Architectural Metals and Galvanic Action

At the moment, I’m working on a façade that uses metals extensively.  Toward this effort, I'm reviewing some basic information about architectural metals.  Here are a few brief notes on metal types and the order of their galvanic action.

Architectural Metals
Lead: Low melting point.  Popular roofing material for centuries
Tin: 2 categories

• The alloying of tin with other metals (Bronze = about 90% Copper + 10% Tin)
• The coating of tin with harder metals (tinnplated iron or steel)

Zinc
Used for roofing, subject to creep at ordinary temperatures.  Creep has been reduced by the introduction of titanium in most architectural zinc in North America.  Galvanization is the process of hot-dipping iron or steel in zinc.
Copper

• Copper: Durable.  The green patina is copper sulfate.
• Copper Alloys
  • Bronze = Copper + Tin
  • Brass = Copper + Zinc

Nickel Alloys

• Nickel silver = 75% copper + 20% nickel + 5% zinc
• Monel metal = 2/3 Nickel + 1/3 Copper (Pioneer to present-day use of stainless steel)

Iron

• Wrought Iron
• Cast Iron (Brittle, but strong in compression, see Bradbury Building as example)
• Sheet Iron (Used for 19^th Cent. ‘tin ceilings’ though no tin was actually involved)
• Steel (Alloy of iron with carbon.  carbon content of 0.2% - 2.1%)
  • Stainless steel ( see previous post on Stainless Steel)

Aluminum

• Most abundant element in the earth’s crust.
• Receptive to high performance coatings.

Galvanic Action
Also called electrolysis, is deterioration that occurs when different metals, or alloys, come in contact.  When this contact occurs in the presence of an electrolyte (H₂O), an electric current will flow from one metal to the other.  Eventually, one metal corrodes and the other stays intact.  Here is the list of galvanic activity; each metal can be corroded by those that follow it.  Metals far apart on the list should not be placed in contact with each other.  In order to prevent galvanic action, metals should be isolated from each other in some way.

1. Aluminum
2. Zinc
3. Iron and Steel
4. Stainless Steel
5. Tin
6. Lead
7. Brass
8. Copper
9. Bronze
10. Gold