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Removing white spots / storage stain on galvanized metal


An ongoing discussion from 2002 through 2009 . . . you can bring it into 2016 and back onto our Hot Topics page by posting here.

(2002)

Q. Hi,

I am wondering if I have an arguing point with the galvaniser over white rust that resulted to black marks on the galvanized surface. As I recall from an article from finishing.com, white rust occurs when the zinc corrodes. And I believe it does rust on these black spots or marks. Right? The galvaniser is telling me that it's something they can't control. The black marks are giving the parts poor appearances.

black spots on galvanizing black spots on galvanizing2

Where I come from - Malaysia, many users are not at all initiated to understand coatings/finishing. Is grinding on galvanized parts to get rid of 'teardrops' from the galvanizing process acceptable/allowed?

I would be grateful if you could give me your opinion.

Thank you.

Regards,

Jeremy Yap
- Malaysia


Faults in Hot-Dip Galvanizing


Hot Dip Galvanizing

(2002)

A. From what I can tell of the pictures the bundles of material appear to be tightly packed and outside. If they are stored flat, any moisture would be trapped on and throughout the material. This causes a chain reaction that can attack the galvanized coating. If you have black areas it is probably bare. If you have areas that are white in color you can brush it away with a wire brush and check for readings with a magnetic gauge to see if it is bare steel. Bundles should be stored inside if possible, but at least stored at an angles in a bundle that is spaced in rows to allow any moisture to drain away from the material.

If you have spikes or "teardrops" they can be evenly filed away without effecting the coating. If you get down to bare metal you will need to repair it with a good zinc rich paint [linked by editor to product info].

Mike Stroia
Gregory Industries - Canton, Ohio


(2002)

A. You can ask your supplier to coat the galvanised sheets with some special rust preventive to prevent this white rust formation. I know a few products used successfully in galvanised products for controlling white rusts.

Indrajit Banerjee
- Kolkata, West Bengal, India


(2002)

A. Hello,

The black spot on galvanized surfaces is commonly called corrosion etching, apart from other factor, aluminum concentration needs to be controlled, drying condition has to be improved and use of PWG grade zinc will help to minimize the formation of black spot. use of certain percentage of tin--<1.3% and antimony < .05% will help to beat the problem of black spot.

Mahbub Hossain
Galco Steel(bd)LTD - Narayangonj, Dhaka, Bangladesh



Readers: To minimize searching & thrashing, and offer multiple viewpoints, we combined multiple threads into the dialog you are viewing. Please forgive any resultant repetition.



The effect of rough white spots on galvanized steel electric masts on durability

(2004)

Q. One contractor has delivered to the site (which is sea port site) a number of 30 m electric masts made of galvanized steel. These were stored near the sea raised on wooden platforms by half a meter. White rust has appeared. Some spots of the white rust areas are rough in texture whereas the other parts of the white rust areas are smooth like the rest of the galvanized steel areas. Measurement of the thickness of galvanization on all spots gave higher results than required . The thickness at the rough spots seems to be more than the rest of the white rust spots. I am concerned about the rough spots. The contractor as well as the material supplier and factory engineers suggest applying a rich zinc paint after cleaning with special chemicals. Please advise of the effectiveness of the proposed treatment.

Dr. Jamal Omari
Ministry of Municipal Affairs & Agric. - Doha, Qatar

A. This white rust is probably also what is known as "wet storage stain" and results from damp parts not getting enough fresh air to form corrosion-resistant corrosion products. While I can't say if these parts are satisfactory or not, I can note that wet storage stain usually isn't nearly as serious as it looks because the corrosion products are so voluminous compared to the amount of zinc that was actually consumed to generate them.

If you search this site or galvanizeit.org for "wet storage stain" you'll find some further input.

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



White Rust treatment

(2005)

Q. I have a lot of galvanized angles, shipped from North America to Indonesia. All of them have White Rust on them. I need to paint them with an Acrylic Latex and am looking for information on surface prep. I have tried pressure washing, Scotch Brite pads with water, wire brushing . . I am wondering - once the white powder is removed is the White Rust process stopped - or will it continue on, under the paint?. . thanks.

Kermit Severson
consumer - Surabaya, Indonesia


(2005)

A. Painting hot dip galvanized steel is not an easy task and I have no direct personal experience. A friend in South Africa had a truckload of galvanized pipes that white rusted and he effectively used dilute acetic acid (vinegar) to clean them. In Northeastern USA a company routinely paints FRESHLY galvanized product and has a trade name for this product. I doubt they would share their technology. In the proceedings of The International Galvanizing Conferences (a series of books) there are several technical articles on painting galvanized products. Also the series of books "Zinc Abstracts" there are likely references to painting and pretreating galvanized steel for painting. There may be a limited number of paint companies with knowledge of pretreating and painting galvanized product.
Regards,

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


(2005)

A. PREPARATION OF GALVANIZED SURFACE IS NOT AT ALL DIFFICULT; CHEMICALS FOR REMOVING SUPERFICIAL AND CHEMICAL IMPURITIES ON THE GALVANIZED SURFACE, AND FOR FORMING ZINC PHOSPHATE COATING ON THE SURFACE ARE AVAILABLE: SPRAY PROCESS OR DIP PROCESS.

