finishing.com -- The Home Page of the Finishing Industry
no popups, no spam
HomeFAQsBooksHelpWantedsForum letter 25997
Serious Education ... plus the most fun you can have in metal finishing.

Prevention and removal of white rust on galvanizing




An ongoing discussion from 2003 through 2014 . . .

(2003)

Q. Dear Sirs,

My question is ; If the hot deep galvanised surface of material has white rust, how can we clean or remove the surface of it? And What can be the actual reason of the white rust on galvanised surface?

Best Regards,

Ozan Karadag
Ms, Mechanical Engineer - Sarýseki , Ýskenderun, Turkey


(2003)

A. White rust consists of zinc-corrosion-products and will grow when fresh galvanized material is stored or transported in a way that condensed water can act upon the zinc-layer without ventilation. When the effect is not that serious, white rust can be brushed off with a soft brush. (it often looks serious because white rust has a large volume - You can measure the zinc layer after brushing off to see if it is serious) Take care for ventilation after brushing.


Dolf van den Berg
- Leusden, The Netherlands


(2003)

A. Dip all galvanizing in a weak Cr solution to passivate after galv. Dipping hot also cools the work for quicker cleaning access. Gives good protection for 2-4 months from white rust. The energy removed from the steel to the passivate solution can be recovered for heating acid to make faster pickling.

Geoff Crowley
Geoff Crowley
galvanizing & powder coating shop
Glasgow, Scotland




(2007) -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. We have some galvanized coils after months now it is rusted (white rust)
How can I deal with this coils ( paint-clean...)
Thank you.

Haytham Kilany
employee - Egypt



December 29, 2008

Q. I am a galvanizing engineer working for a big MNC for the past 4 years. I want to know if white rust could be avoided. Our plant is situated in a coastal area , does the salty air accelerates white rust formation? How can this be controlled ? Is there any passivation treatment for the same?

ARUN KUMAR
PLATING SHOP EMPLOYEE - PONDICHERRY, INDIA


Faults in Hot-Dip Galvanizing


Hot Dip Galvanizing

December 31, 2008

A. Post galvanizing passivation avoids the formation of white rust for a while. (perhaps 6 weeks)
But the options for effective passivation are not so attractive. The best performing include chromate systems which are unattractive from a Health and Environment perspective.

Geoff Crowley
Geoff Crowley
galvanizing & powder coating shop
Glasgow, Scotland


January 3, 2009

A. Sir:

With hex chrome the pH must be kept within narrow limits to give good results. There is an additive for hex chrome quench that gives a perfectly clear coating. The usual effective hex chrome is a few hundred ppm in Cr+6.

Some galvanizers think they can control white rust by:
(1) Use the "dry" kettle method.
(2) Not use a quench.
(3) Use the quench water to make-up for evaporation losses in the hot flux solution and then add only tap water to the quench.
(4) Not allow the galvanized product to get rained on for 2 to 4 days.
(5) Keep the aluminum content of the zinc at or below 0.001%.
(6) Not use bismuth or tin in the zinc or at least limit their use.
(7) Clean the quench water when it contains a cloudy precipitate.

Regards,

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


February 2, 2012

A. This posting relates to this letter and 56143.

In November of 2010, I started my new NiftyGalv process at a plant having 0.1% bismuth in the zinc. During that first trip, I saw virtually no white rust even though it rained about one-third the time I was there. This plant has a relatively warm climate with some air pollution. The galvanized product is not water quenched and goes immediately outside. During my second trip in January of 2011, I again saw virtually no white rust even though it rained.

Yesterday (Feb. 2012), I spoke with the owner and he stated that white rust has been substantially reduced in the last year. I asked him to quantify his "substantially reduced" statement. I asked: "On a scale from 1 (virtually no white rust) to 10 (severe white rust) what were you a year ago and what are you now?" His answer was: "A year ago I was a 6 or 7 and now I am a 0 or 1." The question is: Did NiftyGalv reduce white rust or did the lowering of the bismuth (from 0.1% to 0.02%; no bismuth was added in the last 16 months) reduce the white rust? Because it took 16 months to achieve less white rust, NiftyGalv appears not to be the cause of less white rust. Rather, lowering the bismuth appears to lower the white rust. Without bismuth, lead, or NiftyGalv the galvanizing results are poor. Because lead is toxic its use is not recommended and galvanizers in the USA are phasing it out of their zinc.

My seeing little or no white rust during my two trips is likely due to the lack of unique combined conditions causing white rust. The owner, however, is there every day and he mentioned a year ago during certain night rain events he knew he would arrive at the plant with chalky white all over the product. During the present rain season (now) that chalky white has not reappeared.

Other metals in the zinc that can cause white rust or early corrosion include: too much aluminum, tin, copper, and magnesium.

