Hot Dip Galvanizing for Steel Corrosion Projection (fabricated steel items) By Mike Ainsley International Zinc Association (IZA)

Transcription

1 Hot Dip Galvanizing for Steel Corrosion Projection (fabricated steel items) By Mike Ainsley International Zinc Association (IZA)

2 Steel is and will continue to be one of the most important materials for construction in the modern world 2

3 Steel: a modern construction material of un-paralleled functionality Stable and well understood engineering properties Easily fabricated into a diverse range of shapes Plentiful supply Economical to use 100% recyclable 3

4 BUT. CORROSION CAN DESTROY STEEL S USEFUL AND UNIQUE ADVANTAGES 4

5 Consequences of corrosion Visibly unattractive rusting 5

6 Visibly unattractive rusting 6

7 Consequences of corrosion Loss of structural integrity 7

8 Example of corrosion failure 8

9 Example of corrosion failure 9

10 Costly repairs or replacement Financial compensation Cost of Corrosion Safety risk Interruptions to normal service 10

11 The most commonly used method for corrosion control is to apply a protective coating Types of coatings Metallic coatings (zinc coatings are the most widely used) Non-metallic coatings (such as most paints) 11

12 Galvanizing refers to the application of a zinc coating to steel for corrosion protection Galvanizing methods: Hot Dip Galvanizing Zinc electroplating (electro-galvanizing) Zinc-rich painting ( cold galvanizing ) Zinc thermal spraying (zinc metallizing) 12

13 Hot Dip Galvanizing (HDG) It is the process of applying a metallurgically bonded coating to steel by immersion into molten zinc. The steel items have already been fabricated into the final shape Sometimes this is called after- fabrication galvanizing Molten zinc 13 13

14 Zinc electroplating (-) (+) Zinc Steel Coating Zn ++ Zinc Zn ++ Zn ++ Zn ++ Zn ++ 14

15 Zinc-rich painting Paint contains a high content of zinc powder which forms a semi-continuous zinc layer as the paint dries Electron microscope cross section of the paint coating showing the dispersion of zinc particles 15 15

16 Zinc thermal spraying High velocity molten zinc spray Gas torch nozzle 16

17 Global zinc coatings usage for fabricated structural steel items 100% Hot dip galvanizing ( > 90% ) Usage % 0% Other zinc coatings < 10% 17

18 Hot Dip Galvanizing (HDG) of fabricated structural steel Some introductory comments HDG process HDG coating attributes HDG applications 18

19 Some introductory facts about HDG First used over 150 years ago (in Europe) Globally is now used to protect approximately 25 million tonnes of structural steel per year Used for a wide variety of engineering and architectural applications 19

20 HDG Process Is the process of applying a zinc coating to steel by immersing (dipping) the steel into molten zinc at 450 o C 20

21 HDG PROCESS STEPS 21

22 Example of galvanizing factory 22

23 The following picture sequence shows actual steel items being hot dip galvanized. From commencing the entry into the molten zinc to the time of removal, the total elapsed time is approx 6 minutes 23

24 Steel articles to be galvanized 24

25 Steel articles to be galvanized about to enter the galvanizing bath enclosure 25

26 The galvanizing bath within an enclosure 26

27 Steel articles above the galvanizing bath Steel articles above the galvanizing bath 27

28 Steel articles fully immersed in the galvanizing bath 28

29 Steel articles being withdrawn from the galvanizing bath 29

30 Fully withdrawn 30

31 Water cooling of the galvanized articles 31

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33 How does the coating form? Zinc diffuses into the steel surface and combines with the iron component. This results in the formation of a Zinc Iron alloy layer Zn migration Molten Zinc into the steel Zinc surface Zinc Iron Alloy Layer Steel Steel 33

34 How does the coating form? As the steel emerges from the molten metal, a layer of pure zinc metal solidifies on top of the zinc - iron alloy layer Zinc metal Zinc- iron alloy HDG COATING STEEL 34

35 Cross-section through an actual galvanized coating at high magnification Zn layer Zn-Fe alloy layer Steel base 35

36 Quality Control inspection 36

37 HDG coating quality conformance Examples of applicable HDG Standards are: BS (EN) ISO 1461 ASTM A123 37

38 HDG Standards define: 1. Minimum permitted coating thickness. 2. Maximum permitted size of any area on the steel surface where the coating is not visibly present. Suitable repair methods for small uncoated areas are also defined 38

39 Coating thickness specified by ISO 1461 for HDG structural steel Steel thickness for structural shapes (mm) Coating thickness (microns) < > or = 1.5 & upto < 3 55 > or = 3 & upto < 6 70 > or =

