out-gassing in powdercoating on galvanized steel · ordinary galvanizing plant. powdercoating...
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Out-gassing in Powdercoatingon Galvanized SteelA Galvanizing perspective
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Highland Colour CoatersEstablished : 2002
Galvanizing and Powdercoating
Galvanizing: Kettle 7 x 3.2 x 1.2m. 4tonne lift
Powdercoating: 7 x 3 x 1.2m. 750kg liftMulti-stage dip-tank based
pretreatment
Context:
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Ordinary Galvanizing Plant
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Powdercoating Plant
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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.
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Other names for this phenomenon…
OutgassingPinholesBlisteringBlowholes
CratersVolcanoes
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Coating
Pure Zn
Zn-Alloy layer
Defect
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Objective of the study:
Reduce level of rework due to gassing from between 4 to 5%, to less than 1%
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The Study:
•Three year investigation
•One full time scientist
•University assistance and collaboration
•Powder manufacturer collaboration
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The results…
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Results:
Two areas of influence:
1. Galvanizing operation
2. Powdercoating operation
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How to tackle this issue?
•Consider all the possible factors and prioritise them.•Investigate influence of each•Try to understand the mechanism•Recommend changes.
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Drying time in pretreatment
Etching Time Thickness of steel
Thickness of powder
Curing rate
Surface quality of fettledsteel
Powder formula
Steel surface roughness
Steel chemistry
Powder application
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
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The most important factors
1.Quality of Acid in pretreatment2.Time between Fettling and Powdercoating3.Quality of PC pretreatment rinse water4.Steel Chemistry5.Extent of fettling6.Use of primer7.Time between Galvanizing and
Powdercoating8.Black Steel contamination
Red = Galvanizing. White = Powdercoating
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The Galvanizing Layer and its variability
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Metallographic ExaminationText-book images
Text book Galvanizing Sandelin range steel
Hyper-Sandelin steel
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What we found…
S355J2H0.2% Si
The galv layer is often much coarser than textbook
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We got samples of this steel and did the following:
Examinations of …1. Black steel2. Galvanized steel3. Several fettling grades4. Galvanized, stored outside5. Galv, stored in inert conditions6. Not dried7. Dried after PC pretreatment8. Completed process.
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Text book case Galvanized Light fettling
Standard fettling Heavy Fettling After drying
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•The Galvanizing layer in slightly reactive steels is very rough
•It is even broken in some cases
•Fettling makes it worse.
Initial conclusions
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No Fettle Light Fettle Std Fettle Heavy Fettle
10s Acid Etch
No Acid Etch
90s Acid Etch
300s Acid Etch
Light Micro-poring
Light Micro-poring
Slight Pinholing
Light Micro-poring
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Cross section of a blowhole
Powder
Zinc
Steel
Void
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Crack within the zinc layer
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Conclusion 1
“gas” is water evolving from porosity in the galvanizing layer.
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“Degassing”means the same as
“Drying”
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Surface Contamination before galvanizing
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How can it be that contamination before galvanizing can affect powdercoating after galvanizing?
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Oil on the steel
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Grease on the steel
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Fabricators drilling coolant
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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.
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Clay silicate based marker pen used on steel prior to galvanizing
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These chemicals used by fabricators all have an effect on gassing
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Conclusion 2
Steel contaminated before galvanizing is problematic for subsequent powdercoating
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Last Factor…..
Storage between Galv and Powdercoating….
What conditions? and for how long?
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Outside in the rain ?
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Or in the snow?
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Material stored indoors for as short a period as possiblehas the lowest incidence of gassing problems
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Conclusion 3
Store galvanized steel that is to be powdercoated, inside in the dry between processes.
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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.
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The results…
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Acknowledgements
Researcher: Ricky SpeakmanProduction Director: John MacintoshGlasgow Caledonian UniversityKnowledge Transfer Partnership (Innovate UK)Akzo Nobel’s Felling UK research facilities.
References are listed in the printed paper