable to marine piers and docks. 

The scope of application of CP is enormous and continuously increasing. It is possible to protect vessels and ships, docks, berths, pipelines, deep wells, tanks, chemical apparatus, underground and underwater municipal and industrial infrastructure, reinforced concrete structures exposed to the atmosphere, as well as underground parts, tunnels, and other metal equipments using cathodic protection. Apart from reduction of general corrosion, cathodic protection reduces SCC, pitting corrosion, corrosion fatigue, and erosion-corrosion of metallic materials. 

In general, cathodic protection in seawater is classified into (1) inshore purposes for pipelines, tanks and stmctures, (2) stmctures and ships immersed and active in seawater, and (3) those that are inactive or stationary in seawater. A variety of static stmctures including floating docks, chains, light ships, nets and so forth in various shapes and sizes are protected by (1) anodes directly and closely mounted... 

Cathodic protection can be successful only under immersed conditions and, therefore, only the parts of a ship below the waterline (and holds filled with seawater ballast) can be protected by this method. Ideally, the ship is painted in dry dock to the required standard, and zinc anodes are fitted below the waterline to protect the steel and paint. Since, in general, a zinc anode in seawater will protect all the steel within a radius of about 3 m, anodes are spaced at distances of about 6 m. Because of the turbulence around the stem and because bronze propellers often are present, corrosion rates are higher in this area and more anodes are placed there. 

Fig. 10.28 An example of cathodic protection using sacrificial anodes A newly installed zinc anode on a bilge keel of a vessel in dry-dock. (Courtesy Corrintec (UK) Ltd.)...

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