Metal finishing Q&As since 1989

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Edge treatment for RoHS compliant preplated galvanized sheet parts

2007

I have read that using preplated galvanized sheet steel (RoHS compliant) may be a better alternative than trivalent zinc chromate, but have some questions about the usage.

1. Is the preplated RoHS compliant galvanized sheets readily available in US and China?

2. How are the raw edges (sheared/punched/drilled) treated after forming, etc.?

3. Are the bend radii the same for low carbon sheet steel and the preplated material?

4. Any special handling required in the fabrication shop to avoid scratches, tool marks?

5. Any specific design considerations needed for switching from trivalent chromate to preplated?

Thanks!

Ruth Phillips
Product engineer - Dallas, Texas, US

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January 7, 2008

This is sort of apples & oranges, Ruth. The zinc coating may be applied by either hot dip galvanizing or by zinc electroplating; the items may either be preplated or postplated, and they may utilize either RoHS-compliant trivalent chromates or non-compliant coatings.

1. Preplated material, whether electroplated or hot dip galvanized, should be widely available with RoHS-compliant chromating.

2. It is best practice to roll the sheared and uncoated edges under. Preplating is done to save costs compared to postplating, so as far as I know it is not usual to do any treatment for the punched holes and sheared edges.

3. The coatings are thin and adherent; I do not think there is any need to increase bend radii.

4. An advantage of postplating is that the coating isn't applied until fabrication is done. Obviously you do not want manufacturing processes that follow preplating to scratch the coating but I don't know of a particular checklist of things to do and things to avoid.

5. The biggest concern with preplate is that you will have areas where bare steel is exposed. Obviously, you don't want this to cause functional or aesthetic problems.

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