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Weld joint pre treatment prior to powder coating
Q. Hello,
I am having structural steel powder coated, after salt spray tests we are getting rust at the welded joints. Plating the parts prior to powder coating would be a sure fix however there are cost constraints preventing me from doing this. Is there any products out there (cost effective) to pre treat the weld joints prior to powder coating that would seal the weld joint and prevent rust?
- Arcata, California, USA
2003
"Phosphating of Metals"
by Werner Rausch
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A. Hi Bob, you can certainly e-coat the parts, which will get you coverage in the weld area, but if plating is too costly, e-coating probably would be as well. But I have seen this problem before and believe it is more related to the fact that the tensile stresses in the weld area make them a cathodic hotspot, rather than the problem being due to porosity. Prediction: if you sandblast the welds immediately before treatment that area will not rust.
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
P.S. July 2016: I see that my reply was misread and I see why and apologize for my lack of clarity. When I suggested "sandblasting immediately before treatment", I meant sandblasting before phosphatizing pretreatment, not sandblasting immediately followed by powder coating, in lieu of phosphatizing or after phosphatizing. Sorry!
Q. We have tried sandblasting; however, the results have been worse by doing so, I have been told the silica in the sand actually promotes and accelerates the process of rust. We are going to try a steel shot media blast as recommended by the powder coater, I am very surprised that there is not a low cost treatment out there to seal the weld joint, any other suggestions?
Bob Young [returning]- California
2003
A. Welds usually have problems with adhesion if they are not descaled by either pickling in acid or by mechanical abrasion (sand blasting, wire wheel/brush, etc.). I agree with Ted that you will likely have no problems if you sand blast prior to coating.
Toby Padfield- Michigan
2003
A. We sandblast with aluminum oxide then sometimes I will preheat the part(s), pull part from oven, coat part (starting with welds) and put right back in oven.
Joe corcoranpowdercoating - Cincinnati Ohio U.S.A.
June 4, 2013
Q. Hi
I am sai. For our products we shot blast and ED-coat but still rust appears in welded portions. Give me solutions.
- Chennai Tamil Nadu
July 27, 2016
"Phosphating & Metal Pretreatment"
by D.B. Freeman
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A. Hello Sai. You'll probably need a lot more words in your question to get a useful answer :-)
Please describe your phosphatizing or other pretreatment, what kind of e-coating you are doing (thickness, type), and when you see this rust. I don't even know yet whether you are having a specific problem or are simply holding unrealistic expectations for a minimal coating system. Thanks!
Regards,
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
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