Galvanic cell, creation

Sacrificial Anodes Incontrastto the impressed current technique, the use of sacrificial anodes does not depend on the creation of driven electrochemical cell. Rather, a galvanic cell is formed between the structure and the sacrificial anode in which electrons pass spontaneously from the latter to the former (Fig. 9). Thus, the source of the electrons (the sacrificial anode) must have a more negative electrode potential than the structure. It was for this reason that Humphrey Davy chose zinc or iron to protect copper, and it also explains why magnesium, aluminum and zinc alloys are used to protect steel today. 

Just a few months after the appearance of the Volta pile it was found that the electric current can exert a chemical action. As early as May of 1800, Nicholson and Carlisle carried out water electrolysis. In 1803 the processes of metal electrodeposition were discovered. In 1807 Davy for the first time isolated alkali metals by electrolysis of salt melts. Thus almost simultaneously with the creation of the first electrochemical power source - the "galvanic cell" or "galvanic battery" - many electrochemical processes were discovered and the foundations were laid of the science which to-day we call electrochemistry. 

The second point to emphasize is that the electromotive forces mentioned above are measured at null current. They are the zero cell potentials. This is a reversibility condition. In Daniell s galvanic cell experiment, electrical current can drive an electrical motor, for example. There is then an electrical energy recovery from the free enthalpy initially contained in the chemical system. Due to the current s occurrence, there are thus a release of heat due to the Joule effect as well as an entropy creation. The process is irreversible. However, the current can be very weak with the use of an opposition montage. In this condition, by avoiding any other irreversibility source, the free enthalpy decrease can be fully converted into electrical energy. Then the process becomes reversible. (The opposition montage even permits the reaction course to be inverted in some cases. The cell is then called a reversible one see Chap. 13.)... 

Redox reactions, which involve a transfer of electrons, can occur in acidic and basic conditions. Electrochemistry explains the creation of galvanic and electrolytic cells. You find out about both topics in this part. 

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