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topic 21047

Treating galvanized steel sheets to avoid white rust


A discussion started in 2003 but continuing through 2020

2003

Q. Hello,

Can anybody help in finding solution for treatment of galvanized steel sheets against white corrosion. The sheets are exposed to very humid conditions. I am looking for a treatment which can give prolonged protection without paints. Any suggestion is highly appreciated.

Thanking you,

Ramchander Gegula
- Hyderabad, Andhrapradesh, India


2003

? Hi, Ramchander. Are you already chromate quenching these galvanized sheets?

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


2003

A. Hi,

Hope you have chromate treatment facilities there in your process. Keep chromate solution temperature above 80 °C to dry up the sheet after chromate treatment, next you can use hot air to dry up the sheet after treatment. Corrugate the sheets as soon as possible in the humid condition.Arrange cross ventilation during storage--if necessary use pedestal fan at the storage area.Last of all if the condition does not improve your last resort would be to use oil in the corrugation machine (through special arrangement), which may help you to beat white rust formation.

Regards,

Mahbub Hossain
- Dhaka, Bangladesh



2006

Q. Chromating unit is surely a good solution to prevent Galvanised sheet from white rust formation. But chromic acid is another factor which can arise problem of pollution.
Please suggest whether this problem can be minimised by using chemical coaters? If yes, can coating by chemical coater avoid the use of cleaning section in Color coating line?
Please explain the merits and demerits of this alternative.

Thanks,

Sunil Chauhan
- Indore, M.P, India


2006

A. Hello Sunil. During a panel discussion at Sur/Fin 2006 in Milwaukee, one of the participants claimed that the TCP hexavalent-free chromate conversion coatings developed for aluminum work well on galvanized materials as well. I would suggest that you contact one of the four licensees for further discussions. They are Henkel, Luster-On, Metalast and Surtec. Good luck!

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



October 20, 2018

Q. Dear sir I am starting new galvanizing plant. But starting material looking good, but some few days in, my material comes to white rust peach^patch so how I can control?

sachin PATEL
- Vadodara gujarat india


October 2018

A. Hi Sachin. Please re-check your posting and confirm what word you have misspelled as 'peach' ... sorry but I can't figure it out from the context :-(

Even with a chromate quench, galvanized materials must be stored dry and with good air exposure because carbonates which form from exposure to the atmosphere are one of the factors which discourage white rusting.

Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET
finishing.com - Pine Beach, New Jersey
Aloha -- an idea worth spreading


October 21, 2018

A. I think he means "patch".

Dr. Thomas H. Cook
Galvanizing Consultant - Hot Springs



June 25, 2020

Q. I'm a Galvanizer in South Africa.We make use of a continuous Hot dip Galvanizing line and supply skinpassed chromated steel coils to a drum manufacturer in Cape Town South Africa. The drum manufacturer sends the drum to a juice supplier. The drums are kept under subzero temps (-18 °C) But from time to time they need to defrost to take out the juice to be replaced.During that process of increasing the temps the Galvanized drums started to form condensation and subsequently white rust forming. We did an in depth investigations through labs but could not established pin point root cause of it all.
We used OKEMCOAT F2( hexavalent chrome)to passivate.
Could it be miciro cracking of the CHROMATE LAYER?

Albe Hoffman
- Vredenburg, West Coast,South Africa

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