Hot dip galvanizing flux Q & A's, Problems and Solutions

A. Collins:

Are you "wet" or "dry" kettle? What is too high of rate of dross? How long do your kettles last?

Regards,

A. Ozge,

You answered my question of where you get your inhibitor EXACTLY AS I EXPECTED. Regarding this supplier I respond "NO COMMENT."

Incidentally 30 °baumé FRESH HCl is NOT available, the highest concentration is 20 ° baumé (e.g. about 35.5% HCl). After being fresh the dissolving of iron and zinc causes the baum é to rise to the 30 to 40 ° baum é range. Some iron is required for good pickling in HCl.

In my inhibitor testing of 38 years ago (now published in the magazine METAL FINISHING) I did all my testing at 3 parts inhibitor/10,000 parts mixed acid (e.g. 3 liters inhibitor/10,000 liters mixed acid). For sulfuric acid I used 10% acid, 2% iron, 1% zinc and 160 °F. Only one inhibitor (produced in the USA) did everything I wanted, including NOT slowing the pickling (in a separate test). For HCl I used 1:1 HCl/water 3/10,000 and the same iron and zinc and at 90 °F. Again only one (produced in the USA) did what I wanted.

About 10 years ago I was called into a new start-up plant using sulfuric acid, which previous to my arrival had 3 (55 gallon) barrels of fume suppressant plus inhibitor put into each of three 20,000 gallon pickle tanks. The fumes were so bad that I could NOT walk across the plant floor crosswise. Simply put, my lungs closed up and I could not take a breath. The workers were having nose bleeds and quitting. I drove to a nearby galvanizer and obtained two 5 gallon buckets of the good inhibitor and put one gallon into each acid tank. The next morning I came into the plant and the workers all came up to me and were thanking me for the clean air that they could take a breath. The supplier blamed the first product on a "bad batch." RIGHT!!! With the good inhibitor the air in the plant was the same as clean outdoor air.

A few months ago I went to the East Coast of the USA and noted that the HCl was not properly pickling. A test confirmed that the acids were over-inhibited (same supplier) as the case 10 years ago. This was quite unusual because the plant manager said they had not added any inhibitor for the last 5 months. To increase pickling they had heated up the HCl tanks and the guy was there getting ready to remove and replace the roof. All the acid tanks were then hauled away and new acid brought in and inhibited with the good inhibitor. It was a pleasure having acid that pickled very well and with almost NO fumes. Thus the combination of a good inhibitor and with proper testing, fast pickling, very low fumes, and very long acid tank lifetimes are easy to attain.
Geoff has very good experience.

Regards,

A. Ozge:

30 some years ago I found a wetting agent for flux that I liked. I used vacuum (water aspirated) and reduced the volume of this wetting agent by 95%. I then added water back and got the same good results as a wetting agent. I contacted the maker of this wetting agent and indeed he admitted that he took 2 gallons of the concentrated wetting agent and then added 50 gallons of water to it and sold it by the barrel at about $2,000/barrel. I chose NOT to deal with this guy who liked to dilute a good material and sell it for a huge profit. Later I found a concentrated material that works extremely well at 1 to 3 parts/10,000 parts of tank volume. (e.g. 0.01% to 0.03%). Thus from this posting you should note and understand that if an inhibitor company recommends much higher concentrations it is likely the inhibitor is not very effective or watered down. Also realize that the more inhibitor an inhibitor company sells the more money the inhibitor company makes. Also some inhibitor makers do not disclose on MSDS sheets what is in their products which should make you question why not? In the USA disclosure on MSDS sheets is a requirement of law. An acid inhibitor MUST nearly prevent additional attack of the bare steel after the mil scale is removed, and it MUST NOT slow down pickling (removal of mil scale). Thus two types of testing are required: 1)acid attack on steel in a stressed condition and 2)pickling speed tests on steel with a mill scale coating. Less fumes in the plant come automatically with a good inhibitor. There are fume control agents (which are NOT inhibitors) as well but I rarely use them except in special cases.

My article in the magazine METAL FINISHING about inhibitors is titled something like: "Testing and Use of Acid Inhibitors." From this article you can test and evaluate inhibitors to find the one or two (from many) that suits your needs.

Regards,

A. Sir:

There is no way I can answer your questions because I have no idea what your wetting agent is. I do not know if it would be beneficial or bad.

Regards,

A. Sirs:

In hot dip galvanizing fluxes certain non-volatile chemical compounds may be present (even in brand new fluxes). These may include calcium chloride (CaCl2), magnesium chloride (MgCl2), manganese chloride (MnCl2), sodium chloride (NaCl), and potassium chloride (KCl).

