HDGASA V7E1.pdf

W amoshaPhoto 27. Photo 28. Photo 29.South west corner and eavesPurlinsPhoto 41 is a general view of the southwest end of the building, photo 42 showsthe corrosion concerns on the purlins.Surfaces scraped and residual coatingthickness measured on the rafters andpurlins.The purlins have been manufacturedusing a continuously hot dip galvanizedsheet to SANS 3575/4998. Two places onthe inside of the purlin were scraped asclean as possible and residual coatingthickness readings taken, measuring 18.3and 24.6µm respectively (photos 43 - 46).The horizontal and areas adjacent tothis on the purlins have due to thecorrosivity of the atmosphere and timeof wetness, shown a markeddegradation and subsequent red rust.RaftersThe rafters have been hot dipgalvanized to SANS 121. Two areas ofthe exposed web of the I-Beam werescraped as clean as possible andresidual coating thickness readingstaken, measuring 241 and 237µmrespectively (photos 47 - 50).Difference between continuouslyhot dip galvanized sheeting toSANS 3575 (commercial grade)or SANS 4998 (structural grade)and general hot dip galvanizingto SANS 121Continuously hot dip galvanized sheetingThis is manufactured by ArcelorMittalin Vanderbijlpark or Durfuco in theWestern Cape. Continuous sheetingtravelling at about 140m/min runsthrough a reducing atmosphere atabout 950°C, the sheet travels into ahot dip galvanizing bath under amandrel and as the sheet exits thebath, it runs through a series of air orgas knives where the excess coating,according to the grade of coatingrequired, is mechanically removed.Various coating grades are availablefrom Z100 to Z700, with the mostavailable grade being Z275. The 275represents the mass of zinc ingrams/sqm. To convert the coatingmass to coating thickness 275 must bedivided by 7 (Specific Gravity of zinc)and by 2 (includes both faces), ie.275/2x7 = 19,6µm. The fine print in thespecification allows not less than 40%of the individual value (235 gm/sqm)to be found on one surface, ie.13.4µm.General hot dip galvanized steelThis process is available in most largecentres and in Cape Town there are 6hot dip galvanizers, 5 of which aremembers of the HDGASA, with varyingbath sizes.Pre-cleaned components aredipped into a bath of molten zinc atabout 450°C. A metallurgical reactionbetween the steel and zinc results in acoating comprising a series ofPhoto 30. Photo 31. Photo 32.Photo 33. Photo 34. Photo 35.36 Hot Dip Galvanizing Today Volume 7 Issue 1 2010