Bimetallic corrosion contacts

Finally, it is important to point out that although in localised corrosion the anodic and cathodic areas are physically distinguishable, it does not follow that the total geometrical areas available are actually involved in the charge transfer process. Thus in the corrosion of two dissimilar metals in contact (bimetallic corrosion) the metal of more positive potential (the predominantly cathodic area of the bimetallic couple) may have a very much larger area than that of the predominantly anodic metal, but only the area adjacent to the anode may be effective as a cathode. In fact in a solution of high resistivity the effective areas of both metals will not extend appreciably from the interface of contact. Thus the effective areas of the anodic and cathodic sites may be much smaller than their geometrical areas. 

Protective measures against bimetallic corrosion should ideally start before the particular installation or equipment is built . Reference should be made to tables showing compatibility of metals, alloys and non-metallic materials (5 Table 1.25) and to the literature. However, it must be emphasised that the environment obviously plays a most important role in bimetallic corrosion, and that there are a number of situations in which apparently incompatible materials in contact can be used without adverse effects. 

Both metals are applied to copper-base alloys, stainless steels and titanium to stop bimetallic corrosion at contacts between these metals and aluminium and magnesium alloys, and their application to non-stainless steel can serve this purpose as well as protecting the steel. In spite of their different potentials, zinc and cadmium appear to be equally effective for this purpose, even for contacts with magnesium alloys Choice between the two metals will therefore be made on the other grounds previously discussed. 

Bimetallic Corrosion corrosion of two metals in electrical contact, in which one metal stimulates attack on the other and may itself corrode more slowly than when it is not in such contact galvanic is often used in place of bimetallic. 

In bimetallic corrosion, the anodic and cathodic surfaces are well defined, being different metals, and are established instantly on placing the metals in electrical contact. The corrosion that results can be very vigorous, but one can usually arrange either to avoid using dissimilar metals together or... 

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. 

There is an apparent exception to the necessity for the two metals to be in electrical contact for bimetallic corrosion to occur namely, when a noble metal corrodes slightly and dissolves in water that subsequently flows over a less noble metal, the more noble metal may deposit on the less noble metal, forming a true bimetallic contact. For example, copper can dissolve very slightly in some natural waters and may then deposit on zinc. Bimetallic corrosion may also be experienced when two dissimilar metals, not in direct contact, are nevertheless connected electrically. 

Bimetallic corrosion is more severe under immersed conditions than in the atmosphere. In the latter, attack occurs only when the bimetallic contact is wet this depends on many factors, such as the presence or retention of moisture in crevices, the effectiveness of drainage, and the speed of evaporation. The relative size of the areas of the metals that remain wet in the vicinity... 

After nearly 20 years, the official responsible for maintenance at the War Museum reported that the structure was in perfect condition. Just covered by dust. No need for maintenance in the foreseeable future is envisaged. It should be noted that the roof covering is copper but, because the contacts between the galvanized steel and the copper are in the dry and there is no water present to provide an electrolytic path Joining the two metals, bimetallic corrosion does not occur (see British Standard PD 6484). 

In the previous case, it was a question of a type of bimetallic corrosion without electrical contact between the metals involved. The process initially is difficult to understand it is, however, easy to understand if one considers it as a battery with an external short circuit caused by an electrolyte behaving in the same way. The results of measurements on a model and the action of cathodic protection are described in Ref. 23. 

Aluminum and its alloys are also suitable for use in marine atmospheres. Care must be taken in the design since under these conditions the aluminum is susceptible to bimetallic corrosion when in contact with copper or carbon steel. If the chloride concentration is high, the aluminum may be susceptible... 

The ratio of the area of metals in contact, the duration of wetness, and the conductivity of the electrolyte will determine the severity of corrosive attack. Seawater, which is a highly conducting solution, will produce a more severe bimetallic corrosion than most fresh waters that generally have a lower conductivity. A film of moisture condensed from the air or rainwater can dissolve contaminants conducive to bimetallic corrosion. 

Prevention of bimetallic corrosion can be accomplished by preventing the flow of the corrosion currents between the dissimilar metals in contact. This can be by either insulating the dissimilar metals from each other (breaking the metallic path) or by preventing the formation of a continuous bridge of a conductive solution between the two metals (breaking the electrolytic path). 

Bimetallic corrosion Corrosion of two metallics in electrical contact. The term galvanic corrosion is used generally in place of bimetallic. 

The galvanic corrosion is sometimes defined as bimetallic corrosion and described mainly for engineering structures as they are fabricated from dissimilar materials, which are in electrical contact with a conductive electrolyte. The galvanic phenomena are considered in these latter cases at the macroscopic scale. The galvanic series (widely reported for example in seawater) which report the... 

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.

Macroscopic heterogeneities, e.g. crevices, discontinuities in surface films, bimetallic contacts etc. will have a pronounced effect on the location and the kinetics of the corrosion reaction and are considered in various sections throughout this work. Practical environments are shown schematically in Fig. 1.3, which also serves to emphasise the relationship between the detailed structure of the metal, the environment, and external factors such as stress, fatigue, velocity, impingement, etc. 

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