finishing.com -- The Home Page of the Finishing Industry
A website for Serious Education, promoting Aloha,
& the most FUN smiley you can have in metal finishing.
    no popups, no spam
HomeFAQsSuggested
Books
Help
Wanteds
Advertise
on this site
FORUM
current topics
topic 54709

Dichromate for hot-dip galvanizing




A discussion started in 2007 but continuing through 2018

(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


(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
   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
galvanizing & powder coating shop
Glasgow, Scotland




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



To minimize searching and offer multiple viewpoints, we've combined multiple threads into the dialog you're viewing. Please forgive any resultant repetition or failures of chronological order.



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



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
galvanizing & powder coating shop
Glasgow, Scotland



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
- 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


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
- 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



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 Montanes
TEL - N FERRARIS - Canuelas, 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
- Russian Federation



This public forum has 60,000 threads. If you have a question in mind which seems off topic to this thread, you might prefer to Search the Site

ADD a Comment to THIS thread START a NEW threadView CURRENT TOPICS

Disclaimer: It's not possible to diagnose a finishing problem or the hazards of an operation via these pages. All information presented is for general reference and does not represent a professional opinion nor the policy of an author's employer. The internet is largely anonymous & unvetted; some names may be fictitious and some recommendations may be deliberately harmful.

  If you need a product/service, please check these Directories:

JobshopsCapital Equip. & Install'nChemicals & Consumables Consult'g, Train'g, SoftwareEnvironmental ComplianceTesting Svcs. & Devices


©1995-2018 finishing.com, Inc., Pine Beach, NJ   -   About finishing.com   -  Privacy Policy
How Google uses data when you visit this site.