Bimetallic contact

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. 

Table 1.25 is reproduced from Corrosion and its Prevention at Bimetallic Contacts (H.M.S.O., London, 1958) by permission of the Director of Publications. The reader should note that it is recommended that the Table be used only in conjunction with the Introduction to the original publication. 

Commentary on corrosion at bimetallic contacts and its alleviation , British Standards Institute, PD6484 (1979), confirmed August (1984)... 

Corrosion in tropical environments has been the subject of several papers, some of which deal with corrosion at bimetallic contacts . 

Corrosion of nickel or Bimetallic contact Bimetallic contact... 

Cadmium also provides a sacrificial coating to steel which gives better protection than zinc in applications where strong acids and alkalis may be encountered and those involving immersion in stagnant or soft neutral waters. It should be used in applications involving bimetallic contact with aluminium and in electrical applications where ease of solderability is important. Cadmium has a low torque resistance and should be used as a coating material in cases where bolted assemblies have to be frequently... 

This particular topic remains vital but often controversial especially when attempts are made to codify practice and opinion. The British Standards Institution have published a Commentary on corrosion at bimetallic contacts and its alleviation which represents an important first attempt to produce such a code. It lists 23 metals and alloys coupled to each other in three atmospheric and two immersed environments using a four-point subjective scale to describe behaviour. 

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... 

Some substances that may be present merely as traces in the environment can have a profound effect on behavior at bimetallic contacts. The corrosion at the junction may be accelerated or reduced substantially. 

Hot in hot hard waters, the scale that forms at water temperatures above about 55°C has a coarse-grained structure and adheres less well to the zinc surface. Corrosion of the zinc will still occur locally, as a result of discontinuities in the scale or local electrochemical action. Above about 60°C, zinc usually becomes cathodic to steel, and in bimetallic contacts the steel will then corrode first. Pouibaix (1970) has performed success-fill laboratory simulations of the corrosion of galvanized water distribution pipes. 

British Standards Institution. (1979). PD 6484 Corrosion of Bimetallic Contacts and Its Alleviation. BSI, London, 32 pp. 

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