Impressed current cathodic protection consumable anodes

Cathodic protection by impressed current involves the use of a rectifier connected to a power line. Contrary to sacrificial anodes, which operate at a fixed potential, the use of a rectifier permits to adjust the voltage (or the current) to the particular requirements of a protection scheme. This not only allows one to optimize the electrochemical conditions for protection, but the method is also well suited to protect large surfaces. On the other hand, protection by impressed current needs more maintenance than the use of sacrificial anodes. In order to protect buried structures by impressed currents one uses consumable anodes such as scrap iron or, more often, non-consumable anodes made of iron-silicon alloy, graphite or of titanium coated with noble-metal oxides. 

It will be seen that the impressed current electrode discharges positive current, i.e. it acts as an anode in the cell. There are three generic types of anode used in cathodic protection, viz, consumable, non-consumable and semi-consumable. The consumable electrodes undergo an anodic reaction that involves their consumption. Thus an anode made of scrap iron produces positive current by the reaction ... 

Impressed current anodes have very slow or controlled consumption rates when the anodic reaction occurs on the anode surface. As the reaction consumes alkalinity (equation (7.5)) and generates acid (equation (7.6)), it can attack the anode and the concrete. The level of current is therefore important in maintaining a good anode to concrete bond. Types of anode are described later. The function of the anode is to spread the current to all areas to be protected having converted the electrical current from the transformer rectifier (DC power supply) to an ionic current that flows from the anode to the cathode so that the cathodic reaction will occur on the reinforcing steel surface, suppressing corrosion. 

One of the few impressed current zinc systems in the United Kingdom at the time of going to press is shown in Figure 7.10. There has been some concern about the rise in resistance seen on some systems. This may be due to a build up of corrosion products between the zinc and the concrete, or to treatment of the zinc after application to protect it from atmospheric corrosion. Zinc, of course, is not inert and is consumed by corrosion from the atmosphere and water impingement. The anodic reaction also consumes the zinc and gives rise to the formation of oxides and sulphates at the anode/concrete interface which may increase the electrical resistance between anode and cathode. 

Another approach for reducing corrosion is to employ mechanisms that can modify the electrochemical processes that consume materials. Cathodic protection, either through the use of sacrificial anodes or an impressed current system, can convert a material that normally will corrode quite readily into a material that resists corrosion. This approach, which is the topic of Chap. 13, works very well for protecting fixed assets in contact with potentially corrosive environments such as soils, seawater, or any other electrolytically conducting medium. 

In impressed current systems cathodic protection is applied by means of an external power current source (Fig. 11.7). In contrast to the sacrificial anode systems, the anode consumption rate is usually much lower. Unless a consumable scrap anode is used, a negligible anode consumption rate is actually a key requirement for long system hfe. Impressed current systems typically are favored under high-current requirements and/or high-resistance electrolytes. The following advantages can be cited for impressed current systems ... 

Cathodic protection is a method for protecting metals against corrosion. There are two techniques to achieve this objective. The first approach is to use a sacrificial anode (a less noble metal) and consume it to protect another metal. This technique has been utilized for centuries in marine structures. As pointed out earlier, the galvanic coupling between the two metals results in a current density flowing in the electrolyte as shown in Fig. 6a. In the second approach, known as an impressed current system, the current density is artificially created using an inert electrode. This is depicted in Fig. 6b. Mathematically speaking, the inert electrode can be viewed as a current source where the value of i is assumed to be a known constant at a point. In either case, the intent is to ensure that the... 

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