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



April 1, 2012 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. Galvanized iron wire usually has a galvanizing surface in order to prevent rusting or corrosion. Galvanization can protect galvanized wire underneath from rusting, but after a long time galvanization itself also can rust or corrosion. This will be a white rust instead of a typical red rust. If the corrosion is continued to develop, the underneath galvanized wire may begin to rust. So how to clean corrosion from galvanized wire and improve galvanized iron wire's service life.

Joy Liu
Shijiazhuang city, China


April 6, 2012

A. Sir:

Normally zinc corrodes about 20 times slower than steel. Unfortunately in recent times the addition of bismuth and perhaps tin accelerates white rust formation. Thus if you could provide zinc bath analysis perhaps this question could be addressed. Excess aluminum in the zinc can likely cause white rust.

Also if a quench is used and becomes contaminated (e.g. in a few 1,000 ppm) then this dirty quench will cause white rusting. A simple conductivity meter can determine the level of quench water contamination. Alternatively even putting the quench water into a glass bottle and observing coloration or cloudiness could be useful.

Regards,

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



April 20, 2012

Q. Dear Sir,

May I ask what contaminates the quenching water, and causes white rust / reduced brightness? Is it the residue from flux (ZnCl2 and NH4Cl)? The Zinc ash?

Regards,

CS Leong
- Selangor, Malaysia


April 23, 2012

A. Sir:

You are correct the dissolved material is zinc chloride and ammonium chloride from the flux. The undissolved material is likely zinc oxide. My new process causes the burned flux/ash to not stick to the galvanized product so I need to inform users of this technology to change out their quench tank. In a year or so we can see if the dissolved materials in the quench is less than the usual 5,000 ppm.

Regards,

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


April 27, 2012

thumbs up signDear Dr Thomas,

Thank you for sharing your knowledge with us.

Regards,

CS Leong
- Selangor, Malaysia



August 26, 2013 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. What is the reason behind white rust? Please brief all and give also solutions.

Johur Ahmed Roney
steel - Chittagong, Bangladesh


August 2013

A. Hi Johur. We appended your inquiry to a thread which hopefully answered it for you. If not, please try to detail your actual situation so we can get down to specifics.

This forum is a great way to get advice for highly specific real-world situations, but books might be better if you're seeking insights into broad general concepts. Good luck.

Regards,

Ted Mooney, finishing.com
Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey



Do phosphide fumigants cause galvanizing to white rust?

June 20, 2014

Q. Dear All,
I have situation of complete white rust on the zinc coating, even though the galvanized surfaces are exposed to open atmosphere. But it seems that the galvanized surfaces are exposed to aluminum phosphide fumigation. Could this aluminum fumigation be the cause of this white rust.

Please share your thoughts.

Regards
Sathish

Sathish Kumar
- United Arab Emirates


June 2014

wikipedia
Aluminum Phosphide

A. Hi Sathish. I found your question interesting, so I did some googling of aluminum phosphide fumigation and discovered that it evolves phosphine gas, which is highly corrosive to noble metals, but maybe not to galvanizing. I also learned that the "wheat pills" used for the process often contain chemicals which evolve ammonia to limit the explosivity of the phosphine gas. So thanks for the interesting question. But I have no real clue as to the answer :-(

Hopefully someone experienced with galvanized silos will know right offhand the effect of the phosphide fumigants on it.

Regards,

Ted Mooney, finishing.com
Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey


July 3, 2014

A. This fumigation has NO EFFECT on galvanized surface.

Dalela, Umesh Chandra
- Abu Dhabi, UAE


July 3, 2014

A. Sir:

I presume the fumigation is done in a sea bound container? Aluminum Phosphide in wheat pills is used here on the prairie to kill prairie dogs. As I remember one puts the pills into the hole, covers with dirt, and then adds water. Thus in a container one may put the pills in a dish, adds water and close the container. Take note that phosgene is extremely toxic.

Under proper conditions (e.g. moisture/sea salt), I certainly expect much white rust. I once had a case of extreme white rust in a container which contained tartaric acid (perhaps from a previous shipment).

I do not know if phosgene alone may or may not cause white rust.

Regards,

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



November 25, 2015

Q. We are facing issues of white rust on the parts once they reach the customer overseas.

I am attaching some pictures of parts that have observed white rust once they are received by our customer in the US.

25997-1a  25997-1c  25997-1b  25997-1d 

These parts are put in a box, the boxes are then placed in a pallet and the pallet is stretch wrapped and sealed. How can this occur and how can we address this?

Larger parts are directly placed in pallets and then stretch wrapped.

