Galvanizing flaking off

[PeterCharles]

We have experienced problems recently with galvanizing flaking off our mild steel plate components. It seems to occur on the plate edges.

This is a new problem to us, but someone has suggested it is because the plates are now laser cut rather than sheared and the sharp laser cut edge is the cause of the problem.

Anyone got any comments, experiences?

 

[swall]

First of all, is it really galvanizing (i.e. hot dipped zinc) or is it zinc electroplate? Second, what is the condition of the rest of the mild steel? Is it clean and shiny or does it have an oxide film or scale?

 

[PeterCharles]

1) Yes it is galvanizing
2) Don't know as the platework was done by a sub-contractor who then had it galvanized. But aren't parts degreased, pickled then washed so they are 'clean' prior to immersion in the molten zinc?

 

[swall]

Your assesments of the normal metal surface prep prior to galvanize are right on the money. If these steps were performed, I would see no reason for the galvanize to flake off. At this point, you need to backtrack to the galvanizing source.

 

[PeterCharles]

Backtrack, difficult as the processing was done in eastern Europe :-(

Any comment of the laser cut edge theory??

 

[CoryPad]

Possible, but not highly probable

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[PeterCharles]

Maybe a case for testing a couple of pieces of plate, one lasered, one sheared. Must talk nicely to our UK galvaniser.

 

[PeterCharles]

Cory,

Just talked with a guy at the Galvanising Association. Poor adhering of zinc to the edges of laser cut plate is a known problem. It appears more likely to occur with thinner plates (5mm) than thick ones. The zinc at the cut edges can be peeled back like tinfoil.

The cause of the problem is unclear, but suggestions are a tenacious oxide layer, or a localised hardening of the edge caused by the laser cutting.

The only known solution is to grind back the cut edge!

 

[swall]

It may be that the pickling time is not sufficient to remove the edge oxide. I am hypothesizing that the pickling cleans the flat surfaces OK and the appearance misleads the operator into thinking that the edges are clean as well. Or that they are "clean enough". Lack of surface cleanliness is the probably the number one source of adhesion problems in metal finishing.