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Galvanizing flux problem: contaminants are building up

(-----) 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
Crithwood Ltd.
Westfield, Scotland, UK
crithwood logo

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, USA

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
Crithwood Ltd.
Westfield, Scotland, UK
crithwood logo

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, USA

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
Crithwood Ltd.
Westfield, Scotland, UK
crithwood logo

February 27, 2019

Q. In batch hot dip galvanizing it is normal to use a flux, and today most of this fluxing is done by dipping steel into a heated aqueous solution of the flux. Most commonly that flux is ammonium chloride and zinc chloride mixed. The proportions of the two constituents give rise to the labels "double salt", "triple salt" "quadraflux".
One problem with ammonium chloride is the amount of fume produced. The higher the ratio of this component, usually the more fume is produced.
Some galvanizers use so called "low fuming flux". This substitutes potassium chloride for the ammonium chloride.

Does anyone have experience of using this type of flux, and if so how does it perform? Do you find it difficult to maintain the chemistry? Any quality effects? Any other effects either good or bad? Is temperature of the solution important?
We're considering it for a new plant, as it seems that those using it don't need a filter to remove the particulates from the fume.

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

March 2, 2019

A. Geoff,
The US patent number 3,936,326 (1974) by Wolfgang Muller is likely the inventor of the smokeless flux that you are considering. Because this flux has not been widely commercialized in the last 45 years it is likely not very good. You can easily get a copy of the patent off the internet. It requires 15 minutes drying at 180 °C. This flux depends on a eutectic and is likely hard to maintain sufficient purity. For example, what happens to the melting point as the iron +2 builds in the flux and how to remove iron when required? In one example the flux is used at 33° baumé which would likely give a very large amount of skimmings. I doubt that an MZR machine could recover zinc from the skimmings. In this example the zinc is 1% in lead and the zinc is also 0.2% in aluminum. Such high aluminum is okay because there is no ammonium chloride in the flux. On non-reactive steel I wonder if enough zinc can be on the steel. Let me know what you think after you read the patent.

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


Two short stories.

An employee of a major USA automatic pipes galvanizer told me that when his flux had sodium chloride (NaCl) in it that the iron push down arms were dissolved by the molten zinc in a few days. When he got flux without sodium chloride then the push down arms had a normal lifetime of years.

In about 1985 in South Africa I saw workers put a "potassium chloride" flux which looked like miniature flying saucers (1.25 inches diameter by 3/8 inches thick) onto a pipes galvanizing kettle as a top flux. It melted without much smoke and the workers liked it because it lasted a long time and did not smoke much. I stayed in South Africa a few more weeks, and interestingly that kettle burned out at the corner where the "KCl top flux" had been put. There was some talk that the top flux may have overflowed the kettle and went down the outside. My stay had ended and I do not know what caused the kettle wall to burn through.

Likely wet kettle fluxes have NaCl or KCl or both to make them smoke less and last a long time, however it would explain why wet kettle galvanizers have more dross than dry kettle galvanizers. In the USA we are now having kettles last up to 20 years. I do believe that NaCl in flux increases the reaction between molten zinc and steel to form more dross.

I am now feeding my rainbow trout at -12 °F.


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

March 5, 2019

Q. Low fuming fluxes seem to come is several varieties, and my research so far uncovers that perhaps 25% of UK galvanizers (15 plants) are using one or other of these fluxes, with no bag filter for fume filtration. There is extraction, but this vents to atmosphere and apparently still meets environmental limits (15 mg/m3).
Not all of them contain potassium chloride in place of ammonium, but I'm not sure yet of the actual formulation. One supplier is TIB Chemicals from Germany, who have offered Low Fuming Flux for over 30 years they claim.
At least 5 of these are also using a zinc recovery device (MZR or similar - others are available).
I have only heard of one general galvanizer running such high Al (0.2%) as quoted above, and that was an accident. Very often high Al% results in black spots. A more normal level is 0.001 - 0.002%.
I asked about non-reactive steel, and no one had thought about this at all, suggesting this isn't an issue. But then Si killed steels are far more common here than Al killed, so perhaps we don't see so much non-reactive steel.
On kettle life: there are two determinants on life:
1. Erosion of the kettle wall so that it eventually gets thin enough to be mechanically unsafe and needs to be changed. This is what everyone focuses on.
2 Increased risk of cracking due to thermal cycling. This used never to be an issue, with typical life being 10-12 years, but now with kettles going beyond 12 years, that risk increases and can be the determinant of end of life. While examples of severe cracks have been seen, I haven't heard of a failure yet due to cracking, though when one happens it will likely be dramatic. Unlike a wear hole (leak), where the flow of zinc out the kettle through the hole, is typically small, a crack could open up huge holes and the loss of zinc containment could be "very exciting".

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

March 6, 2019

A. Certainly I was NOT Recommending 0.2% Aluminum in the zinc. I was merely stating what is in the patent, showing that the inventor did not know well hot dip galvanizing. Your figures for Al in zinc are the same as mine. I have located a galvanizer using 2% KCl in his otherwise regular flux. I am trying to get more details. It may be that he has 1% or 2% HCl in his flux which equates to pH 1 or 2.


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

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