Metal finishing Q&As since 1989

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PROBLEM FACED IN GALVANISING LONG LENGTH BY DOUBLE DIPPING

MY BATH LENGTH IS 9.5 MTR BUT JOB IS 12 MTR HENCE HAVE TO GO FOR DOUBLE DIPPING I,E FIRST HALF SIDE AND THAN SECOND HALF. IT OBSERVED THAT THE LENGTH WHICH IS GALVANISED FIRST GETS DARKER SHADE WHILE THE SECOND HALF IS VERY MUCH SHINING. please ADVICE HOW SAME SHINE FOR FIRST AND SECOND HALF PORTION ,I.E HOW TO HAVE SAME SHADE THROUGH OUT THE LENGTH THANKS REGARDS

NARENDRA SHARMA
DIRECTOR - VADODARA, GUJARAT, INDIA
May 11, 2009

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June 22, 2009

I too have noticed this phenomenon sometimes.
But why is it a problem? The corrosion protection of dull grey galvanizing is the same as shiny silvery. And it all goes dull grey after a while anyway?

But why does it happen?
I think its immersion time. Generally the first dip aims to get as much coverage as possible. So it goes deeper (longer immersion, longer withdrawal), and if its hollow sections, it has to fill and to drain (longer still).

the second dip just finishes off the rest of the item, a smaller faster dip.

Geoff Crowley
Crithwood Ltd.
Westfield, Scotland, UK

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July 10, 2009

Sir:
For double dipping, some galvanizers side-grind off the "burn line" prior to the second dip. Other galvanizers have a flux solution (usually quadraflux) and paint the burn line (after it cools a bit) prior to the second dip. By using one of these methods a redip in the flux is avoided.
Regards, Dr. Thomas H. Cook, Hot Springs, South Dakota, USA

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

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Q. We have a 56' dry kettle with bath temp of 832F. We use a zinc ammonium chloride preflux with temps of 157-163F. On a certain product same steel is ran normal dip the zinc does not peel. I say this to void out the bad steel comments. This only occurs when we have to dip, redip the area in preflux that did not dip into the bath and then HDG the rest of the product. When we do this the area in between the orginal splice line and the newly prefluxed line starts to peel off. What and why is this happening?

Sean Zink
- Kenton, Tennessee, USA
February 15, 2012

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February 18, 2012

Sir:

I do not know of a kettle of your size anywhere close to where you live? Where is this kettle that you speak of?

Regards,

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

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See what happens when you look the other way for a moment? Someone goes and builds a 56 ft kettle galvanizing plant while you weren't looking!

Geoff Crowley
Crithwood Ltd.
Westfield, Scotland, UK

February 28, 2012