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Dross control in galvanising

December 28, 2008

Dear Sirs,

We are a newly established Galvanising facility in northern India. I want to know what is the most suited chemical composition of MS so that dross formation is the least.


Sanjay Chowdhary
galvanisers - Alwar, Rajasthan, India

January 2, 2009

You didn't say what "MS" means. It could be a number of things, but assuming it might be Mild Steel ? then this isn't the right direction to take in reducing dross formation.
Its true that some steels are more reactive than others with molten zinc, forming a thicker coating that others (research the sandelin curve on the internet for more), but a galvanizer rarely has any choice in the steel grade.
There are many other factors that can influence dross formation:
Flux composition, cleanliness, temperature
Steel temperature, dryness
Drainage design of hollow items
Zinc temperature, composition
Rinse setup before flux.

Geoff Crowley
Crithwood Ltd.
Westfield, Scotland, UK
crithwood logo

January 2, 2009

I presume that you want to know what MS to use for racking fixtures/and or a drossing device. The answer is a steel low in silicon, phosphorus, and aluminum. These elements in combination above about 0.2^0.02% cause a lot of dross (about 30 to 40 pounds of dross for every pound of steel alloyed, or dissolved).
True dross is primarily an alloy of 94% zinc and 6% iron. It is a semisolid at the galvanizing temperature (like mud in the bottom of a water pond). Actually more like saturated sugar crystals at the bottom of warm water. The temperature of the dross and the zinc are critical in properly removing dross.
A drossing "clam" or "grab" is the poorest drossing equipment resulting in an average of about 2% iron in the dross. The true dross like a sponge with much entrained zinc.
A drossing scoop or shovel properly used with cranes can be effective. The resulting dross can be above 4% average iron in the dross.
When making racking fixtures or a dross scoop, be sure to use welding rod with proper low alloying agents, otherwise the welds will soon disappear in the zinc.

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

January 11, 2009


There is a mistake in my previous response, which should read: "0.02% MAX silicon + phosphorus + aluminum in the steel, NOT 0.2%."


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

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