The authoritative public forum
  for Metal Finishing since 1989
  

  -----

Powder cracking after being applied on GI sheet

We are a commercial refrigerator manufacturer. We are facing a problem in powder coating the GI sheets. After we powder coat the GI sheet, we bend it and then we notice the powder is cracking. this was not happening before. We have testing our curing oven and found that there is no problem. We are suspecting that the problem is in the GI itself as it is a new batch we have received. But we are not able to decide what is the cause of this problem. Is it because the GI sheet is not galvanized properly? And how do we check the GI sheets appon reveiving if it is ik or not? Is there a specific test that we can conduct to know whether the GI has been properly galvanized and that it has the sufficiant layer of zinc. And what about the chemical treatment after galvanization? Does it have any effect on the properties of the GI

Thanks

Alaa Mahayni
Engineer - UAE
2007

---

Although there are many possible causes of your problem including the powder coating process and the pretreatment, if you are confident that you have changed nothing, the galvanizing probably changed. Galvanneal material is best for paint adhesion.

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
2007

---

2007

We do a bend test on 3mm sheet test panel galvanized by us then Powdercoated by us, with every job. The standard bend test bends the sheet round a mandrel of about 10mm diameter, and bends the sheet through about 120 degrees. Cracking of the powdercoating is extremely rare, but is most often associated with improper curing. The thicknes sof the zinc (galvanizing) is only a problem when approaching 500 microns, and that in itself is rare in this thickness of steel (<80u more common).
We have done many informal tests also in bending the sheet 180 degrees round no mandrel (in effect folding it), and even there cracking only occurs when the substrate cracks after repeated bending, until ultimately the sheet breaks at the fold. There should be no lack of adhesion more than about 2mm from the sheet break point.

Cracking of the PC can also be associated with poor pretreatment, where uneven adhesion resulted, and a bend afterward was made in the area of unadhered powder. Your pretreatment mitgh be then suspect?

But as pointed out already, look for a parameter that you changed, and seek clues there. If you changed supply of the sheet, and the problem correlates to that change.... and there was no other change, then its likely to be that.

Geoff Crowley
Crithwood Ltd.
Westfield, Scotland, UK

---

It could also be the paint, have you started using a new batch recently? Paint is a manufactered item that also has tolerances to maintain as the raw materials are mixed. It could be that a recently received batch of paint is "brittle" compared the paint that you've received in the past.

Sheldon Taylor
supply chain electronics
Wake Forest, North Carolina
2007

---

CRACK or FLACK-OFF or PEELING coating is due to two reason (as far I can see your problem):
1) Interfacial contamination
3) Elemental composition coating powder
I would like to do a metallurgical inspection of your part and verify my discussion.

Jose Castellanos
- Minneapolis, MN, USA
January 3, 2008

We have galvanized 15 mm MS round (8 feet length) with a coating thickness of 100 Microns.All necessary pretreatment processes have been followed. In spite of that we are finding crack on the surface during 90 degree bend test. Could you please give us some guide line to avoid cracking during bend test ?

Asoke Das
- Kolkata,West Bengal, India
May 30, 2009