Steel or Aluminum?

Whether in steel or aluminum, Geometrica structures are highly resistant to corrosion.

Zinc is the most effective way to protect steel and Geometrica's steel tubes are always galvanized. For most structure exposures Geometrica's standard galvanizing has an expected life to first maintenance of over 40 years. A duplex system may be used in harsh conditions such as moist enclosed spaces or highly polluted environments.  This system consists of a powder applied paint coating over the galvanized surfaces.  Tubes and purlins as well as washers have fully galvanized surfaces according to ASTM A653 G-90 for the former and ASTM A123 for the latter.  This duplex system provides top level protection against corrosion and extends the service life of the galvanized steel, offering both a cathodic as well as barrier protection against corrosion

Aluminum structures are available for extraordinarily corrosive environments, such as swimming pool covers (subject to chlorine attack), or exposed structures within 50m of the ocean (such as those featuring internal cladding at a port). Aluminum is naturally resistant to corrosion and may be left unpainted. On contact with air it forms a resistant layer of aluminum oxide that arrests any further attack. And Geometrica’s aluminum structures may also be clear anodized to maintain their appearance. Geometrica has built the world's largest aluminum domes. All Geometrica structures may also be powder- or wet- coated in a variety of attractive colors.

Steel and Aluminum!

All Geometrica galvanized steel structures contain some aluminum components: the connectors. This raises the frequent question of whether the two materials are compatible or are subject to galvanic corrosion. By theory, experiments and practice, these two materials are definitely compatible.

  1. Theory predicts that Aluminum and galvanized steel are compatible. These materials are adjacent to each other in the galvanic scale (e.g. Jones [1], pg. 169). The presence of aluminum coupled to the galvanized steel increases the current density (corrosion rate) of the zinc by only 0.1% to 1% (aluminum is the cathode, or protected side of the couple) (Jones [1], table 6.17, pg 184). This increase in the corrosion rate of zinc is negligible, and therefore, the contact between the aluminum and galvanized steel does not accelerate significantly the corrosion of either material.
  2. Testing confirms the theory. For example, Doyle and Wright [2] show that the rate of corrosion of aluminum does not increase from being in contact with galvanized steel. They conclude: "zinc is very compatible with aluminum in all atmospheres, in several cases showing that aluminum was actually cathodically protected by zinc.
  3. Codes have incorporated the above results. For example, the Aluminum Association in its structural code states that there is no need to separate or paint galvanized steel surfaces in contact with aluminum. (Aluminum Design Manual [3], pg. I-B-62).
  4. Many structures and components by Geometrica and others built over the past half century attest to the above. Aluminum cladding is frequently fixed to galvanized structures, and often using zinc coated fasteners. Most exterior grandstand structures in North America have galvanized understructure to which structural aluminum seating and flooring is fixed. And countless Geometrica hubs in service around the world join galvanized steel tubes successfully and permanently.

[1] Jones, D., Principles and Prevention of Corrosion, Mcmillan, New York, 1992.
[2] Doyle, D.P. and Wright, T. E., "Quantitative Assessment of Atmospheric Galvanic Corrosion", Galvanic Corrosion, ASTM STP 978, H.P.Hack, Ed., American Society for Testing and Materials, Philadelphia, 1988, pg. 168.
[3] Aluminum Design Manual, The Aluminum Association, Washington D.C., 2000