Fasteners have long been the weak link in any building structure. The problem lies not in any design or performance of the fastener, but their failure occuring as a result of a weakening over time due to the insidious effects of corrosion.
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, subsequently effecting the long term structural integrity of any construction.
Protective coatings placed on fasteners provide various levels of protection. The Australian Standard for fastener performance (AS3566), which Buildex played a major role in the design, currently lists three classes of corrosion coatings with a proposed fourth:
Class 1 – for general internal use, 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 and moderate industrial and mild marine applications. In moderate marine applications Buildex strongly recommends ZACS 4 coating or Climaseal 4.
The class is intended for roofing and cladding screws in standard applications.
Fastener compliance used to be achieved by accelerated laboratory testing.
The new revision recommends the use of ‘real-world’ testing - exposure on an outdoor test site with a maximum observable rust presence of 5%.
In the case of heavy zinc coatings, 40µm is deemed to comply. Alloy plating of ZINC/TIN 25µm is equivalent to heavy zinc at 40µm.
Buildex coating Climaseal 3 complies with this class.
Class 4 –
For external use in moderate and severe marine environments, classified in accordance with ISO 9223 as generally between 100m from the beach front to approximately 300m inland. In high winds this may extend further inland.
BHP Technical Bulletin TB16 states that fasteners conforming to Class 4 should be used with COLORBOND ULTRA steel and COLORBOND XRW steel(when used 200 - 400m from marine environments).
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 ZACS 4 and Climaseal 4 comply 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, (see pages 14,15 & 78 to 81) To confirm the correct application contact the Technical Help Desk.
