Galvanic electrodes

It is important to note that both the polarographic and galvanic electrodes measure the oxygen tension of the medium in which they are placed (Doran, 2013). So when an electrode is placed in a liquid, it does not measure dissolved oxygen, but rather the dissolved oxygen partial pressure, which is proportional to oxygen... 

Figure 4.3 Schematics showing the typically construction of (a) polarographic and (b) galvanic electrodes. Adapted from Linek (1988).

Define the galvanic electrode potential for the cell shown in Figure 2.4 using the interfacial potentials involved in the electrochemical process. 

After a heat treatment of several hours the electrodes are deposited by sputtering a 50 nm base layer of Ni/Cr or NiAVi followed by 1.5 pm Au-layer generated by galvanization. 

Galvanic cells in which stored chemicals can be reacted on demand to produce an electric current are termed primaiy cells. The discharging reac tion is irreversible and the contents, once exhausted, must be replaced or the cell discarded. Examples are the dry cells that activate small appliances. In some galvanic cells (called secondaiy cells), however, the reaction is reversible that is, application of an elec trical potential across the electrodes in the opposite direc tion will restore the reactants to their high-enthalpy state. Examples are rechargeable batteries for household appliances, automobiles, and many industrial applications. Electrolytic cells are the reactors upon which the electrochemical process, elec troplating, and electrowinning industries are based. 

TABLE 28-1 b Galvanic Series in Sea Water (Approx )/ Volts vs Sat Calomel Ref Electrode... 

Tafel Extrapolation Corrosion is an elec trochemical reac tion of a metal and its environment. When corrosion occurs, the current that flows between individual small anodes and cathodes on the metal surface causes the electrode potential for the system to change. While this current cannot be measured, it can be evaluated indirectly on a metal specimen with an inert electrode and an external electrical circuit. Pmarization is described as the extent of the change in potential of an electrode from its equilibrium potential caused by a net current flow to or from the electrode, galvanic or impressed (Fig. 28-7). 

Fig. 5-4 Cell currents between a coated specimen (5 = 300 cm ) and uncoated steel electrode (5 =1.2 cm ) in NaCl solutions at 25°C. Left shot-peened steel sheet, 150 pva EP-tar. Right hot-dipped galvanized steel sheet, 150 jim EP-tar.

Potential measurements have been carried out at suitable times on platforms with galvanic anodes after the structures have been commissioned. Where impressed current protection was installed, the potential as well as the anode current was measured with fixed, built-in measuring electrodes during the commissioning period. 

The impressed current method with metal oxide-coated niobium anodes is usually employed for internal protection (see Section 7.2.3). In smaller tanks, galvanic anodes of zinc can also be used. Potential control should be provided to avoid unacceptably negative potentials. Pure zinc electrodes serve as monitoring and control electrodes in exposed areas which have to be anodically cleaned in the course of operation. Ag-AgCl electrodes are used to check these reference electrodes. 

The galvanic probe continuously monitors the corrosion characteristics of the drilling fluid. The probe (Figure 4-473) consists of two dissimilar metal electrodes, usually brass and steel. The electrodes are mounted on, but insulated... 

The metal with the more negative corrosion potential in the environmental conditions prevailing (note that the standard electrode potentials are seldom applicable and the galvanic series can be misleading)... 

Serious pitting may occur in the area of welds, particularly in sea-water. Corrosion rates of up to lOmm/y have been reported in weld joints of ice-breakers. The severe corrosion has been attributed to galvanic effects between the weld metal and the steel plate. The use of more noble electrodes for welding are reported to overcome this problem . 

---