Cathodic protection with zinc paints

At first sight, the answer would seem to be to increase the thickness of the zinc layer. This is not easily done, however, because the hot dipping process used for galvanising is not sufficiently adjustable and electroplating the zinc onto the steel sheet increases the production cost considerably. Painting the sheet (for example, with a bituminous paint) helps to reduce the loss of zinc considerably, but at the same time should vastly decrease the area available for the cathodic protection of the steel and if a scratch penetrates both the paint and the zinc, the exposed steel may corrode through much more quickly than before. 

The corrosion of iron is accelerated by the presence of oxygen, moisture, and salt. Corrosion can be inhibited by coating the surface with paint or zinc or by using cathodic protection. 

Iron can be protected from corrosion by coating it with paint or with a thin layer of metal such as chromium, tin, or zinc by alloying and by cathodic protection... 

Aluminum and aluminum-zinc alloy anodes have become the preferred sacrificial anodes for the cathodic protection of offshore platforms. This preference is because aluminum anodes demonstrate reliable long-term performance when compared with magnesium, which might be consumed before the platform has served its useful hfe. Aluminum also has better current/weight characteristics than zinc. Weight can be a major consideration for large offshore platforms. The major disadvantage of aluminum for some applications, for example, the protection of painted ship hulls, is that aluminum is too corrosion resistant in many environments. Aluminum alloys will not corrode reliably onshore or in freshwater [37]. In marine... 

Ship hulls Painting cannot always protect hostile marine conditions, in ships and, areas above keel blocks. Stem and mdder areas suffer erosion and corrosion due to the high turbulence caused by the propeller coupled with the galvanic effects of the noble bronze propeller. Effective cathodic protection of ship hulls and similar marine structures in seawater against corrosion can be apphed using either aluminum or zinc alloy sacrificial anodes. Twenty percent of the anodes required for full hull protection are required for stern protection only. 

The sacrificial protection is typically used for protection of ferrous substrates. It is attained by adding to the paint actual particles or flakes of zinc, which act as anodes with respect to the ferrous substrate [15], providing cathodic protection to... 

On the basis of the electrochemical nature of corrosion, it is clear that metallic zinc must be in direct contact with the electrolyte to protect bare steel regions. Paint or other coatings on top of the zinc coating will therefore eliminate or at best reduce the cathodic protection effect. 

Overactive cathodic protection— which can occur particularly with magnesium anodes—or badly controlled impressed current protection—leads to the reduction of oxygen to form hydroxyl ion (OH ), which can attack paints. Also, the placing of the zinc anodes requires experience in connection with both type and thickness of the paint (system). Since zinc anodes do not allow the production of excessive amounts of alkali, paint failure or the need to use alkali-resistant paints is avoided. 

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. 

A special type of coating for steel is used where exposure to crude oil and seawater is encountered, as in oil tankers. This involves an inorganic paint containing zinc powder which gives cathodic protection against localized corrosion. This has already been discussed in connection with the uses of lithium silicates in Chapter 2. Some further points are as follows. 

Bare inorganic zinc coatings should not be used together with the cathodic protection systems in submerged conditions (sea or brackish water). This applies also to painting of bilges protected with sacrificial anodes. 

The steel hulls of ships are subject to corrosion by the reaction of water and oxygen with iron to form rust. Although painting the surface of the steel can provide some protection, even a small scrape that removes paint can allow corrosion to start. Many ships use blocks of zinc attached to the hull to protect the steel from corrosion. The zinc becomes the sacrificial anode of a cell, losing electrons, and going into solution, while the iron in the steel acts as a cathode, gaining electrons as water is reduced. As the cathode, iron does not corrode. 

Corrosion Inhibitors. A water-soluble corrosion inhibitor reduces galvanic action by making the metal passive or by providing an insulating film on the anode, the cathode, or both. A very small amount of chromate, polyphosphate, or silicate added to water creates a water-soluble inhibitor. A slightly soluble inhibitor incorporated into the prime coat of paint may also have a considerable protective influence. Inhibitive pigments in paint primers are successful inhibitors except when they dissolve sufficiently to leave holes in the paint film. Most paint primers contain a partially soluble inhibitive pigment such as zinc chromate, which reacts with the steel... 

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