Quantcast Relative Area of Anodic Member

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7.8.2 Relative Area of Anodic Member. When joints between dissimilar metals are unavoidable, the anodic
member of the pair should be the largest of the two. For a given current flow in a galvanic cell, the current
density is greater for a small electrode than for a larger one. The greater the current density of the current
leaving an anode, the greater is the rate of corrosion as illustrated by Figure 7-18. As an example, if a copper
strap or cable is bonded to a steel column, the rate of corrosion of the steel will be low because of the large
anodic area.  On the other hand, a steel strap or bolt fastener in contact with a copper plate will corrode
rapidly because of the relatively small area of the anode of the cell.
7.8.3 Protective Coatings. Paint or metallic platings used for the purpose of excluding moisture or to provide
a third metal compatible with both bond members should be applied with caution. When they are used, both
members must be covered as illustrated in Figure 7-19. Covering the anode alone must be avoided. If only the
anode is covered then at imperfections and breaks in the coating, corrosion will be severe because of the
relatively small anode area. All such coatings must be maintained in good condition.
Whichever bonding method is determined to be the best for a given situation, the mating surfaces must be
cleaned of all foreign material and substances which would preclude the establishment of a low resistance
connection. Next, the bond members must be carefully joined employing techniques appropriate to the specific
method of bonding. Finally the joint must be finished with a protective coating to ensure continued integrity of
the bond. The quality of the junction depends upon the thoroughness and care with which these three steps are
performed. In other words, the effectiveness of the bond is influenced greatly by the skill and conscientiousness
of the individual making the connection.  Therefore, this individual must be aware of the importance of
electrical bonds and must have the necessary expertise to correctly implement the method of bonding chosen
for the job.
Those individuals charged with making bonds must be carefully trained in the techniques and procedures
required. Where bonds are to be welded, for example, work should be performed only by qualified welders. No
additional training should be necessary because standard welding techniques appropriate for construction
purposes are generally sufficient for establishing electrical bonds. Qualified welders should also be used where
brazed connections are to be made.
Exothermic welding can be effectively accomplished by personnel not specifically trained as welders. Every
individual doing exothermic welding should become familiar with the procedural details and with the
precautions required with these processes. Contact the manufacturers of the materials for such processes for
assistance in their use. By taking reasonable care to see that the bond areas are clean and free of water and
that the molds are dry and properly positioned, reliable low resistance connections can be readily achieved.
Pressure bonds utilizing bolts, screws, or clamps must be given special attention. Usual construction practices
do not require the surface preparation and bolt tightening necessary for an effective and reliable electrical
bond. Therefore, emphasis beyond what would be required for strictly mechanical strength is necessary. Bonds
of this type must be checked rigorously to see that the mating surfaces are carefully cleaned, that the bond
members are properly joined, and that the completed bond is adequately protected against corrosion.


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