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Galvanic Scale


Galvanic corrosion is an often overlooked problem with roofs. If two metals are in contact with each other with an electrolyte present, then the metal which is less noble, or more anodic, will start to corrode.

The following Galvanic Scale shows how metals rate. Metals that are more electropositive, or anodic, and will corrode more easily are at one end and the metals that are more electronegative, or cathodic, and are more resistant to corroding are at the other end.

GALVANIC SCALE
ANODIC / LESS NOBLE / CORRODED END
Zinc
Aluminum
Galvanized Steel
Cadmium
Mild Steel, Wrought Iron
Cast Iron
Stainless Steel, Types 304 and 316 (active)*
Lead-Tin Solder
Lead
Brass, Bronze
Copper
Stainless Steel, Types 304 and 316 (Passive)
CATHODIC / MORE NOBLE / PROTECTED END
*Active Stainless Steel is stainless steel that hasn’t been chemically cleaned.

Let’s look at an example. Say you took some galvanized nails and used them to secure some copper flashings…and then it rains. (Rain water is a great electrolyte.) Over a period of time, the more noble copper will corrode the less noble galvanized steel, then you have nothing left to hold your copper flashings.

Use the following guidelines in order to prevent corrosion:
  • Avoid contact between metals that are farther apart in the scale.
  • Keep metals separated by using non-conductive, non-absorbant materials such as tapes, sealants, etc.
  • Coat the more noble metal with a nonconductive coating such as an approved paint or a bituminous coating.
  • Prevent moisture from running off of a more noble metal to a less noble metal (i.e. a copper drain line running into a galvanized gutter or collector head).

 

  
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