About 30 years ago I found 13% sodium chloride in a flux from Iceland and the galvanizer had 13% rejects. A reject being product that had to go through the kettle twice. When the flux was replaced with a flux having no sodium chloride the rejects went away. Same thing happened in Madrid except it was 20% sodium chloride in a pipes galvanizing plant with 20% rejects and again rejects disappeared when flux with no sodium chloride was put in the flux tank.

I now know that if the solid flux contains about 3% total non-volatiles (or about 8 grams/liter in the working flux), then the rejects will be about 5%.

I recommend that you have your working fluxes tested by icp/ms and post the results. The results should be in mg/liter of the chlorides of the various metals.

Vedrana-- You cannot neutralize with sodium chloride (NaCl), you must mean sodium hydroxide (NaOH). The proper pH of rinse before flux is 5 or lower, otherwise the product will corrode before you can get to the zinc kettle. Your current procedure is bad and you will contaminate the flux.

Jeet-- Have your incoming (new) flux tested and post the results.

Regards, Dr. Thomas H. Cook, Hot Springs, South Dakota, USA

A. If you would like to try a flux extremely low in non volatile salts I suggest you try PREACT. We manufacture our flux at our plant in Freeport, TX USA from virgin raw materials to insure the lowest possible level of non-volatile salts.

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Thanks for your participation Thorn! We don't recommend one proprietary product over another in this forum, nor can we ask our supporting advertisers who are making this forum possible to bear the costs of postings which steer the potential customers away from them and to non-advertisers unwilling to share in the costs, so we had to abbreviate your posting a bit.

But you are implying that low-volatiles in flux is an important issue, and it sounds to me like a valid technical point. Thanks!

Regards,

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
May 2014

A. Ted, I can appreciate your comments. The best expert to ask regarding the problems of high levels of non-volatile salts in a galvanizing flux can cause is Dr. Thomas Cook. He has done a great deal of work testing fluxes. He is by far the best expert I know on this topic.

Regards,

Thorn Baccich, , President [returning]
Mineral Research and Development - Harrisburg, North Carolina USA
May 15, 2014

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"Hot Dip Galvanizing
A Guide to process Selection & Galvanizing Practice"
by M. J. Hornsby

on AbeBooks

or eBay or

Amazon

(affil links)

Hi again. No question about Dr. Cook's expertise, which has been demonstrated countless times on this site, let alone in many other venues. However, unless there are several suppliers of fluxes with low non-volatiles, the topic becomes a public discussion of a single proprietary product -- which is always a disaster for a dozen reasons ( huh? why?).

I am quite certain that if a responder offered a scathing review of a proprietary product we'd quickly hear from that company's lawyers, whereas all would be well if the readers heard only one side :-) Thanks.

Regards,

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
May 2014

A. Dear Mr. Rashad Ahmed;

Generally double salt flux is used among batch galvanizers. However you can use higher ammonium chloride content but it will cause excessive fume.

If you don't have a dryer you can:

1- Heat up your flux to 60 °C with a simple boiler and heat exchanger coils
2- Use flux surfactants (will be cheap and easy to use)
3- Soprin has an flux additive for exactly that purpose

But please keep in mind that nothing will be as advantageous as a good drying system.

A. Dear Mr. Rashad Ahmed;

For quality issue, actually I have never noticed any difference between double salt and triple salt. Moreover if you can eliminate fume, and I am assuming that your pickling is not perfect (as you want to accelerate your pickling) triple salt can give you a better solution. Although the consumptions are nearly the same, triple salt is cheaper than double.

I have seen a chemical supplier is offering pickling accelerators, but I have never used it or seen any plant using it. I am not sure chemical approach to accelerate pickling can efficiently work. Maybe Dr. Cook can explain that subject to us.

But, kinetic approach for accelerating works perfectly. You can heat up your acids to 30 °C, and give some agitation by a mixer or blowing air in. By applying both, you can double your pickling efficiency. But be careful not to exceed 30 °C much. It will cause excessive acidic gas extraction, and also it makes Dr. Cook so much angry :-)

I am currently installing a galvanizing plant (14.5 x 1.8 x 3 m kettle dimensions) and a fabrication unit (fully automated pole production line and road barrier line). Actually main equipment installation is finished; we are doing the auxiliary fabrications and waiting for governmental approvals.

A. Dear George,

Didn't you filter your flux after adjusting pH and adding peroxide?

