Bimetallic Galvanic Corrosion

11) Unlike in the case of batteries, the loss of driving force is a welcome effect in corrosion. It is well known, for example, that the corrosion rate in poorly conducting solutions is lower than that in highly conducting media. 

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Galvanic corrosion is location specific in the sense that it occurs at a bimetallic couple (Fig. 16.2). It is metal specific in the sense that, typically, corrosion affects the metal that has less resistance in the environment to which the couple is exposed. Hence, in principle, we would anticipate galvanic corrosion of relatively reactive metals wherever they are in physical contact with relatively noble metals in a sufficiently aggressive, common environment. Experience has shown, however, that all such couples do not necessarily result in unsatisfactory service. This is because of the interplay of various critical factors that influence galvanic corrosion. These critical factors are discussed in the next section. 

Figure 16.2 Galvanic corrosion along plane of bimetallic contact.

Galvanic corrosion of dissimilar metals can be minimized by controlling humidity near such bimetallic connections. In general, continuously dry bimetallic joints do not corrode. 

It also follows that if the solution is stirred the rate of arrival of oxygen at the cathode will be increased. This will result in a corresponding increase in the rate of bimetallic corrosion as is shown in Fig. 1.63 for the aluminium-mild steel couple in stirred 1 - On NaCl solution . The increase in galvanic corrosion rate will be in the inverse relation to the slope of the anodic polarisation curve of the more negative metal, provided that the cathodic reaction is not totally diffusion controlled. 

Bimetallic corrosion in atmospheres is confined to the area of the less noble metal in the vicinity of the bimetallic joint, owing to the high electrolytic resistance of the condensed electrolyte film. Electrolytic resistance considerations limit the effective anodic and cathodic areas to approximately equal size and therefore prevent alleviation of atmospheric galvanic corrosion through strict application of the catchment area principle. 

Most of the published data on galvanic corrosion concern solid metal couples rather than bimetallic coating systems, and it is important to bear... 

Galvanic Corrosion corrosion associated with a galvanic cell (often used to refer specifically to Bimetallic Corrosion). 

Even single metals, however, are subject to aqueous corrosion by essentially the same electrochemical process as for bimetallic corrosion. The metal surface is virtually never completely uniform even if there is no preexisting oxide film, there will be lattice defects (Chapter 5), local concentrations of impurities, and, often, stress-induced imperfections or cracks, any of which could create a local region of abnormally high (or low) free energy that could serve as an anodic (or cathodic) spot. This electrochemical differentiation of the surface means that local galvanic corrosion cells will develop when the metal is immersed in water, especially aerated water. 

Corrosion occurring at bimetallic junctions may not in fact be galvanic corrosion but may be due to insulating or other effects. 

Formation of a Galvanic Cell. When a metal or alloy is electrically coupled to another metal or conducting nonmetal in the same electrolyte, a galvanic cell is created. The electromotive force and current of the galvanic cell depend on the properties of the electrolyte and polarization characteristics of anodic and cathodic reactions. The term galvanic corrosion has been employed to identify the corrosion caused by the contact between two metals or conductors with different potentials. It is also called dissimilar metallic corrosion or bimetallic corrosion where metal is the conductor material. 

Some of the CRP participants saw no corrosion of their aluminium alloy coupons, while others saw significant pitting. Pitting, crevice and galvanic corrosion were the main forms of corrosion observed. Crevice corrosion was not always accompanied by pitting of the aluminium surfaces within the crevice. Bimetallic corrosion of aluminium alloys coupled to stainless steel generally resulted in accelerated corrosion with pitting. 

Galvanic corrosion, also called bimetallic corrosion, results from the formation of an electrochemical cell between two metals. The corrosion of the less noble metal is thus accelerated. 

The corrosion-like electrochemical process of material removal refers to spatially uniform general corrosion of the metal surface. However, the wet CMP environment can also support certain other types of undesirable electrochemical corrosions, such as localized pitting, and bimetallic/galvanic decomposition that contribute to surface defects. The considerations for mitigating these defects constitute a major aspect of slurry (additive) selection, which in turn can be facilitated by the use of electrochemical techniques. 

It must be noted that the above guideUnes are very qualitative and do not provide specific iniformation as to corrosion rate, nor do they make distinctions about the performance of different materials, or the effect of bimetallic or other galvanic corrosion cells. For more information, the reader is referred to Refs 2,16, and 17. 

Corrosion caused by the connection of two or more different metals also occurs underground. This electrochemical corrosion cell is commonly referred to as bimetallic or galvanic corrosion. Typical examples include brass or bronze valves connected to steel or cast iron pipes and stainless steel fasteners coimected to steel or cast iron. These couplings of dissimilar metals will locally affect the corrosion rate. Aluminum can be severely corroded if directly connected to most other engineering alloys, such as steel, iron, copper, or stainless steel—dielectric isolation must be used. 

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