Galvanic 

Designs sometimes cause dissimilar metals to be in contact within an assembly.  Increased variance of the two metals’ anodic index cause accelerated corrosion of the more anodic metal.  The variance in anodic index can be controlled somewhat through coatings (paint, epoxy, powder coat, etc) and plating (zinc, chrome, etc).

The table below provides a generalize guide for predicting the anodic corrosion two dissimilar metals are prone to.  The service environment also plays a role:

• For harsh environments: outdoors, high humidity, and salt environments fall into this category. Typically there should be not more than 0.15 V difference in the “Anodic Index”. For example; gold - silver would have a difference of 0.15V being acceptable.
• For normal environments: storage in warehouses or non-temperature and humidity controlled environments. Typically there should not be more than 0.25 V difference in the “Anodic Index”.
• For controlled environments: where the temperature and humidity controlled, 0.50 V can be tolerated. Caution should be maintained when deciding for this application as humidity and temperature do vary from regions.

Anodic Index