Out-gassing in Powdercoating on Galvanized Steel. A Galvanizing perspective

Transcription

1 Out-gassing in Powdercoating on Galvanized Steel A Galvanizing perspective

2 Context: Highland Colour Coaters Established : 2002 Galvanizing and Powdercoating Galvanizing: Kettle 7 x 3.2 x 1.2m. 4tonne lift Powdercoating: 7 x 3 x 1.2m. 750kg lift Multi-stage dip-tank based pretreatment

3 Ordinary Galvanizing Plant

4 Powdercoating Plant

5 Definition of the problem: Gassing Common in powdercoating on galvanized steel. Occasionally on aluminium castings. Usually results in the need to strip and re-work the job.

6 Other names for this phenomenon Outgassing Pinholes Blistering Blowholes Craters Volcanoes

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13 Coating Defect Pure Zn Zn-Alloy layer

14 Objective of the study: Reduce level of rework due to gassing from between 4 to 5%, to less than 1%

15 The Study: Three year investigation One full time scientist University assistance and collaboration Powder manufacturer collaboration

16 The results

17 Results: Two areas of influence: 1. Galvanizing operation 2. Powdercoating operation

18 How to tackle this issue? Consider all the possible factors and prioritise them. Investigate influence of each Try to understand the mechanism Recommend changes.

19 Drying time in pretreatment Etching Time Thickness of steel Thickness of powder Curing rate Surface quality of fettled Powder formula Steel surface roughness Steel chemistry Powder application steel Powder condition Weather conditions Geometry of steel Hanging orientation Galv pretreatment chemistry Acid etch in PC pretreatment Drainage hole placement Stacking of items after fettling Extent of fettling Time between Galv and PC Time between fettling and PC Position in curing oven Moisture in compressed air Drying time in galv between flux and Zn Time between PC pretreatment and spraying Rate of withdrawal in Galv kettle Curing oven cycles Drying Oven temperature Use of Primer Black steel contamination Recycling of powder Quality of rinse water in PC Nature of work on the steel Ambient humidity Ambient temperature

20 The most important factors 1. Quality of Acid in pretreatment 2. Time between Fettling and Powdercoating 3. Quality of PC pretreatment rinse water 4. Steel Chemistry 5. Extent of fettling 6. Use of primer 7. Time between Galvanizing and Powdercoating 8. Black Steel contamination Red = Galvanizing. White = Powdercoating

21 The Galvanizing Layer and its variability

22 Metallographic Examination Text-book images Text book Galvanizing Sandelin range steel Hyper-Sandelin steel

23 What we found S355J2H 0.2% Si The galv layer is often much coarser than textbook

24 We got samples of this steel and did the following: Examinations of 1. Black steel 2. Galvanized steel 3. Several fettling grades 4. Galvanized, stored outside 5. Galv, stored in inert conditions 6. Not dried 7. Dried after PC pretreatment 8. Completed process.

25 Text book case Galvanized Light fettling Standard fettling Heavy Fettling After drying

26 Initial conclusions The Galvanizing layer in slightly reactive steels is very rough It is even broken in some cases Fettling makes it worse.

27 300s Acid Etch Light Microporing 90s Acid Etch Slight Pinholing 10s Acid Etch Light Microporing No Acid Etch Light Microporing No Fettle Light Fettle Std Fettle Heavy Fettle

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29 Cross section of a blowhole Powder Zinc Void Steel

30 Crack within the zinc layer

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35 Conclusion 1 gas is water evolving from porosity in the galvanizing layer.

36 Degassing means the same as Drying

37 Surface Contamination before galvanizing

38 How can it be that contamination before galvanizing can affect powdercoating after galvanizing?

39 Oil on the steel

40 Grease on the steel

41 Fabricators drilling coolant

42 These samples were all degreased. 6.5 minutes in NaOH based degreaser at 60 degrees C None of the samples would have been rejected for poor quality galvanizing.

43 Clay silicate based marker pen used on steel prior to galvanizing

44 These chemicals used by fabricators all have an effect on gassing

45 Conclusion 2 Steel contaminated before galvanizing is problematic for subsequent powdercoating

46 Last Factor.. Storage between Galv and Powdercoating. What conditions? and for how long?

47 Outside in the rain?

48 Or in the snow?

49 Material stored indoors for as short a period as possible has the lowest incidence of gassing problems

50 Conclusion 3 Store galvanized steel that is to be powdercoated, inside in the dry between processes.

51 Summary (Galvanizing issues) Reactive steel has porous galvanizing which traps water Contamination on black steel causes gassing Severe fettling opens up porosity Storage in rain causes gassing All of these are not a problem in galvanizing only, but they become a problem for the powdercoater who coats that steel.

52 The results

53 Acknowledgements Researcher: Ricky Speakman Production Director: John Macintosh Glasgow Caledonian University Knowledge Transfer Partnership (Innovate UK) Akzo Nobel s Felling UK research facilities. References are listed in the printed paper