Pretreatment of Steel Electronic Chassis 2

[tmech77]

I was going to use a zinc electrodeposited pretreatment per ASTM B633 including a clear chromate, on an electronic chassis that will be powder coated. Some of the flanges on the chassis will not be powdercoated to maintain electrical conductivity between them. The fabricator suggested that we use Galvanneal instead of the zinc coating. The chassis will be in a climate controlled room temperature environment. Which process is better?

 

[MikeHalloran]

I don't know about B633 specifically, but I think most electro-zinc coatings are fairly bright, and may not take coatings well until/unless they have weathered a bit. Maybe that's what the chromate is for.

Galvanneal typically provides a dull gray surface that does take finishes well without a lot of fuss. It's worth considering.




Mike Halloran
Pembroke Pines, FL, USA

 

[CoryPad]

The benefits of galvanneal for you are twofold:

1) the surface provides for higher adhesion strength of coatings compared with electroplating
2) the zinc layer is applied to the initial steel sheet (which is a small cost increase for the raw material), so you avoid the need to send a finished chassis out for electroplating (a much larger cost decrease)

 

[tmech77]

Are you saying that you can purchase steel with the Galvanneal already on it?

 

[CoryPad]

Yes, that is how the steel coils used for automotive structures are made. If you are in the American Midwest, then Alro is one source that stocks these:

http://www.alro.com/divSteel/MetalsGridCarbon_PlateSheet.aspx

 

[tmech77]

So on a drawing of the part, you would not specify Galvanneal as a finish process. In stead it would be part of your raw material callout, right? So would it possibly be specified as: Cold rolled galvannealed steel sheet per A1008?

 

[btrueblood]

ASTM A653, A-grade material is how I usually designate it. Tensile properties are closer to hot rolled mild steel, due to post-coating anneal process. If you need higher strength material, with ASTM A446 you can get 50 ksi min. yield.

 

[CoryPad]

Right, specify the correct raw material with the coating. If you need higher strength, you can use ASTM A1079.