A. Dear Sir...
Better you do continuous flux filtering system which helps to remove Iron content from Flux bath.

A. George,

The reason you got so much foam when you added H2O2 to the flux was that your pH is too low. You must also FIRST add aqua ammonia ⇦this on eBay or Amazon [affil links] (ammonium hydroxide) to provide OH- groups for the Fe+3 that is formed to make Fe(OH)3. Without the ammonia, peroxide self destructs to oxygen (the foam) and water.

Regards,

A. Sir,
Best titrant for Fe+2 is potassium dichromate with sulfuric acid/phosphoric acid buffer and sodium diphenylamine sulfonate. End point is from light green to deep blue.
Regards,

Q. Dear Dr. Thomas
Thanks for response regarding Fe+2 analysis.
Can you describe a complete method to analyse hot dip galvanizing flux for iron content using potassium dichromate?

Thanks awaiting reply

A. Sir:

Nickel in the zinc is effective ONLY with silicon semikilled steels (e.g. silicon between about 0.04% and 0.14% Si). There is very little steel like this in the USA and nickel in the zinc at 0.05% nickel is not so popular as it was some years ago.

I am not aware of any proof that nickel chloride in the flux is effective in controlling silicon semikilled steels.

I did some experiments with auto catalytic nickel plating (a few microns before a regular flux) and it did work on silicon semikilled steels. There were problems in controlling the nickel content in the auto plating solution depending on temperature, pH, and some other variables. A flux company in Germany said they did not make nickel containing flux because after it was contaminated there was no way to get rid of it.

Because nickel causes lung cancer, I NEVER recommend the use of nickel in galvanizing because it is quite likely to become airborne.

Regards,

A. Sir:

I presume your numbers are percent?
How was the Zinc determined? ICP or AA
How was the ammonium ion determined? Kjdahl or Specific Ion Electrode
How was the iron determined?

IF!! your numbers are correct?? There is not nearly enough ammonium chloride for good galvanizing. The iron is a bit high.

Regards,

A. You will certainly get a lot of fume.
It might work, but it could also be that you don't get full wetting and fluxing and that results in bare patches.

You could try it in a small batch on some sample plates and see what happens, that way you don't waste a whole flux tank of materials.

Q. Thanks sir
I also want to know the ratio. How much NH4Cl to use in 1 liter?
Please advise me.

 Hi cousin Thaw. I am not a galvanizer, but I try to listen and learn -- and I think in your eagerness to get started you have not slowed down to listen closely :-)

I think the answer was essentially: "It's a bad idea and liable to not work well for several reasons; but if you insist on trying it, then run some experiments on a few small parts". Demanding that people provide the details on exactly how to do what they've repeatedly suggested that you not do might be considered badgering, but good luck :-)

Regards,

A. Sir:

45 years ago I did experiments using NH4Cl alone and ZnCl2 alone for hot dip galvanizing fluxes. NH4Cl dried very quickly on the steel but corroded the steel very badly. ZnCl2 never dried and the steel did not corrode. The NH4Cl flux gave much skimmings (ash) and smoke. The ZnCl2 spattered badly when going into the molten zinc. In the end both steel samples galvanized fairly well with about 3 mils of zinc (75 microns). Do not expect good results from ammonium chloride alone or zinc chloride alone.

Experiments with hot 160 °F (71 °C) triple or quadraflux dried quickly and did not corrode and gave an excellent result. The steel must be in the hot triple salt solution long enough to heat the steel completely so the heat of the steel dries the flux from the inside.

Regards,

A. Sir:

If you are using ammonium hydroxide (aqua ammonia) to raise the pH of your flux then this is the same as adding ammonium chloride. Each U.S. gallon of ammonium hydroxide is the same as adding 11 pounds of ammonium chloride.

If this is not the "problem", then perhaps whoever is dong the testing may be doing it incorrectly.

Regards,

A. Sir:

By adding 1,000 liters of 25% aqua ammonia (expressed as percent NH3), and 275 Kg of zinc chloride, you are indeed increasing your ACN.

The Kg of ammonium chloride in 1,000 liters of 25% NH3 is:
0.25 * 1,000 L * 0.91 * (53.45/17) = 715 Kg ammonium chloride.

