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Ted Mooney, P.E. RET
Pine Beach, NJ
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for Metal Finishing since 1989
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Treating galvanized steel sheets to avoid white rust
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,
- Hyderabad, Andhrapradesh, India
2003
? Hi, Ramchander. Are you already chromate quenching these galvanized sheets?
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
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,
- Dhaka, Bangladesh
2003
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,
- 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, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
2006
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?
- Vadodara gujarat India
October 20, 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,
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
October 2018
A. I think he means "patch".
Dr. Thomas H. CookGalvanizing Consultant - Hot Springs
October 21, 2018
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 investigation through labs but could not establish pin point root cause of it all.
We used OKEMCOAT F2( hexavalent chrome) to passivate.
Could it be micro cracking of the CHROMATE LAYER?
- Vredenburg, West Coast,South Africa
June 25, 2020
A. The chromate layer is thin, soft, and not very resistant to acids. It's probably gone, or at least compromised by acidic orange juice.
Jeffrey Holmes, CEF
Spartanburg, South Carolina
July 16, 2020
A. I assume that Albe is talking about the outside of the drums (which may indeed have juice dripped or spilled on them), and assume the inside is lined with something. I don't think chromated galvanized steel would be an appropriate surface for any drink or wet food.
Luck & Regards,
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
July 2020
You're correct Ted.The juice is inserted into double layered bags and not in contact with the drum surface. The atmosphere within the warehouse where they cool down the frozen juice maybe too acidic and can contribute to the white rust formation.
Albe Hoffman [returning]- Vredenburg,South Africa
July 18, 2020
A. Hi. Although there are things that can be done in the galvanizing process and chromate quench process that may improve resistance to white rust, it seems that galvanizing may not be the ideal finish for these conditions.
Ideally, per my understanding, galvanizing in open air very slowly reacts with the carbon dioxide and forms glassy zinc carbonate reaction products which tend to seal the underlying zinc away from the atmosphere, and which are relatively inert. When the environment is too wet, possibly too cold, too still, too acidic, and too enclosed, galvanizing cannot perform at its best. Painting or powder coating the drums may be called for in this application.
Luck & Regards,
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
July 2020
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