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E-coat issues when painting welded zinc coated steel

July 1, 2010

I'm welding Galvannealed steel plates of 1.4 mm thickness, which after welding need to be painted by e-coat.

During welding, a thin "white" layer is formed around the seam; then once painted, there are several "yellow stains" which looks like Silicon precipitation issue but in a high amount. My assumption is that this is caused due to my wire filler composition.
Process description is as follows:

100% robotic MIG
Welding type: Fronius CMT (Cold Metal Transfer)
Robots: Motoman
Welding speed: 23-25 in/min
Gas: 100% CO2
Material: 1.4 Thickness, Galvannealed
Coating Layer Thickness: 7 microns
Filler Metal: ER70S6

Does anyone have experience in this issue?, any recommendations to avoid the formation of those "yellow stains"?

Thanks in advance

Alex Heredia
Manufacturing engineer - Stockton, California

July 6, 2010

Ecoat performance over zinc-coated steels (galvanize, galvanneal) depends on the quality of the phosphate layer that is deposited after cleaning. In order to obtain a good phosphate coating with appropriate crystal size, etc., the welded parts must have a minimum of oxidation and slag. Best performance is achieved when using weld wires with low Si & Mn, which means using ER70S-3 rather than ER70S-6. In combination with the weld wire, the shielding gas should be mostly Ar, with 1-5% CO2 maximum. You also want to minimize the heat input into the weld, so that oxidation in the HAZ is minimized.

Toby Padfield
Automotive component supplier - Michigan, USA

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