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"Dichromate for hot-dip galvanizing"





An ongoing discussion beginning back in 2007 ...

2007

Q. Dear Sir,

I want to know what should be the concentration and temperature of chrome bath (hex or tri) to completely passivate the zinc coating on hot dipped galvanized pipe. Please let me know it in terms of percentage or ml/liter.

Atul Mishra
galvanizer - Anjar, Gujrat, India
^


simultaneous 2007

A. Sir:
The common dichromate passivation solutions are normally between 300 and 3000 ppm sodium dichromate dihydrate. Acetate ion added as acetic acid greatly improves the appearance in that the surface has no color and does not show chromate stains. The product should not be too hot when chromated to avoid distortion and discoloration. Also hot hexavalent chromium gives of hex-chrome into the atmosphere and is known to cause cancer in humans. The higher the oxidation number (e.g. +6 in this case, the more covalent and the more volatile the ion).

pH control is essential for long term trouble free use and effective white rust control.

I have no first hand experience with trivalent chromium, but there are some newer US patents which read well (e.g. very long term salt spray testing).

Regards,

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


2007

A. Between 10 and 100 parts per million by mass is commonly used. That's 0.001 to 0.01 per cent by mass. Since the chromate salts are solids, your optional unit of mL/L is not applicable.

Bill Reynolds
Bill Reynolds [dec.]
consultant metallurgist - Ballarat, Victoria, Australia
We sadly relate the news that Bill passed away on Jan. 29, 2010.

^


2007

A. The required concentration depends greatly on the temperature at which you passivate.
If you are using passivation as a quench - that is you plunge hot steel, straight from the galvanizing molten zinc into a tank of passivation chromate mix, then a very low concentration (like 0.2%) is required. Higher concentrations will colour the material yellow, brown or even the colour of chocolate.

If passivating cold, then higher concentrations like 2-5% are often used.

An advantage of doing it hot is the lower concentration, and hence lower hazard (who wants Chrome anyway, if you have to have it, have less of it, until someone proves an alternative), and also you have the opportunity to recover some of the energy from the steel, which you could use elsewhere - for example heating your flux tank.

But read the makers recommendations. Any strength depends on the strength of the original material.

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



Using DI water in quench tank to minimize White Rust on galvanize

September 9, 2009

Q. Sir,

I am Arunkumar from Pondicherry - India

Currently I am using de-mineralized water for quenching tank , this seem to reduce the reaction rate of white rust.

Kindly give your opinion in this regard.

Thank you

Arunkumar Rajasekar
plating shop employee - Pondicherry, India
^


September 10, 2009

A. I would expect that since it will have trace amounts of chloride,sulfides and other nasty anions vs tap water.

James Watts
- Navarre, Florida
^


September 28, 2009

A. I feel the Raw water / Softened / DM water does't make any difference in quenching or does not influence the quality of the finishing of Hot dipped steel. the chlorides/carbonates or any minerals of the quenching water does not have time to react with the quenched metal parts, which are suspended while quenching that too only for a shorter period.
Water may have its influence only when the hot dipped metal is allowed to stay into quenching bath for a much longer duration.

Ravi Shankar
- Chennai, Tamil Nadu, India
^


October 1, 2009

A. Sir:
I recently had a workshop here on the ranch and a company employee came here, for which I consulted many years ago. They had put two ion exchange canisters for the water for their quench (no chromate). It was a fairly expensive operation costing about $0.04/gallon (I think around $10,000/month). Since that time this company has been purchased by another company and now they use hex chrome in their quench and the ion exchange tanks have been removed. I had the guy bring tap water and the conductivity meter registered 200 microSiemens (about 160 ppm total dissolved solids). I doubt seriously that they needed DI water. In your case if you take the conductivity or TDS of your tap water you may get a high (or low) value and help you decide if DI water is worth the money.
Regards,

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



To minimize search efforts and to offer multiple viewpoints, we combined previously separate threads onto this page. Please forgive any resultant repetition, failures of chronological order, or what may look like readers disrespecting previous responses -- those other responses may not have been on the page at the time :-)



April 1, 2010

Q. I want to know a little bit about different concentrations of dichromate in applying with hot - dip galvanizing for milled steel . What is the suitable and economic concentration for the chromate? I am trying to use Kalium Dichromate (potassium) K2CR2O7 for the coating. Is this material suitable ?

chinnappan savariyappan
plating shop employee - India
^


April 9, 2010

A. Sir:

Most North American galvanizers use sodium dichromate dihydrate, likely it is less expensive than the corresponding potassium salt. Also the usual concentration is around 500 to 1000 ppm and I like to use the color of the solution (e.g. standards in long, big test tubes) to estimate concentration. In use the pH of the solution drifts upward and must be carefully controlled using the proper chemicals. It is quite wise to have a water tank for quenching prior to the hex chrome tank.

