Hot Dip Galvanizing of Sheet Metal - hdgasa

Featureproper measures should be taken toprevent contamination by moisturewhile the material is still bundled ornested in stacks (figure 5).If not required for immediate use, coilsor packs of sheets must be stacked onsite under properly designed cover,clear off the ground and protected fromwind-driven rain (figure 6).Figure 6.Plastic tarpaulins which completelyenvelop packs of sheets or coilsshould not be used, as a sudden dropin ambient temperature may causecondensation of water vapour, whichcan easily be drawn in between nestedsheeting by capillary action.Ideally, deliveries of galvanized andcolour-coated steel sheet to thebuilding site should be scheduled fora storage period of not longer thantwo weeks prior to installation. Inspectthe storage site regularly to ensurethat moisture does not penetrate thestock.Removal of Wet Storage StainWet storage stain should rather beprevented than cured.Although in extreme cases theprotective value of the coating may beimpaired, wet storage stain attack isoften superficial despite the relativebulkiness of the corrosion product.Where surface staining is light andsmooth without growth of the zincoxide layer as judged by lightlyrubbing fingertips across the surface,the staining will gradually disappearand blend in with the surrounding zincsurface as a result of normalweathering in service.When the affected area will not befully exposed in service or when it willbe subjected to a humid environment,wet storage staining must be removed,even if it is superficial. This isessential for the basic zinc carbonatefilm to form. The formation of this zinccarbonate film is necessary to ensurelong term service life.Light deposits can be removed bycleaning with a stiff bristle (not wire)brush. Heavier deposits can beremoved by brushing with a 5%solution of sodium or potassiumdichromate with the addition of 0.1%by volume of concentrated sulphuricacid. Alternatively, a 10% solution ofacetic acid can be used. Thesesolutions are applied with a stiff brushand left for about 30 seconds beforethoroughly rinsing and drying.Unless present prior to shipment fromthe galvanizer, the development of wetstorage stain is not the responsibility ofthe galvanizer. The customer mustexercise proper caution duringtransportation and storage to protectagainst wet storage staining.BrandingAll hot dip galvanized andCHROMADEK® strip produced byIspat Iscor is marked every metre withinformation regarding the materialorigin, date of manufacture anddimensions. Coating mass is alsoprovided (see table 2 for theconversion from mass to coatingthickness).Because of the reputation of IspatIscor’ galvanized and CHROMADEK®sheeting in the South African market,both names are now used genericallyto describe other metallic and colourcoated roof sheeting, although theyare not necessarily the same. It is thusimportant to establish with certaintythe quality and relevent specificationof the product at procurement stage.12 Hot Dip Galvanizing Today Volume 2 Issue 1 2005