IMOA Stainless Solutions: Sunscreen Design

Each month, Stainless Solutions from IMOA covers a different stainless steel issue with tips on design and specification, and links to technical resources. This month IMOA provides resources for designing energy-saving, long-lasting exterior sunscreens.

 

Exterior building shading can substantially lower or even eliminate air-conditioning costs, increase occupant comfort, and reduce the building’s total environmental impact. It can also update the appearance of existing buildings and create interesting aesthetic options for new building design. IMOA’s Bioclimatic Second-Skin Facade Page provides more information about the benefits of sunscreens and additional project examples.

Researchers around the world have studied hundreds of buildings to look at actual performance, using sophisticated testing facilities to develop software that predicts the benefits of interior and exterior screening as part of fenestration modeling. One example is COMFEN, which was developed by the Lawrence Berkeley National Laboratory (LBNL) in the U.S. in cooperation with similar government-sponsored research facilities in other parts of the world. This free software automatically imports much of the data needed for an assessment including information specific to the material and finish. The Chinese and Indian governments have adopted and customized this software and testing laboratories have been constructed in China and Singapore to test fenestration products in cooperation with LBNL.

While the benefits of sunscreens are numerous, designers and engineers are not always aware of the unique material performance challenges associated with them. Unlike sealed curtainwall systems, both sides of the panels and their structural supports are constantly exposed to the moisture (rain, fog, condensation, and humidity) and the corrosive surface deposits necessary for corrosion. From a material selection and design standpoint, the following factors must be considered:


Atmospheric Corrosion
The building-facing side of sunscreens is often very visible to occupants and will accumulate higher levels of corrosive deposits and dirt than the rain-washed outer side. This can make a regular manual cleaning program, smoother finishes that retain less dirt, and/or more corrosion-resistant metals for panels and structural supports necessary to avoid an unsightly appearance. IMOA’s selection system and resource pages provide guidance. Sheltering makes the service environment of affected components more corrosive in most parts of the world.
 

Galvanic Corrosion

Galvanic corrosion is a significant concern when different metals are used for structural supports, fasteners and panels. The presence of moisture (even humidity) on a regular basis, the relative surface area of different metals, and their location in the “galvanic series” determine whether the corrosion rate of one metal will be accelerated. If they are not in contact with other metals, aluminum or carbon steel structural supports and fasteners have higher corrosion rates than stainless steel and that limits the service life of stainless steel sunscreen panels. If they are in direct contact with stainless steel or any other more noble metal in a sunscreen, they will corrode at a highly accelerated rate. Stainless steel structural supports and fasteners are therefore a much better long- term solution. The duplex stainless steels also provide higher strength levels than either aluminum or the common carbon steels.

Maintaining metal separation to prevent direct contact between dissimilar metals on a long term basis is unlikely. Guidelines for Corrosion Prevention provides information on galvanic corrosion, the galvanic series, and how to design to avoid the problem.

The Nordic Embassies in Berlin, Germany made creative use of copper, wood and glass sunscreens which are supported by stainless steel. The different materials delineate buildings. Stainless steel was used to support the copper to avoid a galvanic corrosion problem.

 

Crevice Corrosion
All metals are subject to crevice corrosion. It can occur even when the metal might not otherwise have a corrosion problem. With stainless steel, this type of corrosion can only occur if salts are present. With low levels of salt, Type 316 provides good performance. When there is exposure to higher levels of deicing or coastal salt, a more corrosion-resistant stainless steel should be considered. Mechanical joints, woven metal fabrics and gratings that are swaged together and not welded are examples of design components with crevices.

Mechanical joints are exposed to the weather in sunscreen designs and may not remain sealed if the structure’s components have different coefficients of thermal expansion. The movement of the structure and coating loss over time due to abrasion should also be considered.

If the weave of a stainless steel panel is tight, crevice corrosion can occur wherever wires cross. Type 904L (UNS N08904) and other alloys with improved corrosion performance are available for locations with higher salt exposure.

 

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