47096

Black stains on hot deep galvanized Z purlin [Trinidad] 

November 20, 2007

I wonder if you can help me to find out what is going on with a galvanised Z-Purlin that is showing surface defects as dark strips running parallel to each on one side of the piece only and at specific locations. Micrographs show that the coating is cracked and that the cracks run parallel to the substrate surface, more or less at mid coating thickness. The cracks never reached the base metal, so adhesion seems to be not a problem. Could you give any indication of the probable cause of this problem? I have to add that this is a new Purlin.

Thanks in advance for any help you can give me.

Mark Nunes
hobby - Trinidad

There are 2 main sorts of galvanizing, post fabrication hot dip galv, and continuous sheet galvanizing.

This purlin is of the latter. The sheet is then slit and folded (rolled) into the desired shapes.

The problems in the two processes are often quote different. Different metal mixture (typically higher Al), and much thinner coating.
None of the above helps much, but the cracking isn't concentrated on the bends, so not a forming issue.
Mid coating cracking might be between the alloy laters (there are several) but these tend to be very thin in sheet galv. Causes could include poor cooling after emerging from the molten zinc. (The speed of the line in sheet galv is very high).

Geoff Crowley    galvanizing &    powder coating shop Glasgow, Scotland

 

I think,it is directly related excess aluminium amount.Check your molten zinc composition and increasing the zinc amount gonna prevent this appearance,Or this case can be related in Tin amount,but I am not sure,you can chek the tin amount in molten zinc,Due to dentritic structure(spangle formation)cracks can formate...

Hakan Esen
- Bursa, Turkiye

Sir:
If you would post the zinc bath chemistry it could help to determine the cause of your problem. Also if you use a flux, then the composition of the flux solution could be helpful.
Regards,

Dr. Thomas H. Cook
- Hot Springs, South Dakota, USA