Featuring: - hdgasa

More documents

Recommendations

Info

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
Duplex CoatingsRelative humidity and “time of wetness”are further factors that will promotecorrosion of steel or zinc especiallycombined with the presence of chloridesin humid air.Time of wetness has a significant effect onthe life of a hot dip galvanized coatingespecially combined with chloridedeposits. Full exposure to sunlight has adirect effect of reducing corrosion whilepermanently shaded surfaces will corrodefar quicker as they can remain damp formuch of the day.In all the conditions mentioned above asuperior duplex system could beconsidered essential. Hot dip galvanizingor painting on their own may not providethe lifetime required but a combinationprovides a synergetic effect where the sumof 1 + 1 = 3.Other environments that requireduplexing would be in salt water, corrosivefresh waters and certain soil typesespecially underground in some mines.One side of the sign post, due to the full sunhas no corrosion.We also recommend where hot dipgalvanizing is partially cast into concreteor is fixed onto concrete that the surfaceof the hot dip galvanized coating ispainted to prevent differential aireationand possible crevice corrosion especiallyin moist marine conditions as these areastend to remain permanently damp.Superior duplex systems are 2 or 3 coatpaint systems applied onto carefullyOne side of pole the hot dip galvanizing istotally corroded due to a combination of seasalt, moisture and lack of drying from the sun.cleaned hot dip galvanized coatings.The closer to the sea the greater thetotal dft of the paint system, i.e. inaggressive marine conditions, the totalorganic coating over hot dip galvanizingshould be at least 150µm DFT, toprevent early breeching of the paintand the formation of zinc oxides andhydroxides.By Iain Dodds of Cape Galvanizing.Cape Galvanising (Pty) LtdZinc Metal Spraying & Coating (Pty) Ltd• Hot DipGalvanizing• Grit Blasting toSA3• Duplex Systems• Metal Spraying• SABS ISO 1461• SABS ISO 2063• IndustrialPaintingCAPEGALVANISINGPO Box 1551 Parow 7499Juliana Street, Beaconvale Industria, Parowvalley 7500Tel.: (021) 931-7224 Fax: (021) 931-9490Email: capegalv@mweb.co.za Website: www.capegalvanising.co.zaZINC METAL SPRAYINGAND COATINGCBT (011) 868-3408 1114/10/06Volume 5 Issue 4 2008 Hot Dip Galvanizing Today 21
Page 1: 2008 Volume 5 Issue 4HOT DIP GALVAN
Page 4: CommentsExecutive Director’sComme
Page 7 and 8: Awards EventVolume 5 Issue 4 2008 H
Page 9 and 10: Tubes, pipes and scaffoldingneutral
Page 11 and 12: ABECO TANKS - THE NAME THAT HOLDS W
Page 13 and 14: Water storageBenefits of the BOTANK
Page 15 and 16: TechnicalTechnical differences betw
Page 17 and 18: Duplex CoatingsDuplex coated Lula p
Page 19 and 20: Duplex CoatingsUnderstanding qualit
Page 21: Duplex Coatingsdocumenting and reso
Page 25 and 26: EducationMeasuringcoatings on metal
Page 27 and 28: Member’s CornerGalvadip has their
Page 29 and 30: Coating ReportThe specification req
Page 31 and 32: Coating ReportGeneral commentIn man
Page 33 and 34: Bread bakery exchanger coilinvestig
Page 35 and 36: W amoshaIt is noteworthy that sever
Page 37 and 38: Miss Conceptionsreserves were 200 0
Page 39 and 40: Walter’s CornerWalter’s CornerS
Page 41 and 42: Case StudyNo. 16/2008Case StudyRais

Featuring: - hdgasa

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?