Galvanic anodes supports

The quality control of galvanic anodes is reduced mainly to the analytical control of the chemical composition of the alloy, to the quality and coating of the support, to an adequate joint between support and anode material, as well as to restricting the weight and size of the anode. The standards in Refs. 6, 7, 22, 27, 31 refer to magnesium and zinc anodes. Corresponding specifications for aluminum anodes do not exist. In addition, the lowest values of the rest potentials are also given [16]. The analytical data represent the minimum requirements, which are usually exceeded. 

Figure 8.16 shows an equivalent electrical circuit that simulates the pipeline cathodic protection depicted in Figure 8.9. Both pipeline and sacrificial anode (galvanic anode or inert anode) are buried in the soil of uniform resistivity. The pipehne is connected to the negative terminal and the anode to the positive terminal of an external power source (battery). The arrows in Figure 8.16 indicates the direction of the ciurent flow from the anode to the pipehne. The electron flow is also toward the pipehne to support local cathodic reactions and the protechve current (Ip) flows from the pipehne to the power supply. The soil becomes the electrolyte for complehng the protective electrochemical system or cathodic protechon circmt [24].

Design of cathodic protection for marine structures in both fresh and salt water require special techniques. Galvanic systems usually employ zinc or aluminum alloy anodes. Impressed current systems frequently use high silicon, chromium bearing iron, platinized niobium, or mixed-metal oxide/titanium anodes. The structure being protected affects the design. Stationary facihties such as bulkheads and support piles require different techniques from ship hulls [55]. 

Galvanic cathodic protection systems have been used extensively since the early 1990s in Florida on prestressed concrete bridge support piles in the sea. One of the reasons the galvanic system is used there is because concrete resistivity is low due to the marine exposure conditions. The Florida systems frequently incorporate a distributed anode of zinc fixed on the atmospherically exposed concrete and bulk zinc anodes in the water which pass current through the low resistance sea water to protect the submerged area as shown in Figure 7.4. 

When two dissimilar metals are in electrical contact with one another and both are contacting the same electrolyte, one of the metals will preferentially corrode, a process known as galvanic corrosion (also the principle by which certain types of batteries function). The more active metal will corrode, which is the metal having the more negative open-drcuit (or corrosion) potential, when immersed all by itself in the electrolyte the more noble metal (having the more positive open-circuit potential) will support the reduction reactions. The more active metal is, therefore, the anode and corrodes faster than it would all by itself, whereas the other more noble metal becomes the cathode and corrodes slower than it would alone (or maybe not at all). The electrolyte resistance, important in all corrosion processes, may play a particularly influential role in this type of corrosion process. 

Figure 3.3 (a) Schematic drawing (not to scale) showing a typical mixed potential process of metal dissolution, using the example of Cu in Eqn (3.4). Transport of dissolved ions closes the (corrosion) current loop in the electrolyte. Reactions (3.9) and (3.10) are coupled here, as also noted in Figure 3.2(a—c). (b) A simple mixed potential scheme, commonly found in the corrosion literature to describe galvanic decay of an anode metal A in the bimetallic couple of A and C (cathode). The cation charge of A is z+. The cathode metal supports ORR, and the electrons necessary for this reaction come from the anode metal s dissolution. 

Mg anodes can deliver sufficient current density 21.5 to 32.28 mA/nF to protect the inner wall of a galvanized steel hot water tank. It is reported that galvanized hot water tanks either corrode within few years or, if they last longer than this, remain sound for 15 to 20 years. This is supported by the fact that scale formation has a preservative or corrosion preventive effect of galvanized steel in hard water. Soft water can prevent the formation of this scale due to localized galvanic corrosion that can lead to perforation. It is suggested that if a tank is cathodically protected for the first year or... 

Thermodynamic principles can help explain a corrosion situation in terms of the stability of chemical species and reactions associated with corrosion processes. However, thermodynamic calculations cannot be used to predict corrosion rates. When two metals are put in contact, they can produce a voltage, as in a battery or electrochemical cell (see Galvanic Corrosion in Sec. 5.2.1). The material lower in what has been called the galvanic series will tend to become the anode and corrode, while the material higher in the series will tend to support a cathodic reaction. Iron or aluminum, for example, will have a tendency to corrode when connected to graphite or platinum. What the series cannot predict is the rate at which these metals corrode. Electrode kinetic principles have to be used to estimate these rates. 

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