Chromium trioxide (chromic acid) flake (dark purple flakes) is NOT suitable for hex chrome treatment of galvanized product. This chemical causes terrible stains on the product. Keep in mind that Europe has banned the use of hex chrome and with good reason. Hex chrome is a terrible cancer causing agent, and it easily comes out of solution into the air (e.g. the higher the oxidations number (e.g. +6 in this case)) the more covalent (more volatile) is the compound). The biggest problem is when a massive galvanized product is pulled out of hex chrome quench while it is still quite hot. At 300C, I measured 17000% the maximum CDC hex chrome in the air 4 feet above the hex chrome tank as a massive steel piece was being withdrawn.

Regards,

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



Sludge in passivation quench of hot dip galvanising line

July 31, 2012

Q. Hi guys

I have a customer how has questions relating to the sludge build up in the passivation quench after his kettle. He has no water quench prior to his passivation and uses his passivation as a quench and passivation tank, he find that sludge builds up relatively quickly in the tank and asks the following:

1) What is the sludge caused from, is it ash, etc.?
2) Is there a way to reduce the amount of sludge build up in the passivation tank
3) Does the chemical composition play a role in how quick sludge is built up ie, do non chrome containing passivation/ quenches build up sludge quicker than chrome based ones?
4) What is the rule of thumb on the frequency of dumping this tank

Thank you
Donavan

Donavan Jones
Supplier - Germiston, South Africa
^


August 3, 2012

A. I've seen the same. Using a combined quench passivate, various ash drops off the work and settles in the tank.
Accumulation rate dependent on ash present, tonnage processed, and perhaps other factors too?

One method of removal is to use your drossing equipment, and at about half the frequency.
So if you dross fortnightly, then every second drossing, use the dross spoon to clean the tank. Saves pumping!
And it will settle using the same down time that the zinc requires after drossing.

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


simultaneous August 7, 2012

Q. Thanks Geoff,

Any idea if the chemical composition would have any effect on the amount present, i.e., would a chrome free passivation produce less than a hex - chrome one? I personally wouldn't think so, but just checking

Regards
Donavan

Donavan Jones [returning]
- Germiston, South Africa
^


August 7, 2012

A. Donavan:

By using sodium dichromate at about 500 ppm (light yellow) and a pH of 4.2, the chromate quench will have the longest lifetime. Using these methods the chromate quench should last 3 to 5 years. Otherwise too high or too low of pH will give bad coloration to the product and not protect from white rust. Fast deterioration of the quench is usually caused by very bad flux solution or in the case of a "wet" kettle, a very bad top flux.

Geoff:

Your response is very unusual to me because of the apparent large quantity of zinc droplets and ash/flux sticking to the product. This reminds me of the (bad) old days. There are now ways to get excellent zinc drainage and virtually NO ash/flux sticking to the product. This was a surprising "by-product" of my work and, in fact, I just advised my clients to change-out their quench tanks because the impurities will build up very much slower. Thus the usual build up is about 5000 ppm (total dissolved solids) every year or two. It will be interesting to see what the total dissolved solids are after a year or two with the new technology.

Regards,

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


August 13, 2012

Q. Thanks Dr Cook

500 ppm, what would that relate to in % by vol of sodium dichromate?

Why or how would a "bad flux solution" impact on the quench and in what way? In terms of dissolved solids?

Could you expand on your comments to Geoff with the new ways to improve zinc drainage and the new technology that you were referring to?

Thank you

Donavan Jones [returning]
- South Africa
^


August 14, 2012

A. Donavan:

At 500 ppm (0.05%) % by weight and % by volume are virtually identical. I also used a few hundred ppm of acetic acid. Another guy thinks using phosphoric acid to control pH is better, especially if the product is to be painted. He is likely correct.

Bad flux (especially low ACNV and high baumé) causes more flux carry-over into the quench resulting in more chloride in the quench tank (at a faster rate).

Additional comments on my new process are not possible at this time.

Regards,

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



Rinsing scheme for trivalent & silane galvanizing quenches

May 25, 2018

Q. Hello! Dear sirs, would you tell me, please, what kind of washing scheme is needed after passivation of hot-dipped zinc coatings in a solution based on trivalent chromium. And after passivation in silane-containing solution?
Kind regards

Maxim Alekseevich
- Russian Federation
^


June 11, 2018

A. Hello Maxim,

I think silane based passivation systems are not mature for consistent results yet, so I would go for trivalent chromates for hot dip galvanization process.

