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G a l v a n i c S c a l e
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| 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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