40 40 Measuring coating thickness using portable meters is simple, quick, and accurate

41 HDG Standards define maximum permitted size of area where the coating is not visibly present Sufficiently small uncoated areas can be on-site repaired instead of re-galvanizing the item 41

42 Important comment on coating thicknesses as specified in Galvanizing Standards ASTM123 or BS EN ISO 1461 ) Specifications for minimum average coating thickness defined in Galvanizing Standards are not based on achieving a required service life Instead they are based on the realistic capabilities of the galvanizing process. For example, the ASTM 123 specified coating thicknesses were established following tests on many thousands of steel items of different thicknesses that were galvanized using the traditional galvanizing process. The results of these tests formed the basis for defining the average minimum coating thickness for different steel thicknesses 42

43 43 Requests for greater coating thicknesses than specified in Galvanizing Standards ASTM123 or BS EN ISO 1461 ) As stated previously, the specified coating thickness as defined in Galvanizing Standards is the natural thickness due to the technical nature of the galvanizing process. Greater thicknesses (such as microns) which may be requested to give extra corrosion protection may therefore be impossible to achieve in the normal conventional galvanizing process. It may be possible to achieve extra thickness coatings by including a sandblast of the steel surface before galvanizing. This is usually at an added cost and there is a risk that extra thick coatings will not be as adherent to the steel as normal thickness coatings

44 HDG coating attributes The HDG coating provides a UNIQUE COMBINATION of advantages which is not available from other coatings Resistant to damage Excellent corrosion protection Proven long service life Reliable and predictable service performance Assured coating integrity Good corrosion protection economics Environmental compatibility Versatility (able to galvanize most steel shapes and sizes) 44

45 HDG coating attributes Resistant to damage due to strong adhesion, high hardness & toughness. Zinc Zn - Fe Alloy STEEL Harder than steel Bond strength 10x more than paint 45

46 HDG coating attributes Excellent corrosion protection due to both barrier protection + cathodic protection Barrier protection Zinc has a naturally low corrosion rate (1/20 th to 1/80 th that of steel) ZINC STEEL Cathodic protection Zn ++ Zn ++ protection ZINC e _ STEEL ZINC e _ 46

47 Comparison with conventional paints No cathodic protection means the corrosion will spread beneath the paint coating. 47

48 Side by side performance comparison of HDG and a conventional paint coating 48

49 HDG coating attributes Proven long service life Typical service lives are: Rural location: up to 100 years Urban & mild coastal location: years Severe coastal & industrial location: years 49

50 HDG coating attributes Reliable protection: The fact that HDG coatings have been used for over 150 years testifies to its reputation for reliability 50

51 HDG coating attributes Predictable service life Rate of coating loss is approximately linear. Service life expectancy can therefore be extrapolated Coating thickness (microns) 0 Initial coating thickness 10 Measured coating thickness after 10 years Service Life (years) Projected service life 40 51

52 HDG coating attributes Assured coating integrity It is often stated that the HDG process is an honest process since there can be no short-cuts to achieving a successfully applied coating 52

53 HDG coating attributes Economic corrosion protection HDG s long service life means little or zero maintenance is required. Therefore HDG often provides lower cost corrosion protection than a paint coating on a life-cycle cost basis Coating cost HDG Cost Savings 3 rd maintenance paint 2 nd maintenance paint 1 st maintenance paint 1 st paint coat 53

54 HDG coating attributes Environmentally compatible 1. Zinc coatings are not toxic 2. Galvanizing conforms to the needs for green building and construction practices. This has been demonstrated in Life Cycle Assessment studies which compare galvanizing and paint coatings 54

55 LCA case study performed on a multi-storey steel car-parking structure Comparison of Hot Dip Galvanizing vs 3-coat epoxy paint system Study undertaken by the Institute of Environmental Protection Technology (University of Berlin, 2006) Assumes 60 year service life, maintenance painting required at 20 and 40 years 55

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57 Results of the study 100% 50% Resource and Energy Consumption Paint Pollution effects - greenhouse gas - reduction of ozone layer - atmospheric acidification Paint 0% HDG HDG

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59 HDG coating attributes Versatility Diverse range of steel shapes and sizes can be galvanized 59

60 HDG coating attributes Versatility Diverse range of steel shapes and sizes can be galvanized 60

61 Summarizing the benefits of Hot-Dip Galvanizing Provides reliable, long-term corrosion protection Provides economical corrosion protection Is environmentally compatible Can be applied to a diverse range of steel shapes and sizes 61