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How to prevent rust formation in the galvanize materials



(-----) January 3, 2008

Q. Good day. We are experiencing formation of white rust (zinc oxide) in our galvanize items after 2 weeks of exposure. I read in one article that sodium dichromate is added to the water rinse bath to prevent rust formation in galvanize items. What will be the concentration of sodium dichromate that I will add to our rinse bath? Will the addition of sodium dichromate have an effect in the color of our galvanize item? We also observe some greenish rust in our galvanize items during rainy days, what will be the cause of these greenish rust? Thank you very much

Sincerely,

Rose Ann N. Alvarez
supervisor, galvanizing plant - Bulacan, Philippines
^


November 6, 2009 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. Dear sir,
We are facing the problem of chromate marks on the galvanize pipes. We are chromating in sodium dichromate.
How to check the concentration and what should be the concentration of the solution.
Thanks

sanjay garg
- hisar,haryana,india
^


January 4, 2008

A. A chromate addition to your rinse / quench tank of suitable concentration will prevent white rust for about 2 months, but not for ever.
The chemical manufacturer will advise the strength, but 0.15% chromic acid is common.

There are other ways to prevent white rust:
It forms mainly where items are touching or very close in damp conditions.
So either keep dry or separate the items.

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


January 18, 2008

A. Dear Rose Ann:
Some galvanizers in North America use chromate, others have found ways around chromate and do not have white rust. Those using chromate usually use between 500 ppm (light yellow) and 5000 ppm (deep orange). The chromate concentration is measured using stoppered glass bottles with made-up standards and compare the color of the quench with the standards.
Much more important than the chromate concentration is the pH of the chromate quench which must be controlled weekly. Without pH control the chromate quench usually quits working after 3 or 4 months. With pH control the chromate quench works for 5+ years.
There is another chemical required to prevent yellow or orange or green staining of the product. This material gives a very light sky-blue color wherever the quench dries. It is a very pleasing color but you have to look very hard to even see it. The product is a bright silver and it is not possible to determine that it was even quenched in chromate.
Keep in mind that hex chrome causes throat and lung cancer and the chromate quench must not be allowed to get hot because hex chrome is quite volatile (easily put into the gas phase). For proper use of chromate quench the product should be put into water first to cool the product and wash off carried over flux.
Regards,

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


January 19, 2008

Dr Cook posted ..."Keep in mind that hex chrome causes throat and lung cancer and the chromate quench must not be allowed to get hot because hex chrome is quite volatile (easily put into the gas phase). For proper use of chromate quench the product should be put into water first to cool the product and wash off carried over flux."

But the safety aspects of chrome are related to concentration. It is possible to run a hot quench tank with low safety risk. Its possible to have all the cooling done in the passivation tank, so that one tank does both cooling (quench) and passivation using chromate. The tank will typically run at 80 -90 °C, and the steam that come off contains no detectable Cr.

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


January 22, 2008

Geoff,
-From the June 1994 NIOSH Pocket Guide to Chemical Hazards, page 70 the following exposure limits are placed on hex chrome: NIOSH 0.001 mg/cubic meter (as Cr+6); OSHA 0.1 mg/cubic meter (as CrO3); IDLH 15 mg/cubic meter (as Cr+6). IDLH is "Immediately Dangerous to Life or Health."
About 25 years ago I tested a 0.1% solution of hex chrome and made a video of the results. The detection equipment was 3 feet from the surface of the solution. Results are:

Temp C Conditions Hex Cr in mg/cu.Meter

55 Stirred Detected by Paper Chromatography
97 Easy Boil 0.023
97 Easy Boil 0.026
97 Easy Boil 0.066
150 Dripped onto Galv Steel at 150C 19
300 Dripped onto Galv Steel at 300C 119

The greatest problem to avoid is to be sure that the galvanized steel is cooled down prior to withdrawal from the chromate quench. A two inch thick piece of steel may take 10 minutes or more to cool properly and many galvanizers just do not have time to wait for this cooling.
A lesser but significant problem in a single chromate tank (for cooling and treating) is that carried over flux contaminates the quench and then cost of disposal is high. With a two tank system (e.g., water for cooling and then chromate for treating) the plain water can be used for flux evaporation make-up and only tap water put into this water quench to give very, very long lifetime to the chromate treatment.
Regards,

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


January 24, 2008

Thanks for the info from the 1994 US limits (14 years ago?) and the results of testing from 25 years ago.

