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Duplex CoatingsDuplex systems and why they are necessaryHot dip galvanized steel is painted forseveral reasons.1) In highly aggressive environments itis necessary to provide a superiorcorrosion resistant coating byapplying several coats ofappropriate paint. Any structurewithin one kilometre of the seafront can be classified as being inan aggressive environment. Thisenvironment will be discussed laterin this article.2) Galvanizers may consider the zinccoating to be reasonably decorativebut for many it may be necessary toprovide a more decorative colourscheme especially in retail and highprofile areas.Inaccessible areas.3) For safety reasons it may be necessaryto overcoat the hot dip galvanizing towarn of potential dangers such as inaviation and traffic, railway structures,light houses, beacons, cranes andtowers.4) It may be important to camouflage thestructure or to ensure it blends in withthe countryside to appear lessobvious, and aesthetically morepleasing.5) Structures that are inaccessible formaintenance require extra longduration of protection againstcorrosion. Examples of these could beunderground mining, jetties inmarinas and harbours or stadiumroofs in soccer and rugby grounds.Many other examples could becategorised here.Atmospheres are usually classified asrural, urban, industrial or marine or acombination of these. In rural and urbanconditions hot dip galvanizing rarelyrequires painting as the zinc coating willprovide a remarkable corrosion freelifetime. In industrial areas wherepollutants such as sulphur dioxide andnitrogen dioxide are present it may benecessary to paint the hot dip galvanizedsteel to prevent attack by “acid rain” inthe form of moisture combining with thepollutants. Heavy rainfall is beneficial tosurfaces as it washes away pollutantswhile soft rainfall may merely wet thesurface and accelerate the corrosionProximity from the sea.attack. This condition is very importantfor salt deposits from the sea as forexample in the Western Cape, rain isprevalent from the North West andprovides relief on one side of steelstructures. However the prevailing windis the South Easter which is responsiblefor severe salt deposits all over thepeninsula on the other side of thestructure. No rain originates from thisNote the wave action.wind and structures on the east coastsea front are subjected to a salt spraybattering with very little wash downcleaning and subsequent removal of thesalt deposits.Due to heavy wave action off the coast ofthe majority of South Africa, chlorides aredispersed several kilometres inland bystrong prevailing winds.Decorative duplex coating.20 Hot Dip Galvanizing Today Volume 5 Issue 4 2008