Homogeneous galvanic corrosion

Homogeneous galvanic corrosion may also occur on the surface of steel components that are covered or partially covered with mill scale (magnetite, Fe304) or iron sulfide corrosion products. Both mill scale and iron sulfide are noble with respect to steel. Significant galvanic corrosion can occur where breaks or holidays in these corrosion products expose unprotected metal. 

Another form of microstructural galvanic corrosion, graphitic corrosion, is unique to gray and nodular cast irons. It may be encountered in cast iron pumps and other cast iron components. It is a homogeneous form of galvanic corrosion, not requiring connection to a different metal. 

Materials of construction for the zinc/mercuric oxide cells are limited not only by their ability to survive continuous contact with strong caustic alkali, but also by their electrochemical compatibility with the electrode materials. As far as the external contacts are concerned, these are decided by corrosion resistance, compatibility with the equipment interface with respect to galvanic corrosion, and, to some degree, cosmetic appearance. Metal parts may be homogeneous, plated metal, or clad metal. Insulating parts may be injection-, compression-, or transfer-molded polymers or rubbers. 

Except in rare cases, there is no homogeneity of materials, metals or alloys in buildings or in mechanically joined structures. For example, metallic fittings in aluminium are always joined with screws in stainless steel, while accessories such as hinges, filters, lift-off hinges are in stainless steel or chrome-plated steel, or even in brass. Under appropriate conditions, there is a risk of galvanic corrosion of aluminium. 

Wood, concrete, and soil are porous materials, generally exposed to the atmosphere, and they can come in contact also with metallic materials. Their porosity, not occluded, allows the diffusion of cathodic reagents the presence of water allows the ionic conduction for which these solid environments may allow electrochemical corrosion. They are intrinsically heterogeneous environments, for which the localization of the anodic and cathodic areas may occur also on homogeneous metallic materials. The water and the dissolved substances determine the value of the electrical resistance that the environment can oppose to the galvanic couplings. 

---