Fastener Selection Guidelines for Use with Galvalume� Sheet Although there are many selection criteria to consider when choosing a fastener for use on a metal building, one of the most important is the compatibility of the fastener with the panel material. Another aspect of the selection process should also include the expected service life of the fastener relative to the other components used in the building construction. When these other components are made of Galvalume sheet, where service life in excess of 20 years is to be expected, equivalent durability should be expected from the fastener. Although the best advice for fastener selection will come from your fastener supplier or manufacturer, we offer these guidelines based on our own accelerated laboratory tests and on-site building inspections. Compatibility:Exposed corrosion resistance is generally a well understood concept: If a material rusts or otherwise fails in a given environment in a relatively short time, it is not acceptable. Compatibility, however, is more complex in that it not only involves corrosion resistance in the local environment, but also the interaction of materials in intimate contact. Contact can either be direct with the materials physically touching each other, or the contact can be made with an electrically conductive solution such as ponding water. An example of this is exposed fasteners on a metal roof, especially if a rubber or polymeric sealing washer is not used. Galvanic Corrosion: When a more active metal is in electrical contact with another less active metal, the more active component will sacrifice itself to prevent the other from corroding. This is known as dissimilar metal contact, or galvanic corrosion and, under the right conditions, can be very aggressive. For instance, if one were to install an unprotected steel screw in a Galvalume sheet building panel, the zinc in the coating at the area of contact with the screw head would sacrifice itself to protect the screw. Aside from the natural rusting of the screw head itself over time, the panel coating in the area of the screw would be consumed and the panel would begin to rust. In a corrosive environment (acid rain, for example), the process is speeded up considerably due to the increased conductivity of the electrolyte (rain water). There are many other factors which can affect the degree of galvanic corrosion in addition to the basic explanation given above, such as: the relative exposed area ratio of the metals, their polarization characteristics, the nature of the corrosion products and the total time of exposure to wetness. For more general information on the compatibility of building materials, request Bethlehem's Technical Bulletin 204: Compatibility of Materials Used in Metal Building Construction. Galvalume� is a registered trademark of BIEC International, Inc. The Tests:Laboratory testing examined the performance of fasteners on bare and prepainted Galvalume sheet in two corrosive environments: standing water and the 2-liter Kesternich SO2 (DIN 50018) cabinet. The fasteners were screwed into prepunched holes in Galvalume sheet panels taken from standard production material. The paint system of the prepainted Galvalume sheet consisted of a Bethlehem-approved primer with a silicone-modified polyester topcoat. Standing Water: A standing water environment can simulate the conditions sometimes found on areas of low-slope roofs and at drip edges. The test panels with mounted fasteners were placed flat in shallow plastic trays which were in turn placed in a humidity cabinet at 100% relative humidity at 140�F. The trays were deep enough so that distilled water completely covered the panels and fasteners. After 2000 hours of constant immersion, the panels were removed for evaluation. Kesternich Test: The Kesternich test is widely used in the industry for studying the corrosion resistance of materials. It most closely simulates a severe industrial environment, incorporating 8 hours exposure to an atmosphere of sulfur dioxide gas and 100% relative humidity at 100�F with 16-hour periods of drying. The panel assemblies were tested for a total of forty 24-hour cycles. The fastener materials included: 300-series stainless steel (in both solid and other configurations), 400-series stainless steel (with and without a zinc-plated coating), a zinc/aluminum cast alloy, a cast nylon plastic, electroplated zinc and electroplated cadmium. Painted fasteners were not tested due to the large variety of paints available. During the tests, only the exposed heads and washers were evaluated: Other aspects, such as the shanks or the strength of the head, were not taken into account. Building Inspections:Accelerated tests can quickly evaluate and compare materials, but there is no substitute for in-service field examinations. Bethlehem personnel have examined many buildings over the years and continue this practice as an ongoing evaluation of the performance of Galvalume sheet. In addition to studying roof and sidewall panels, these inspections were used to observe the performance of trim elements used on metal buildings and the exposed fasteners in a wide variety of environments. So, What Fasteners Should I Use?Combining the results of our laboratory tests and field inspections has revealed some clear precautions when choosing a fastener for long-term durability and compatibility with Galvalume sheet building products. The table below summarizes our findings and ranks the materials in order of descending preference.
Other Precautions:Building inspections have revealed another area of concern: fastener installation. When a screw is overdriven or driven in at an angle, two potentially harmful things can occur: A sealing washer can be destroyed causing a leak, and, the screw head can cut through the coating of a steel panel. Although zinc and 55% aluminum/zinc coatings will protect the damaged area for a time, the damaged area can rust prematurely depending on how much steel is exposed and on the local environment. Overdriving a fastener can also cause a depression in the panel depending on the force used during installation. These depressions can trap and hold water for extended periods resulting in a localized ponding condition. Fasteners come in a wide assortment of configurations, sizes, materials and strengths depending on the end use. Most fastener manufacturers offer technical, applications and installation assistance for the use of their products. When you need advice for a specific problem or use, take advantage of the expertise offered by the manufacturers. |
