Cathodic protection electrical basis

The electrode potential may be lowered into the region of immunity by making the specimen the cathode in a suitable galvanic cell (in which dissimilar metals are in electrical contact) or an electrolytic cflL This is the basis of cathodic protection (section 10 5.3) A lowering of the potential from the FeOj" zone into the passive zone is an example of incomplete cathodic protection ... 

Galvanizing iron sheet is an example of useful application of galvanic action or cathodic protection. Iron is die cathode and is protected against corrosion at the expense of the sacrificial zinc anode. Alternatively, a zinc or magnesium anode may be located in the electrolyte close to the structure and may be coimected electrically to the iron or steel. This method is referred to as cathodic protection of the structure. Iron or steel can become the anode when in contact with copper, brass, or bronze however, iron or steel corrode rapidly while protecting the latter metals. Also, weld metal may be anodic to the basis metal, creating a corrosion cell when immersed (Fig. 5). 

Hydrogen embrittlement is similar to stress corrosion, in that a normally ductile metal experiences brittle fracture when exposed to both a tensile stress and a corrosive atmosphere. However, these two phenomena may be distinguished on the basis of their interactions with applied electric currents. Whereas cathodic protection (Section 17.9) reduces or causes a cessation of stress corrosion, it may, however, lead to the initiation or enhancement of hydrogen embrittlement. 

When two different metallic materials are electrically connected and placed in a conductive solution (electrolyte), an electric potential exists. This potential difference will provide a stronger driving force for the dissolution of the less noble (more electrically negative) material. It will also reduce the tendency for the more noble metal to dissolve. Notice in Table 1.2 that the precious metals gold and platinum are at the higher potential (more noble or cathodic) end of the series (protected end), while zinc and magnesium are at the lower potential (less noble or anodic) end. It is this principle that forms the scientific basis for using such materials as zinc to sacrificially protect the stainless steel drive shaft on a pleasure boat. 

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