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topic 60826

Contaminants building up in galvanizing flux

August 22, 2018

Q. What are the most likely contaminants that could build up in batch galvanizing flux? We find variable performance in our flux which according to the simple analysis done seems in spec. Sodium chloride suspected.

Geoff Crowley
Geoff Crowley
galvanizing & powder coating shop
Glasgow, Scotland

August 26, 2018

? Geoff,

The elements are: calcium, magnesium, manganese, potassium, and sodium. One of my clients has (all expressed as respective chlorides):

calcium chloride (CaCl2) = 3,601 ppm
magnesium chloride (MgCl2)= 1,893 ppm
manganese chloride (MnCl2) = 190 ppm
potassium chloride (KCl) = 300 ppm
sodium chloride (NaCl) = 3,480 ppm

Total NTNVs = 9,464 ppm in the working flux solution.

This client has periodic pimples on his product. My experience is that non-traditional, non-volatiles above 7,000 are a problem.

I have another client who has a total NTNVs of 3,189 who thinks that iron (+2) above 0.5% causes pimples.

What do you think Geoff?

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

August 28, 2018

Q. I suspect Na and the most common.
The others are not so commonly available, though this depends a lot on water quality. We have relatively pure water, with low Ca, Mg, Mn, K, but Na is in the process already in NaOH degreaser. So it can much more easily react with Cl to form NaCl and stay soluble in the flux solution.
The others could be problems where there's harder water.

Geoff Crowley
Geoff Crowley
galvanizing & powder coating shop
Glasgow, Scotland

August 29, 2018

A. Geoff,

Mn comes from the steel and is prevalent in fluxes following HCl. Many fluxes from Europe are made from secondary sources like galvanizing plants and all five that I have mentioned are prevalent.

Why not have your "working" flux solution tested for the five elements that I posted?

To get a correct result the testing company needs to "spike" the flux with a known addition (for example sodium) to determine recovery and make a proper correction. Most testing companies do not do this and results are usually NOT correct. AA, AE, and ICP are "fooled" because they are non-linear and are affected by matrix effects.

Caustic is not the only source for sodium ion. Some flux companies put sodium in flux, either to make "non-fuming" flux or use cheap secondary starting materials. Thus you may want to have your new incoming flux tested also.

At one time I thought you told me you use a magnesium compound to neutralize your acid rinse water?

At present, I think calcium or magnesium may cause flux on the product to collect moisture from the air and cause zinc spatter at the kettle.

You say your water is "good." What is the TDS of your tap water? Is there a reason you think the flux makers have good tap water?

Some new fluxes have substantial potassium chloride in them, which causes galvanizers to use a higher concentration of flux and results in excessive "dry" ash.

TESTING of the flux is what needs to be done.

Geoff what do you think about pimples?

Respectfully, Tom Cook

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

September 13, 2018

Q. Thanks for input on this.
We typically run about 1000 ppm total dissolved chlorides in flux in the tank. Tests on bought flux (mostly German manufacture)don't show any concerning trends of contamination, but I know there are suppliers whose flux is rather poor in this respect.
Yes in one of two plants we used a bed of mangnesia to neutralise acid in rinse water (in the first only of two rinses between aid and flux), and in the other plant this is not done. There's no significant Mg contamination of that plants flux compared to the other.
We're really lucky in Scotland with very pure towns supply water. Out of the tap conductivity (a different way to measure TDS) is typically 20-30µS/cm. (in our powder shop we further clean that down to <5µS/cm). In galv this is pretty good. In the south of England they have poor water full of Mg, Mn, Ca, etc.
I have no information on flux manufacturers water supplies, but have reason to believe that its either good or that they clean it - to get such low contamination levels in the liquid flux they supply and we purchase.
We operate a zinc recovery system from the kettle ash, and this is the point where any changes in flux chemistry are first noticed, being far more sensitive than the at kettle observations.
While we've not used so called "low fuming flux", I've seen it used in several plants. In most cases it's K they use rather than Na. That is they substitute potassium chloride for ammonium chloride, that latter being the major source of fume in that operation.

On "pimples": that's a rather generic term for several issues. I've seen them resulting from reactive steels, from poor flux, from excessive immersion time, and perhaps other causes. I think reactive steel is the most common cause I've seen.

Geoff Crowley
Geoff Crowley
galvanizing & powder coating shop
Glasgow, Scotland

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