October 08, 2010 AdminHBA Design, Technology, Wisdom (Lessons Learned)

Material, Design, and Galvanic action…

Screen wall design for Portsmouth Courts Complex
– Sungjae Lee

I am involved in material research and the detail design for several areas on the new Portsmouth Courts Complex.

One example is the screen wall enclosure that hides the chillers, generator and transformer outside the building. Through my experience for the last few projects, this usually constitutes a solid wall built up with a brick veneer and concrete masonry units. The design team for this project selected a perforated, corrugated, anodized aluminum panel with brick piers as the best solution.

The aluminum perforated, corrugated panel is not only used for the purpose of visual design and aesthetics, but also serves an important technical function.  The perforated wall panels are designed to be 6″ from the ground to enhance air circulation to aid in peak performance of the equipment; it allows air movement around the electrical equipment and mechanical chiller units.  There are a number of perforation size and pattern options; however the 10% option was selected because it provided the greatest visual screening while allowing for optimal air movement.  The use of this type of the panel also provides a cost savings to the project over an all-solid masonry wall.

Each aluminum panel is structurally supported by  a steel tube frame that is anchored to the brick piers. Special care was taken in the detail design of the connection between the steel tube and the aluminum panel due to potential galvanic action.

Galvanic corrosion is an electrochemical process in which one metal corrodes preferentially when in electrical contact with a different type of metal and both metals are immersed in an electrolyte.  This can be as simple as the moisture in the air The electrolyte provides a means for ion migration whereby metallic ions can move from the anode to the cathode. This leads to the anodic metal corroding more quickly than it otherwise would; the corrosion of the cathodic metal is retarded even to the point of stopping.  What this really means, is the less noble metal, Steel in this case is susceptible to higher rate of corrosion.To prevent the action, different metals should be isolated from one another, or compatible metals should be used. For this condition, as shown on the illustration, we specify an asphalt coating on the steel tube in contact with the aluminum panel to isolate one from the other material and to prevent the galvanic action.