I guess each country decides its own limits, and perhaps change them as more information comes to hand, and also that practices change with time too.

We run a quench strength only 1/10 of what you trialed, and have conducted tests of that in normal actual operation, annually for some years.
The most recent data showed volume concentrations of 1.18 x 10E-5 8 hour TWA in mg.m3.

Current exposure limits in the UK are 0.5 mg/m3 for Cr VI, and 0.05 mg/m3 for Cr.II, & Cr.III (all 8 hour TWA)

Its not so common for galvanizers to be processing two inch (50mm) thick plates (though not unheard of), the more typical material is in the 6-12mm (1/4 to 1/2 ") ranges for typical jobbing galvanizing.

The difference between the tested exposure levels and the limits is significant, and enough to be fairly satisfied that despite Cr being a "nasty" substance, it can be used safely in a controlled environment.

As soon as some chemicals manufacturer comes up with an alternative that works consistently, reliably and sustainably, then I'm sure they will have ready sales. But meantime Cr is the one chemical that does the job, and can be used safely.

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


January 25, 2008

Geoff,
Thank You.
Tom Cook

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


January 26, 2008

Dear Geoff,
In your Jan. 4 response you say 0.15% chromic acid is common. In your Jan 24 response you say you use 0.01% chromium (e.g. 1/10 of my testing of 0.1%). Which do you recommend? Do you control pH and if so at what value and with what chemical(s)? Do you recommend a surfactant? To be sure my book is outdated, but my testing is not because the chemistry/physics do not change. How often do you find it necessary to dump your chromate quench? It is always a pleasure to correspond with you on this site.
Regards, Tom Cook

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


January 28, 2008

Re: Cr strength. This is very dependent on the temperature. We use 0.01% - 0.02% and it works at about 85-90C (24 hours operation plant, so tank always hot).

Under about 60C and this strength is useless and is almost like a water only quench.

0.15% is common for a passivate at ambient temp. My post was ambiguous - sorry.

We don't control the pH, but a titration (I forget exactly what, I don't do it myself) shows the need to add more concentrate. (The conc is where the hazard lies)

We dump the tank at about 2 year intervals, but not all of it. At Christmas time (only real break) we decant the settled liquor to another tank, then clean out the sludge, which is largely washed off ash etc.
About 4 times a year, we use the drossing spoon and get the worst of the sludge (mud) out.

Its visually easy to tell is the passivate strength is getting low, the work starts to look dull immediately. At that time 1-2 liters of concentrate (in a 35000 litre tank) is usually enough to fix it, but the titration gives a more accurate assessment.

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


February 5, 2008

Geoff,
25 years ago while in Montreal I formulated a chromate quench that works extremely well. First I tried chromium trioxide (also called chromic acid) and sodium dichromate dihydrate. The chromium trioxide (chromic acid) gave extremely bad results by giving a dark green, dark brown, or yellow coatings on the galvanized product.
Then while using sodium dichromate, I found another chemical in commercial chromate quenches (by smell) and used some pure chemical with the same anion. After a few tries the results were very good in that no discoloration of the product was present. Next I worked on the pH and found that with use the pH rises naturally to about 6. Thus over time I tried various acids (all but one discolored the product) to control the pH to 4.2. Meanwhile HydroQuebec was testing the samples and all were passing the 40 hour salt spray tests.
Thus for a perfectly clear coating sodium dichromate, plus another chemical, plus maintaining pH at 4.2 with the right acid gives a chromate quench that lasts 5+ years and passes salt spray testing.
Regards, Tom Cook

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


July 22, 2010

Messrs. Crowley and Cook,

Please confirm that there are no associated safety and health risks involved in quenching galvanized items directly in a chromate solution followed by water rinsing. I had experimented quenching galvanized specimen plates with water, air, acidified sodium dichromate, and non-acidified sodium dichromate solution where the chromate quenches were followed by water rinse to prevent chrome powder from drying on the surface of the material. The plates quenched with acidified sodium dichromate solution (about 1.0% sodium dichromate w/w) gave favorable results on luster and coating thickness.

I am more concerned on the concentration of tri or hex chrome in the steam or heat evolved during the quenching process for operator or worker safety since we do not have (yet) an ambient air chrome analyzer. Your data would be very helpful in our endeavor.

