HDGASA V7E1.pdf

FastenersAll Class 2 fasteners:Coating Type Minimum local metallic Passivationcoating thicknessElectroplated zinc 12μm Type CMechanically platedzinc 17μm Type CMechanically platedzinc/tin 12μm Type CAll Class 3 fasteners:Coating TypeMinimum local metallic Passivationcoating thicknessExamples of Class 2 electroplated screws.Electroplated zinc 30μm Type AHot dip galvanized 30μm Type AMechanically platedzinc 40μm Type AMechanically platedzinc/tin 25μm Type AAll Class 4 fasteners:Coating Type Minimum local metallic Passivationcoating thicknessHot dip galvanized 50μm Type A: Examples of Class 3 mechanically plated screws.Mechanically platedzinc/tin 45μm Type AIn addition to the plating / coating types and thicknesseslisted above, fasteners can be protected with a sheradized zinccoating EXCEPT for self tapping roofing screws and selfdrilling roofing screws. These cannot be sheradized as thecooling process which forms part of the sheradizing processhas a detrimental effect on the drilling and tappingperformance of the screws.Where the fastener can be hot dip galvanized, this can beapplied according to SANS 121 (ISO 1461).The big advantage of the classification of the environment andassociated plating / coating specifications is that the expectedlife span of the fasteners can be calculated.Example:An electroplated self drilling screw (Class 2) with 12µm zinc isused in a rural environment with low corrosivity (zinc lossmaximum 0.7µm per year as per ISO9223). The expected lifespan of the fastener is 12µm ÷ 0.7µm = 17.14 years.If the same fastener is used in a Class 3 moderateindustrial environment (zinc loss maximum 2.1µm zincper year as per ISO9223) the expected life span of thefastener will only be 12µm ÷ 2.1µm = 5.7 years. Thiswould clearly be an unacceptable situation and a Class 3Examples of Class 4 mechanically plated screws.fastener which would offer between 14 to 19 yearscorrosion protection should be used.The short comings of the old SABS 1273 standard made itvirtually impossible for architects and specifiers to specifyroofing fasteners according to a South African standard. Selfdrilling screws are by far the most commonly fasteners usedto fix roof sheeting and cladding on commercial, industrialand other large projects.As these screws were not specified in the old SABS 1273standard, the quality and especially the corrosion protectionof the self drilling screws used in South Africa was of a verypoor quality. It is hoped that the revised standard will go along way in ensuring that the correct type of fasteners and thecorrect corrosion protection will be specified and that thestandard will be embraced by the roofing industry.12 Hot Dip Galvanizing Today Volume 7 Issue 1 2010