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Coating ReportWet storage stain and subsequent redrust to the hot dip galvanizedsheeting on a slag crushing plantAs part of the Association’s effort to educateand impove communication between all, weinclude for your reading, this coating reportby the Association.For obvious reasons names of parties havebeen witheld but the article might beinvaluable to others in order to avoidsimilar situations in future.The Hot Dip Galvanizers Associationwas asked to comment on the wetstorage stain and subsequent redrusted areas to the roof, side andend sheeting of a slag crushingplant. We report as follows:ReportWet storage stain (white rust)Wet storage stain is the name given tothe white bulky deposit, which mayform on the surface of closely stackedfreshly galvanized articles, whichbecome damp (from exposure to rainor condensate) under poorly ventilatedconditions during storage or in transit.Although in extreme cases theprotective value of the coating may beimpaired, attack is often superficialdespite the relative bulkiness of thecorrosion product. Where surfacestaining is light and smooth withoutgrowth of the zinc oxide layer asjudged by lightly rubbing fingertipsacross the surface, the staining willgradually disappear and blend in withthe surrounding zinc surface as aresult of normal weathering 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 beremoved, even if it is superficial. Thisis essential for the basic zinccarbonate film to form. The formationof this zinc carbonate film isnecessary to ensure long-term servicelife. In general terms light depositscan be removed by cleaning with astiff bristle (not wire) brush or scotchbrite pads and industrial vim. Heavierdeposits can be removed by brushingwith a 5% solution of sodium orpotassium dichromate with theaddition of 0.1% by volume ofconcentrated sulphuric acid.General view of the end sheeting.Alternatively, a 10% solution of aceticacid can be used. These solutions areapplied with a stiff brush and left forabout 30 seconds before thoroughlyrinsing and drying. Wet storage stainis not necessary a cause for rejection,provided that the specified minimumcoating thickness is present.Unless present prior to shipment fromthe sheeting manufacturer or stockist,the development of wet storage stainis not necessarily the responsibility ofthese two parties.General view.Close up of the side sheeting.The customer must exercise propercaution during transportation and30 Hot Dip Galvanizing Today Volume 4 Issue 1 2007
Coating ReportGeneral view of the roof sheeting showingdeposits of wet storage stain and subsequentred rust.Deposits of wet storage stain have beenremoved from the sheeting.Coating thickness readings have been takenand are still above the requirements for aZ275 class of coating.subsequent storage to protectagainst wet storage staining.Coating thicknessGenerally local specifications call forthe use of continuously galvanizedsheeting according to SANS 3575 /4998 coating grade Z275. Thisspecification requires a coating massof 275g/m 2 as a triple spot test but235g/m 2 on a single spot test. Thespecification also states that not lessthan 40% of the single spot test will befound on either surface. This equatesto a nominal zinc coating thickness ofabout 20µm with a minimum coatingthickness of 13.5µm.The equivalent thickness iscalculated from the followingformula:Thickness in microns (µm) =Mass per unit area, g/m 22 x 7(2 includes both sides and 7 is theapproximate specific gravity of zinc).Volume 4 Issue 1 2007 Hot Dip Galvanizing Today 31
Page 1: 2007 Volume 4 Issue 1HOT DIP GALVAN
Page 4 and 5: CommentsExecutive Director’sComme
Page 6 and 7: Continuous hot dip galvanized sheet
Page 10: Continuous hot dip galvanized sheet
Page 15 and 16: ADVERTORIALSHAPING THE FUTURE OF ST
Page 20 and 21: Safety and security including conti
Page 26 and 27: Duplex CoatingsA life cycle costing
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Page 30 and 31: Duplex Coating ReportDuplex coated
Page 34 and 35: Coating ReportRed discoloration has
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Page 42 and 43: Personality Profilethat nobody woul
Page 44 and 45: Guest WriterBob Andrew, our guest w
Page 46 and 47: Case Studyperformance of hot dip ga
Page 48: HOT DIP GALVANIZING MEMBERSGALVANIZ

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