The resulting ACN of your additions are:

ACN = (715 Kg/275 Kg) ACN = 2.60

In the above calculation: 0.25 is the % NH3.
The 0.91 g/ml is the density of the NN3 solution.
The 53.45 is the molecular weight of ammonium chloride.
The 17 is the molecular weight of ammonia

The real question here is WHY would you need to add 1,000 liters of NH4OH to neutralize excess acid in your flux? This is enough ammonia to neutralize about 2,000 liters of 17% HCl. You must not have a rinse tank between your HCl tanks and your flux OR you have an old rinse solution that is stagnant for many years? Perhaps clean water is costly in your country.

I do not normally give any advise without knowing more about a plant, however skimmings or ash possibly could be added to your flux to both neutralize acid carry-over and to keep your ACN down.

What is the density (in Be' units or g/ml) of your flux.
Your operation is not in control.

And by the way I do not like double salt.

Regards,

A. Sir:

The best results that I have found for flux is: 12 to 14 degrees baumé (measured at room temperature; 4.2 pH; temperature 160 °F (71 °C); below 1% sulphate; non-traditional non-volatiles below 7,000 (Ca, Mg, Mn, K, Na expressed as chlorides). The ACN which should be 1.6.

Unfortunately, many commercial fluxes sold in India, China, and Europe are made from recycled materials or poor starting materials and give poor results in galvanizing. Only by testing the actual process flux can suitability be determined. Fortunately in the USA there is one excellent flux manufacturer.

Hi Yashwant. Can you please post more words, so we can be clear exactly what the problem is before the waters get muddied by people misunderstanding your question? :-)

You're saying that after 5 min in the flux tank (why that long?), purlins (bare steel? what grade?) made of 2 mm thick material turn red (bright red or the color of rust?) for some unknown reason? Then they galvanize correctly anyway, or they galvanize unsatisfactorily, or you stop the process and don't even attempt to galvanize them? Thanks!

Regards,

A. Sir: The red color is Fe(OH)3 and is likely from too high of pH. Check the on-line library on this website for a better understanding.

Regards,

A. Degreasers: It's quite possible to galvanize clean steel without a degreaser, but if quality of finish is important, not advisable. A good degreaser will remove oils that you hardly thought were there. but which will inhibit the acid from cleaning the steel. Sources include: MIG welding sprays, lubricant in a bandsaw, drilling fluid, and lastly one often not thought of - dirty hands handling the steel. Degreasers come in several formats. Perhaps most common and low cost is an alkali tank of say NaOH heated to about 40-60 °C, and full immersion for 203 minutes. The downsides of this are energy cost (though recovered heat from the kettle might be used), the need to rinse off the alkali before putting the steel into acid, and the space in the shop. Advantages: perhaps lowest cost degreaser. Improvements include chelates and surfactants, available as proprietary brand degreasers.
Next are acidic degreasers. Reputed to not work as well, but often sufficiently, this removes the acid-base problem requiring rinsing.
And perhaps lastly, in acid degreasers. Additives to add to acid tanks to remove a little oils. Downside here is the oil has to go somewhere, and often concentrates on the surface of the tank, and then on the upper tank walls as something that looks like road tar. And that can then come off, onto the steel, and stay there until the zinc, where it burns off, and the smoke causes problems in the fume filter.

Fluxes: usually a mixture of zinc chloride and ammonium chloride. The ratio of these two is varied, and you will find as many opinions on what this should be as people you ask. Common ratios are double salt (2:1 ammonium to zinc chlorides), triple salt, and quadruple salt. The ammonium causes the fume, so higher the ratio, more fume. Probably the most common is double salt, but this varies by region and continent (which really means is habitual and gets copied in local areas).

A. The black resinous stuff on the zinc surface is flux. If you take straight flux powder and sit it on the zinc it will go like that.
You were at 28 Be, but that's too strong and too cold; and then you went to 18-20 and 60 °C and that's too weak but temperature OK. A good target is 23-27 Be, or 1.19 to 1.22 SG.
The black deposits on the job are flux residues. It can be washed off with water.
But its presence there in those places on the job indicates poor skimming practice.

A. Not quite. That is gets onto the steel and left there like that can be avoided by better skimming. But the formation on the zinc surface of the black sludge (molten flux) is probably also avoided.
The ratio of the ammonium chloride to zinc chloride is important, and should be somewhere between 2:1 and 3:1 (double salt, triple salt as they are called).
Most galvanizers can't tell the difference between double and triple salt in operation. Triple salt makes more white fume (its the ammonium chloride that makes it and higher concentration of that means more fume). But get beyond those limits especially towards the 1:1 range and more black deposit on the kettle. Have you had this measured in your flux? If it gets out of ratio, its easy just to add the low component until back to specification.