And, it's only my opinion, for hot dip galvanized parts the customer is not looking for "better white corrosion resistance" but a good thickness for long-lasting parts in the field.

So, I hope you take the best choice for your product and your customers!!

Best of luck!

Daniel Montañés
TEL - N FERRARIS - Cañuelas, Buenos Aires, Argentina
^


June 13, 2018

thumbs up sign Thank you very much, Daniel, for your response. I agree that zinc mainly provides protection for parts and I suppose these two characteristics (protection and pretty good look) are not mutually exclusive. Besides, in the future, maybe, passivation methods without chromium will be preferable. I am concerned about the environmental friendliness of the process with respect to both trivalent chromium and silanes.
Best of luck for you, Daniel!

Maxim Alekseevich [returning]
- Russian Federation
^



September 26, 2019

Q. Hello I'm Hata and I got a problem with our final appearance of guardrail (whitening runs) which suspected from our chromate passivation bath because the problem not appear on batch which skipping the passivation. we control our passivation at 0.02% and pH 4.8

Mohamad Hatta
Galvanizing - Johor malaysia
^


September 26, 2019

A. Not much detail to go on to answer here..
Is all work affected by the white marks after quench? Or just these handrails?
What's in the tank? 0.02% what?
What else is in there? NaCl? KCl?
What temperature does this tank run at?

geoff_crowley
Geoff Crowley
Crithwood Ltd.
supporting advertiser
Bathgate, Scotland, UK
crithwood logo
^


September 27, 2019

A. Hata: You are running chromate at 200 ppm so it should be a light yellow color. Is the solution clear or cloudy? How old is the chromate solution? Is it made up from sodium dichromate or potassium dichromate or hopefully NOT chromic acid (CrO3) which stains the product. Hopefully you adjust the pH using conc. HCl. pH 4.2 is better. What is the TDS (total dissolved solids) in the chromate quench. Above about 4,000 TDS white rust is common. What is the TDS of your tap water? Sodium or calcium in the chromate quench can cause white rust. Do you use a water quench prior to putting the product into the chromate quench, so as to wash off the flux residue in water rather than the chromate quench?

Regards,

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


October 1, 2019

thumbs up sign  Thanks for your attention Mr Geoff & Dr Thomas. I'm using sodium dichromate at 0.02% , acetic acid to control pH around 4.5-4.8 and the temperature is 65 °C. Anyway I've tried to increase the conc. of sodium dichromate to 0.04% with pH 4.2 as told by Dr. Thomas and the problem seem to be settled but the thicker items got a bit yellowish.
Frankly I've no strong background in technical or chemistry but since following you guys it helped me a lot.

Mohamad Hatta [returning]
Galvanizing - Johor malaysia
^


October 2, 2019

A. Hata: The Ka for acetic acid is about 10 to the minus 5 which means that no matter how much acetic acid you put in your quench the quench will not have a pH lower than 5. Acetic acid "buffers" the quench to pH 5. You will need a small amount of HCl to lower the pH to 4.2. Perhaps 1/2 liter or 1 liter will do the job.

Regards,

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


October 3, 2019

thumbs up sign  Really appreciate on your valuable advice sir. I will try to work on it

Mohamad Hatta [returning]
Galvanizing - Johor , Malaysia
^


October 4, 2019

A. Hatta: To calculate the exact amount of conc. HCl to add to the chromate tank, put 50 ml of chromate solution in a conical glass flask, add 15 drops of 0.1% bromophenol blue sodium salt and titrate with 1:100 HCl (1 ml conc. HCl + 99 ml of distilled water). When the solution changes from a dark color to the normal yellow chromate color stop adding the 1:100 HCl. By proportion calculate how much conc. HCl to add to the chromate tank.

Regards, Tom Cook

Dr. Thomas H. Cook
Galvanizing Consultant - Hot Springs
^



July 21, 2020

Q. We recently had complaints from a customer that slit our hot dip galved material. They're complaining about black dust that formed on their felts(use for back tension) when they slit the coils.

We analyzed the dust and it shows Zinc oxide, which is normal. But why some coils and not all?

Is it possible that the sludge build up in the chrome tanks and then sprayed onto the strip surface+ squeezed be the reason thereof?

Please advise

Albe Hoffman
- Vredenburg, South Africa
^


July 21, 2020

Zinc Oxide (a/k/a 'white rust') is white. That's not the answer. If the black material is magnetic, then it's either iron or iron oxide (magnetite).

tom_rochester
Tom Rochester
CTO - Jackson, Michigan, USA
Plating Systems & Technologies, Inc.
supporting advertiser
plating systems & technologies banner ad
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