Sacrificial protection, galvanics

Having introduced matters pertaining to the electrochemical series earlier, it is only relevant that an appraisal is given on some of its applications. The coverage hereunder describes different examples which include aspects of spontaneity of a galvanic cell reaction, feasibility of different species for reaction, criterion of choice of electrodes to form galvanic cells, sacrificial protection, cementation, concentration and tempera lure effects on emf of electrochemical cells, clues on chemical reaction, caution notes on the use of electrochemical series, and finally determination of equilibrium constants and solubility products. 

Cathodic protection, provided that the material is not susceptible to embrittlement. Sacrificial zinc (galvanized) coatings. 

Scientists have developed a number of methods for protecting metals from oxidation. One such method involves the use of a sacrificial metal. A sacrificial metal is a metal that is more easily oxidized than the metal it is designed to protect. Galvanized iron, for example, consists of a piece of iron metal covered with a thin layer of zinc. When galvanized iron is exposed to oxygen, it is the zinc, rather than the iron, that is oxidized. 

It is not practical to galvanize large objects, such as a ships or pipes. Instead a block of a reactive metal, such as magnesium (or zinc), is attached to the large object and, again, preferentially loses electrons to oxygen. This method of protecting the metal is known as sacrificial protection. 

SACRIFICIAL PROTECTION - Reduction of corrosion of a metal in an electrolyte by galvanically coupling it to a more anodic metal. A form of cathode protection. 

Fig. 9 Cathodic protection for underground pipe — sacrificial or galvanic anode.

Cathodic protection (CP) is an electrical method of mitigating corrosion on metallic structures that are exposed to electrolytes such as soils and waters. Corrosion control is achieved by forcing a defined quantity of direct current to flow from auxiliary anodes through the electrolyte and onto the metal structure to be protected. Theoretically, corrosion of the structure is completely eliminated when the open-circuit potentials of the cathodic sites are polarized to the open-circuit potentials of the anodic sites. The entire protected structure becomes cathodic relative to the auxiliary anodes. Therefore, corrosion of the metal structure will cease when the applied cathodic current equals the corrosion current. There are two basic methods of corrosion control by cathodic protection. One involves the use of current that is produced when two electrochemically dissimilar metals or alloys (Table 19.1) are metallically connected and exposed to the electrolyte. This is commonly referred to as a sacrificial or galvanic cathodic protection system. The other method of cathodic protection involves the use of a direct current power source and auxiliary anodes, which is commonly referred to as an impressed-current cathodic protection system. Then cathodic protection is a technique to reduce the corrosion rate of a metal surface by making it the cathode of an electrochemical cell [3]. 

SECTION 20.8 Electrochemical principles help us understand corrosion, undesirable redox reactions in which a metal is attacked by some substance in its environment The corrosion of iron into rust is caused by the presence of water and oxygen, and it is accelerated by the presence of electrolytes, such as road salt. The protection of a metal by putting it in contact with another metal that more readily undergoes oxidation is called cathodic protection. Galvanized iron, for example, is coated with a thin layer of zinc because zinc is oxidized more readily than iron, the zinc serves as a sacrificial anode in the redox reaction. 

There are two forms of cathodic protection, impressed current and sacrificial. The impressed current system has been described above and is the system conventionally used for atmospherically exposed reinforced concrete structures. An alternative method is to directly connect the steel to a sacrificial or galvanic anode such as zinc without using a power supply. This anode corrodes preferentially, liberating electrons with the same effect as the impressed current system, e.g. 

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].

Anticorrosion protection of metal structures using sacrificial magnesium, zinc, and aluminum anodes is the oldest and, at the same time, the simplest method of electrochemical protection. Galvanic anodes are mainly used in cases where the structure is covered with a good insulation coating and low currents are required for protection, and also when lack of a power supply makes realization of cathodic protection impossible. 

The basic disadvantage of sacrificial protection is the irreversible loss of anode material and the resulting need of its replacement in addition, corrosion products of the anode can pollute the environment. Also, the range of application of galvanic anodes is limited by the resistivity (the specific resistance) of the environment and relatively small values of the protective current. A schematic diagram of sacrificial anode protection is presented in Fig. 8-23. 

Sacrificial Protection sa-kro-Ifi-shol pro-Itek-sh9n n (1) The use of a metallic coating, such as a zinc-rich paint, to protect steel. In the presence of an electrolyte, such as salt water, a galvanic cell is set up and the metallic coating corrodes instead of the steel. (2) Zinc or aluminum anode. 

Zinc based metallic coatings (e.g. galvanizing) have been used for some time to provide corrosion protection for steel substrates. The basis of this protection is due firstly to the sacrificial protection afforded by the zinc coating to the steel substrate, and secondly to the slower rate of zinc corrosion compared to that of the steel. 

Fig. 15 Cathodic protection for underground pipe, (a) Sacrificial or galvanic anode, (b) Impressed-i rent anode, ac, alternating current...

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