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Ted Mooney, P.E. RET
Pine Beach, NJ
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Color changes on weld seam of pipes during hot dip galvanizing
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"User's Guide to Hot Dip Galvanizing for Corrosion Protection in Atmospheric Service" by NACE (1997 only rarely avail.)
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Q. Hi,
We are faced with color changes on weld seam of hot dip galvanized ERW steel pipes DURING the STEAM BLOWING process (only outside surface). Pipe length is 6 meters. Coating thickness; 45-55 micron in the body of the pipe, 55-65 micron on the darkened weld seam. Can anyone help me to find the reasons of this problem?
Regards,
Deniz AkinEngineer - Istanbul, TURKEY
February 21, 2011
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February 24, 2011 A. Sir, Galvanizing Consultant - Hot Springs, South Dakota, USA A. The seam weld has had the swarf removed by a scraping process which re-profiles the steel, which in turn allows faster alloying reaction in galvanizing. This means the composition of the coating at that point is higher iron alloy than surrounding area which has purer zinc on the surface of the coating.  Geoff Crowley Crithwood Ltd. Westfield, Scotland, UK  February 24, 2011 |
A. In ERW continuous pipemaking, there's not usually any welding wire. The edges are continuously welded together by RF after being formed such that the two edges of the originally flat steel are now touching together at a point of the diameter of the tube. That weld often leaves a ridge, which is removed as the continuous tube moves along, by a stationary knife edge tool.
If it's not sharp, that tool leaves a rough surface profile where the weld ridge was removed.
Geoff Crowley
Crithwood Ltd.
Westfield, Scotland, UK
February 27, 2011
Q. Thank you for your reply Mr. Cook and Mr. Crowley.
Mr. Crowley, as you mentioned above, we do not use welding wire. We weld the edges together by high frequency resistance method. Then we remove the weld bead.
I removed the galvanize layer from the pipe surface by acid and couldn't see any rough surface profile where the weld bead removed.
I understand that from your answer, there is no free zinc layer on the top of the coating where the problem reveals. So it seems dark-grey colored. I want to examine the coating layers under the microscope. Normally I have an etchant for galvanize layers but this time it did not work on this sample. Do you suggest any etchant solution to see clearly the galvanize layers under the microscope?
Regards,
- Istanbul, Turkey
March 30, 2011
A. Deniz,
I'm not sure how you could examine this weld area closely.
Have you measured the thickness of the galv layer on the weld (grey ) area versus the thickness of the other area? I suspect you might find it slightly thicker on average (over many measurements). This would tend to indicate alloying right through the thickness of the coating layer.
Another way to tackle this problem is with the customer. Why do they see this as a problem? The corrosion resistance of the grey is equal to the silver (providing similar thickness of coating). And all the coating will turn grey with time anyway, and so the grey stripe will disappear.
Is it just an appearance issue? Galvanizing doesn't have a good reputation as an aesthetic coating, it's a functional one.
Your coating (provided thickness of coating is OK over weld) meets the relevant standards, so what really is the problem?
Geoff Crowley
Crithwood Ltd.
Westfield, Scotland, UK
March 30, 2011
Q. Deniz,
I wonder if you could let me know which etchant you were using for galvanize layers?
Indeed I need the solution!
Someone recommend 2% nital, do you think would it work?
Thanks in advance
- Hatay, Turkey
July 22, 2011
Galvanized pipe after hot dip is burned at the weld line.
Ta Van Thinh- viet nam
September 23, 2020
? Hi cousin Ta. Please explain in what way you feel your problem is significantly different than the question asked by Bekir, or try to phrase your ongoing questions in terms of the detailed answers already offered. Or were you answering the question? Thanks!
Luck & Regards,
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
September 2020
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