Electrons can flow from one metal to another, which may contribute to metal corrosion under the right conditions. When two metals are placed in contact during construction it is necessary to take into account the galvanic compatibility of the materials or a serious structural problem may occur with time.
Parameters:
(1) difference in the anodic index between 2 metals
(2) the environment that the materials will be in
The anodic index (AI) is a voltage scale that ranges from 0 (gold) to 1.85 (beryllium). Gold is considered cathodic and does not corrode. The higher the anodic index the more anodic the material and the greater its propensity to corrode.
difference in the anodic index =
= ABSOLUTE((anodic voltage for one material) - (anodic voltage for the second material))
Environment
Example
Allowed Difference in AI
controlled
temperature and humidity controlled
<= 0.50 V
normal
indoors but without temperature and humidity control
<= 0.25 V
harsh
outdoors, high humidity, salt
<= 0.15 V
If the difference in the AI exceeds the environmental conditions then
(1) different materials must be used
(2) the metals must be coated or separated
(3) a "sacrificial anode" metal must be provided
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