Cathodic protection with sacrificial anodes

The reason for the absence of crevice corrosion in such cases is often misunderstood. It is an example of cathodic protection with sacrificial anode. 

Figure 10.14 Potential-current diagram for cathodic protection with sacrificial anodes.

Figure 10.15 Schematic potential-distance diagram for cathodic protection with sacrificial anodes. = electrical potential in the metalhc materials (structure, anodes and connections), Ea = electrode potential of the anode, = electrode potential of the structure (the cathode), AE = E - E, = electrical potential drop in the corrosive medium (the water, the electrolyte) from the anode to the cathode.

Cast iron alloys have been widely used in soil many gas and water distribution pipes in cities are still in use after decades of service. These have been gradually replaced with steel (coated and cathodically protected) and also with polymeric pipes. While cast irons are generally considered to be more resistant to soil corrosion than steel, they are subject to corrosion damage similar to that described above for steel. Coatings and cathodic protection with sacrificial anodes tend to be used to protect buried cast iron structures. 

Cathodic protection, with sacrificial anodes or impressed currents, of underwater structures. 

The forms of corrosion which can be controlled by cathodic protection include all forms of general corrosion, pitting corrosion, graphitic corrosion, crevice corrosion, stress-corrosion cracking, corrosion fatigue, cavitation corrosion, bacterial corrosion, etc. This section deals exclusively with the practical application of cathodic protection principally using the impressed-current method. The application of cathodic protection using sacrificial anodes is dealt with in Section 10.2. 

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. 

As is well known, high-purity zinc corrodes much less rapidly in dilute acids than commercial purity material in the latter instance, impurities (particularly copper and iron) are exposed on the surface of the zinc to give local cathodes with low hydrogen overpotentials this result is of practical significance only in the use of zinc for sacrificial anodes in cathodic protection or for anodes in dry cells. In neutral environments, where the cathodic... 

Cathodic protection with a sacrificial anode that is less noble than either member of the couple is frequently used to reduce the severity of bimetallic corrosion, particularly that resulting from the use of bronze propellers in steel ship hulls. 

Corrosion in these areas is sometimes effectively controlled by cathodic protection with zinc- or aluminium-alloy sacrificial anodes in the form of a ring fixed in good electrical contact with the steel adjacent to the non-ferrous component. This often proves only partially successful, however, and it also presents a possible danger since the corrosion of the anode may allow pieces to become detached which can damage the main circulating-pump impeller. Cladding by corrosion-resistant overlays such as cupronickel or nickel-base alloys may be an effective solution in difficult installational circumstances. 

The best way to overcome the limitations of cathodic protection related to changes in the environment is to monitor the potential and adjust the cathodic currents accordingly. This cannot be readily done with sacrificial anodes, and the method of impressed-current cathodic protection is sometimes preferred, in spite of its higher cost. 

Fig. 7-17. Cathodic protection with (a) impressed current and (b) magnesium sacrificial anodes.

Jetties are individual or multiple piles interconnected together to form a structure in the seabed and support a deck. The piles of a jetty usually have half of their length in the seabed and the rest in the high tide and splash zones up to the jetty deck. They are often concrete structures reinforced with steel. Cathodic protection using sacrificial zinc or aluminum anodes is installed after the completion of the jetty. With a deepwater jetty the suspension of more than a single anode or placing of alternate anodes at different levels is necessary. A few and larger anodes are necessary while impressed current method is employed. An... 

A potential-log current diagram for a system with sacrificial anodes is shown in Figure 10.14. The flat anodic curve of the sacrificial anode is typical for a suitable anode material. There is a potential difference AE between the structure and the anode due to a potential drop in the water. When the structure is completely protected, the galvanic current Ig = Ic = E, where E is the cathodic current on the structure, and E is the anodic current on the sacrificial anode, implying that the relatively small cathodic current on the anode is disregarded. 

There are several methods that can be used to control corrosion of steel reinforcements in concrete. First, the design of the structure should provide for drainage of salt-containing waters away from the reinforced concrete. Second, concrete of adequate thickness, high quality, and low permeability should be specified to protect the reinforcements from the environment. Third, chloride content of the concrete mix should be kept to a minimum. For further protection, the steel reinforcements can be epoxy-coated. In many parts of North America, steel reinforcements used in bridge decks are now epoxy-coated as a standard construction procedure. Cathodic protection is also being used, both with impressed current anodes and with sacrificial anodes [61]. (See Chapter 13.)... 

Figure 13.2. Cathodically protected pipe with sacrificial anode.

Cathodic protection can be applied by connecting sacrificial anodes to a structure. Basically, the principle is to create a galvanic cell, with the anode representing the less noble material that is consumed in the galvanic interaction (Fig. 11.5). Ideally, the structure will be protected as a result of the galvanic current flow. In practical applications a number of anodes usually have to be attached to a structure to ensure overall protection levels. The following advantages are associated with sacrificial anode CP systems ... 

Galvanic corrosion is the enhanced corrosion of one metal by contact with a more noble metal. The two metals require only being in electrical contact with each other and exposing to the same electrolyte environment. By virtue of the potential difference that exists between the two metals, a current flows between them, as in the case of copper and zinc in a Daniell cell. This current dissolves the more reactive metal (zinc in this case), simultaneously reducing the corrosion rate of the less reactive metal. This principle is exploited in the cathodic protection (Section 53.7.2) of steel structures by the sacrificial loss of aluminum or zinc anodes. 

Cathodic protection (CP) is an electrochemical technique of corrosion control in which the potential of a metal surface is moved in a cathodic direction to reduce the thermodynamic tendency for corrosion. CP requires that the item to be protected be in contact with an electrolyte. Only those parts of the item that are electrically coupled to the anode and to which the CP current can flow are protected. Thus, the inside of a buried pipe is not capable of cathodic protection unless a suitable anode is placed inside the pipe. The electrolyte through which the CP current flows is usually seawater or soil. Fresh waters generally have inadequate conductivity (but the interiors of galvanized hot water tanks are sometimes protected by a sacrificial magnesium anode) and the conductivity... 

The modern procedure to minimise corrosion losses on underground structures is to use protective coatings between the metal and soil and to apply cathodic protection to the metal structure (see Chapter 11). In this situation, soils influence the operation in a somewhat different manner than is the case with unprotected bare metal. A soil with moderately high salts content (low resistivity) is desirable for the location of the anodes. If the impressed potential is from a sacrificial metal, the effective potential and current available will depend upon soil properties such as pH, soluble salts and moisture present. When rectifiers are used as the source of the cathodic potential, soils of low electrical resistance are desirable for the location of the anode beds. A protective coating free from holidays and of uniformly high insulation value causes the electrical conducting properties of the soil to become of less significance in relation to corrosion rates (Section 15.8). 

Zinc should give a potential of -1 - 05 V vs. CU/CUSO4 and should have a driving potential of about -0-25 V with respect to cathodically protected steel. Zinc is therefore sufficiently negative to act as a sacrificial anode, and its first use for such purposes was on the copper-sheathed hulls of warships more than a century ago. The first attempts to fit zinc anodes to steel hulls, however, were a complete failure, for the sole reason that it had not been realised that the purity of the zinc was of paramount importance. The presence of even small amounts of certain impurities leads to the formation of dense adherent films, which cause the anodes to become inactive. 

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