HDGASA V7E1.pdf

W amoshaRaftersPurlinsPhoto 41. Photo 42. Photo 43.Photo 44. Photo 45. Photo 46.Photo 47. Photo 48. Photo 49.magazines on www.hdgasa.org.za.Conclusion and recommendation:Due to the prevailing off sea chloridecarrying winds (South Easterly), highwave action, humidity, low lying landmass and lack of rainfall from the seaside, the False Bay coastline from andincluding Glencairn to just west of theStrand up to about 2 to 3km from thesea is considered to be extremelycorrosive.The specifying and use of a materialsignificantly inferior (for corrosionresistance purposes), is unacceptableand although our organisation, has forthe past 13 years tried to educate usersand specifiers in all the major centresas to the differences, there are stillmany who innocently mis- specify,accept the incorrect materials and whenthey fail prematurely are significantlyembarrassed by the situation.As discussed previously, tocomprehensively remove the tenaciouszinc chloride layer on the galvanizedpurlins, abrade the red rust areas (manyof which are directly below the sheetingand in inaccessible areas –shape of thepurlin), followed up by patch priming ofthe latter surfaces, with subsequentlayers of paint, within a short period ofcleaning, will in my opinion beextremely difficult and expensive tosuccessfully achieve.Most of the fasteners, which have alsofailed are only zinc-electroplated. Theycan often be confused by uninformedpersons with hot dip galvanizedequivalents.The difference essentially between thetwo methods of coating is in thecoating thickness and appearance,where the zinc electroplated fastenerswould have a coating thickness ofabout 5 to 15µm and be shinywhereas, by the hot dip galvanizedmethod will rarely be less than 55 to65µm and matt grey in colour. As lifeis proportional to thickness, it standsPhoto 50.to reason that a thicker coating willlast that much longer.My recommendation would be that thepurlins be removed and replaced withhot dip galvanized purlins to SANS 121.As stated this area off the False Baycoastline is extremely corrosive and ifour opinion was sought at the outset ofthe project, we would have specified aduplex coating (hot dip galvanizing plusan appropriately thick paint coatingsystem) for all external components toachieve a durable service free life.Terry Smith38 Hot Dip Galvanizing Today Volume 7 Issue 1 2010