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One of the world's leading manufacturers and suppliers of self-drilling screws and fasteners. |
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Why Corrosion is a Problem.
The problem lies not in any design or performance of the fastener, but their failure occurring as a result of a weakening over time due to the insidious effects of corrosion.
Corrosion is normally associated with non-precious metals such as steel, zinc and aluminium. These metals only exist as they are refined from ore. In the presence of air, water or salt, these metals will corrode rapidly. The presence of oxygen will cause oxides to form, which gives the metals a disfigured appearance and will eventually cause them to be unsafe or useless. Corrosion is the natural process of reverting the metal to the ores from which it came.
Corrosion of fasteners can be caused by salt laden air from the ocean, airborne acids from industry and chemical sprays, U.V from the sun or humidity in tropical and moist areas. Corrosion dramatically affects the performance of fasteners over their lifetime, and subsequently effecting the long term structural integrity of any construction. Exposure to corrosion can cause a weakening, cracks will form and grow where the fastener was carrying the load, and a failure will occur.
Protective coatings placed on fasteners follow the principles of “galvanic protection.” This occurs when two dissimilar metals are in contact in a corrosive environment the more active metal will corrode in preference to the lesser active metal. If steel is coated with zinc, then the steel is protected until all the available zinc has corroded away.
There are many different ways this can be achieved to give various levels of protection.
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This photo highlights under-roof corrosion of roofing fasteners.
The fasteners are in a dangerously advanced state of deterioration due to corrosion, and in one case have broken off completely the connection between the roofing profile and the metal batten.
The structure is approximately 12 years old and located in an area classified as mild to moderate marine, with strong prevailing winds.
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The Australian Standard for fastener performance (AS3566) was introduced in 1988, amended in 1990 and ratified in 2002.
Buildex® has played a major role in the design of this standard, which serves as a benchmark for industry performance.
It currently lists four classes of corrosion resistance.
CLASS 1
For general internal use where corrosion resistance is of minor importance.
Most ZINC/YELLOW drywall and chipboard screws are in this category.
CLASS 2
For general internal use where significant levels of condensation occurs.
Electroplated ZINC/YELLOW is generally used to meet this class.
CLASS 3
For general external use in mild industrial and marine applications.
The class is intended for roofing and cladding screws in
mild applictions.
CLASS 4
For external use in marine and moderately severe corrosive environments, generally within
1 kilometre from marine surf, although topography and /or strong prevailing winds may extend this distance.
Compliance is to be assessed through real world testing “when testing at an approved outdoor test site there shall be no red rust present on the significant surfaces of 95% of the fasteners tested”. These outdoor test sites must be located less than 500m from the mean high water line, in a coastal area with surf for most of the year.
Buildex® coating Climaseal®4 complies with this class.
Note: When fixing polycarbonate, stainless steel and aluminium roofing profiles in highly
corrosive environments, it is recommended that stainless steel fasteners should be used.
*Colorbond ULTRA® and XRW® is a registered trademark of Bluescope Steel
Pty. Ltd. |
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