Galvanic anodes forms

Figure 7.8 (a) Galvanic anodes for installation in cored holes. Anodes are linked with titanium wires to form a zone, (b) Galvanic anodes being installed in core holes in a wall. Courtesy Fosroc Ltd. 

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. 

It will be evident from the underlying thermodynamics and kinetics (Fig. 10,25) that successful anodic protection of a metal relies upon the maintenan of a stable passivating film. A simple example of galvanic anodic protection is the addition of alloying elements (0,1% Pd or 1% Cu) to stainless steel which form local cathodes (Fig. 10.34),... 

Localized corrosion, which occurs when the anodic sites remain stationary, is a more serious industrial problem. Forms of localized corrosion include pitting, selective leaching (eg, dezincification), galvanic corrosion, crevice or underdeposit corrosion, intergranular corrosion, stress corrosion cracking, and microbiologicaHy influenced corrosion. Another form of corrosion, which caimot be accurately categorized as either uniform or localized, is erosion corrosion. 

It is somewhat less corrosion resistant than tantalum, and like tantalum suffers from hydrogen embrittlement if it is made cathodic by a galvanic couple or an external e.m.f., or is exposed to hot hydrogen gas. The metal anodises in acid electrolytes to form an anodic oxide film which has a high dielectric constant, and a high anodic breakdown potential. This latter property coupled with good electrical conductivity has led to the use of niobium as a substrate for platinum-group metals in impressed-current cathodic-protection anodes. 

Anodic oxide formation Lakhiani and Shreir have studied the anodic oxidation of niobium in various electrolytes, and have observed that temperature and current density have a marked effect on the anodising characteristics. The plateau on the voltage/time curve has been shown by electron microscopy to correspond with the crystallisation of the oxide and rupture of the previously formed oxide. It would appear that this is a further example of field recrystallisation —a phenomenon which has been observed previously during anodisation of tantalum" . No significant data on the galvanic behaviour of niobium are available however, its behaviour can be expected to be similar to tantalum. 

A detailed discussion of galvanic corrosion between dissimilar metals in contact in a corrosive environment has been given in Section 1.7, but in the case of coating discontinuities the effect of the anode/cathode area relationship and the nature of any corrosion products formed at small discontinuities may modify any choice made on strict considerations of general galvanic corrosion theory based on the potentials of the coating and substrate in the environment under consideration. 

As noted previously, dealloying also affects pre-boiler components manufactured of cast iron, such as FW pumps and valves. These components may, under certain (long-term) circumstances, suffer a selective form of preferential leaching galvanic corrosion. Specifically, graphitic corrosion may take place when the (anodic) iron matrix con-... 

A form of corrosion resulting from the presence of two dissimilar metals such as steel and copper in an electrolyte such as water forming a galvanic couple, whereby the less noble anodic metal (in this case steel) corrodes. 

---