Best regards.
barlow campano
Barlow Campano
galvanizing chemist - Jeddah, KSA
^


July 23, 2010

Dear Messrs. Crowley and Cook,

I am a chemist of a 10-m and 15-m long galvanizing kettles in Jeddah, KSA. I have been following your threads on chromate finishing such as letter 47463 and I need clarification whether the data you provided - 1.18 x 10E-5 8 hour TWA in mg/M3 was for Cr, Cr III, or Cr VI taken from ambient air sample and how far from the steam or heat evolved during quenching of the galvanized material in sodium dichromate.

I have been experimenting various quenching methods such as air, water, acidified sodium dichromate (about 1% Na2Cr2O7 w/w), and non-acidified sodium dichromate (at the same concentration). The articles quenched in acidified sodium dichromate gave favorable results in terms of luster and coating thickness. The items were water rinsed after the quench to remove excess chrome that dry up and become yellowish powdery surface deposits. Drawbacks however are personnel safety from heat released (if there are any hence the query) and frequency of disposal for the chromate quench rinse water.

Kind regards.
barlow campano
Barlow Campano
galvanizing chemist - Jeddah, KSA
^


August 1, 2010

The quoted result was Cr total, sample device was worn by the operator to simulate operator exposure over a shift.

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


August 2, 2010

Sir:

The best hex-chrome quench that I developed has the following formulation:

28.7 grams Na2Cr2O7.2H2O

52.5 mL glacial acetic acid

Both dissolved in 10 liters of water.

And then HCl added to a pH of 4.2.

pH's below 3 or above 5 must be avoided.

This hex-chrome solution is NOT rinsed off afterward with water rinse. If the product is water quenched prior to hex-chrome, then the lifetime of this hex-chrome is very long and is much safer to use because it does not become hot.

The 2007 CDC handbook lists hex-chrome limits as:

NIOSH as TWA 0.001 mg/m3

OSHA as TWA 0.005 mg/m3

This handbook lists the TARGET ORGANS AS:

blood, resp sys, liver, kidneys, eyes, skin [lung cancer]

For chromium II and chromium III the exposure limits are higher.

Much has changed in the last few years due to the reduction of galvanizers using prime western zinc (with 1% lead). Thus the greater use of high grade zinc (with no lead) has caused some galvanizers to use various alloying agents in the zinc including: bismuth, nickel, tin, varying amounts of aluminum and others. These alloying elements have profound effects on galvanizing including likely more (or in some cases less) white rust. These alloying agents are being used to reduce the surface tension of pure zinc, to decrease viscosity, to reduce the coating thickness on reactive steels, and to improve the appearance of the product.

The ban by Europe on hex-chrome has also promoted the use of alternative galvanizing quenches including trivalent chrome and molydates. Some of these quenches contain high concentrations, low pH's and solid "blockers" like silicon dioxide (sand) and titanium dioxide (paint pigment). These new materials make the results of salt spray testing ambiguous.

Quite soon I will go to a galvanizer and perform many tests on alloying agents to sort-out the effects of many zinc alloying elements to determine the best alloy.

In your question, you asked if hex-chrome is safe to use. As a PhD chemist, I cannot answer that question. Today, for me the best answer to your question lies in the CDC (Center for Disease Control) handbook and with the medical physicians who update that handbook.

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

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


August 4, 2010

Mr. Campano: You ask:

"Please confirm that there are no associated safety and health risks involved in quenching galvanized items directly in a chromate solution"

No one should confirm that. Chromates can be, and are, dangerous and carcinogenic. You need adequate protective gear, ventilation, and engineering to protect employees. It can be done safely, but requires proper knowledge, equipment and procedures. Please employ someone who understands the problems and solutions.

And never, never, rely on answers from an internet forum. There are many true experts here, but the ultimate responsibility is yours.

jeffrey holmes
Jeffrey Holmes, CEF
Spartanburg, South Carolina
^


August 5, 2010

As a galvanizer in Europe I am unaware of "The ban by Europe on hex-chrome...".

Certainly there are moves in that direction.

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


August 10, 2010

Thank you kind sirs for the responses. I'm sort of evaluating which is better between rinsing chromate quench/finish against leaving it powdered dry. I guess we have to do the testing ourselves to confirm your answers and/or suggestions.

I did wrote CDC but they haven't come up to a data or solid backing yet to date.

Regards.

Barlow

barlow campano
Barlow Campano
galvanizing chemist - Jeddah, KSA
^

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