Rishabh Maheshwari
- Delhi, India


simultaneous November 28, 2015

A. The image shows classic white rust, sometimes also called "wet storage stain". The clues are in that second name.
To avoid this staining it is necessary to keep the galvanizing dry, and to store it so that no one part touches another. The white stuff is carbonates and oxides of zinc. It is not faulty galvanizing, rather a problem with fresh galvanizing being stored inappropriately.
It can be reduced by passivating the steel after galvanizing, using chromate passivation. This can be done hot or cold. It often gives 3-6 weeks protection. There is a company in India making such a passivation product.
If you cannot passivate (and some places cannot use Cr6+ chemicals due to local legislation), then the way you stored the steel is the only other way to prevent the problem.
Note: this problem is aesthetic only. It has no detrimental effect on the protection the galvanizing gives.

Geoff Crowley
Geoff Crowley
galvanizing & powder coating shop
Glasgow, Scotland



December 2, 2015

A. First of all, what you are seeing is white rust caused by water, almost certainly in a confined system. It can be removed by a weak acid or a very weak solution of a strong acid, followed by rinsing. Naval Jelly is used frequently for such applications. It is phosphoric acid-based, so it leaves a thin film of zinc phosphate on the surface.

It has always surprised me that hot-dip galvanizers seldom use any post-galvanizing passivate. But that seems to be the way most hot-dip galvanizers do it, and you see the result.

The classical post-galvanizing treatment is described by Neish in U.S. Patent 2,665,232 (you can get it from Google Patents), which describes a solution of an alkali metal silicate and an alkali metal chromate or dichromate.

Today, with the EU requiring so-called 'trivalent passivates,' there may be other (trivalent) alternatives at a higher cost. There is no technical reason that I know of why they would not work.

tom_rochester
Tom Rochester
Plating Systems & Technologies, Inc.  

Jackson, Michigan, USA



December 3, 2015

A. It's not so much that the EU require trivalent chromates, they seem intent to ban all chromates, from 2017.

We have tried many non-chrome passivates and not found one that works yet. Our test is simple. Freshly galvanize some test plates, about 4-5 for each chemical under test. Passivate with the material being tested. Now stack the plates flat to flat, in a pile and leave this out in the weather for a couple of weeks. It must rain several times (not hard here in Scotland), and dry at least once.

Some tested so-called passivates make a worse job than water.

Geoff Crowley
Geoff Crowley
galvanizing & powder coating shop
Glasgow, Scotland




February 25, 2016 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. What should be added in galvanizing kettle with lead to prevent or lessen white rust formation? We are facing problem of rapid white rust after galvanization.

Armaghan Rauf
- Lahore, Pakistan


thumbs up signHi Armaghan. As you see, we added your query to a long and detailed thread on the subject. Please review it and then try to cast any remaining questions in terms of what has been offered. Thanks!

Regards,

pic of Ted Mooney
Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey
Striving to live "Aloha"



July 19, 2017


Q. For Geoff Crowley

We are looking at non-chromate quenches as well and were wondering if you had any updated information or results on viable alternatives.

Angelo Palucci
Corbec - Lachine, QC, Canada


July 2017

thumbs up sign Hi cousin Angelo. Please don't turn the conversation "one way" because it tends to scare away those 1 in 20 readers like Geoff who answer questions instead of just asking them. Please also tell us what you have been studying & doing, and what if anything you've learned so far. Thanks!

Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey
Striving to live "Aloha"


sidebar2 July 21, 2017

Ted, I am afraid I am strictly here seeking knowledge. I am new to the industry. I know our technical department is working on trying to eliminate chromates from the quench process. I am looking for information that can help them in terms of alternative materials. I am not sure if this is the right forum for that. Hopefully someone can point me in the right direction.

Angelo Palucci
Corbec - Lachine, QC, Canada


July 2017

thumbs up sign Hi again, Angelo. It's the right forum, just a slightly imperfect approach to how public forums work best :-)

What we'd prefer to see is your technical department sharing in the exchange of info: "we've read that ...", "we've found an article that claims ...", "we tried ... "

Not trying to single you out, sorry; countless readers do the same, and I've done it myself on software forums! I just took this opportunity to explain how the forum would ideally work because I didn't want Geoff to feel pressured by being addressed by name :-)

Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey
Striving to live "Aloha"


July 23, 2017

A. Sir:

At one time I thought that calcium chloride in the flux (from hard water, or contaminated flux) was a cause of white rust. Now I am investigating aluminum chloride in the flux as a source of white rust.

To reduce white rust, I recommend a metallic aluminum level in the molten zinc to be no higher than 0.0005%.

Regards,

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

ADD a Q or A to THIS thread START a NEW THREADView This Week's HOT TOPICS

adv. pointer    

Disclaimer: It is 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; 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


©1995-2017 finishing.com     -    Privacy Policy
How Google uses data when you visit this site.