Out-gassing in Powdercoatingon Galvanized SteelA Galvanizing perspective

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:

Ordinary Galvanizing Plant

Powdercoating Plant

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.

Other names for this phenomenon…

OutgassingPinholesBlisteringBlowholes

CratersVolcanoes

Coating

Pure Zn

Zn-Alloy layer

Defect

Objective of the study:

Reduce level of rework due to gassing from between 4 to 5%, to less than 1%

The Study:

•Three year investigation

•One full time scientist

•University assistance and collaboration

•Powder manufacturer collaboration

The results…

Results:

Two areas of influence:

1. Galvanizing operation

2. Powdercoating operation

How to tackle this issue?

•Consider all the possible factors and prioritise them.•Investigate influence of each•Try to understand the mechanism•Recommend changes.

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

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

The Galvanizing Layer and its variability

Metallographic ExaminationText-book images

Text book Galvanizing Sandelin range steel

Hyper-Sandelin steel

What we found…

S355J2H0.2% Si

The galv layer is often much coarser than textbook

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.

Text book case Galvanized Light fettling

Standard fettling Heavy Fettling After drying

•The Galvanizing layer in slightly reactive steels is very rough

•It is even broken in some cases

•Fettling makes it worse.

Initial conclusions

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

Cross section of a blowhole

Powder

Zinc

Steel

Void

Crack within the zinc layer

Conclusion 1

“gas” is water evolving from porosity in the galvanizing layer.

“Degassing”means the same as

“Drying”

Surface Contamination before galvanizing

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

Oil on the steel

Grease on the steel

Fabricators drilling coolant

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.

Clay silicate based marker pen used on steel prior to galvanizing

These chemicals used by fabricators all have an effect on gassing

Conclusion 2

Steel contaminated before galvanizing is problematic for subsequent powdercoating

Last Factor…..

Storage between Galv and Powdercoating….

What conditions? and for how long?

Outside in the rain ?

Or in the snow?

Material stored indoors for as short a period as possiblehas the lowest incidence of gassing problems

Conclusion 3

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

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.

The results…

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