S. MEENAKSHISUSNDARAM
- CHENNAI, INDIA


(2007)

A. I have achieved a measure of success at my seaside home using a locally produced product made by a local company called Plascon. they sell a galvanized iron cleaner that is scrubbed on using scotch brite pads, rinse well with clean water and allow to dry. Prime with a coat of their yellow galvanized iron primer. Then two coats of Wall and All. The two latter products are water based. I used this treatment on new iron railings around my property about 4 years ago and they are still looking good. I have no connection whatsoever with Plascon company.

Peter Flicker
Retired - Cape Town, South Africa



(2006)

Q. If I use white passivation and white rust still formed where is the problem and bad storage can lose the passivation effect, if white rust formed on pipes how could I remove it easily?

Mouhmad Hammad
asp - Jordan


(2006)

A. Sir:
About 20 years ago an industrial friend in South Africa told me how he cleaned a truckload of white rusted hot dip galvanized pipes using vinegar (5% acetic acid). He said it worked well.
Regards, Dr. Thomas H. Cook

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



Readers: To minimize searching & thrashing, and offer multiple viewpoints, we combined multiple threads into the dialog you are viewing. Please forgive any resultant repetition.



White rust and black marks on hot dipped galvanized

(2007)

Q. I am also interested in finding about the black spots appearing on galvanised steel; however, I have not been able to find much. My initial guess was that it is Fe3O4, however I'm not fully satisfied. If anyone could help me by telling the reason for black spots appearing and how can it be reduced I would be glad.

PRASHANT KUMAR SINGH
student - Roorkee, Utarranchal, INDIA



(2007)

Q. I have some galvanized sheet metal that has white rust spots on the surface. They have been stored outside for at least 12 months. Is there a way to remove the white rust spots and leave the galvanized surface. I have a phosphoric acid pickle bath, but I am nervous about removing the galvanized layer. Any help would be appreciated.

Adam Adkisson
Operations - custom coater - McAllen, Texas


(2007)

A. White rust forms on unpassivated galvanized surfaces, particularly where they are touching face to face and wet. Its sometimes called "wet storage stain".

Its mainly zinc carbonate, and it a white powdery deposit. It can be brushed off with a stiff hard-bristle brush (nylon bristles or similar). It severe cases, you could use a fine grade of sandpaper, but this will produce an alternate surface finish inconsistency, unless you treat the whole surface in the same way.
Using acid runs the risk of wrong strength, stripping the zinc, leaving acid behind in crevices, etc.

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



(2007)

A. Sir:
A friend in South Africa had a load of white rusted galvanized pipes. He cleaned them in acetic acid. I think he used a few percent (perhaps 2% to 5%). This would be similar to vinegar. He then washed them in clean water and allowed them to air dry. He said the results were good.
Another friend used some sort of automobile tire cleaner to clean off some white rusted galvanized products. It took some labor but the claimed results were good.
Regards,

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


January 5, 2009

Q. Dear Sir,
we are facing problem of wet storage stain even in the month of March & April ,which are dry months in India. I think it has nothing to take with humidity or anything.The problem looks to be different. Can you suggest something on that.
Regards,

Rajesh Mishra
- Vadodara, Gujrat, India


June 19, 2009

Q. I recently got a batch of surplus galvanized metal roofing...then found a significant amount of white powdery stuff sticking on the stacked sheets. I assume the whole stack had gotten wet and stayed wet.
Any recommendations as to whether this white stuff should come off if I don't plan to paint right away? Weather is mild and we're away from the coast, so these roofs last many years without paint...but I'm worried now that these panels have the white stuff on major portions (front and back).
Thanks in advance for your help.
Sam

Sam Wonderly
- Merced, California, USA



White rust on galvanised railings at a waterfront house

January 20, 2017

Q. Reading some of the postings leaves me wondering whether one should paint galvanized, or just leave it as is.
If left unpainted, will the galvanising eventually fail?

Or is it just a matter of re cleaning white rust off occasionally?

anthony roger toomey
retired and a gardener - oyster bay, Australia


Latex Self-Etching Primer

January 2017

A. Hi anthony. Everything will eventually fail of course, but galvanizing, even in salt air, should offer a pretty good life. Painting it may be problematic but you can try a primer made for galvanized metal.

Regards,

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


January 23, 2017

A. Bridges exposed to salt water usually have galvanized steel which is then painted with at least one coat of paint and usually two. That would probably be the best practice. I'd recommend a solvent based paint as the water in water based paint reacts with zinc.

BTW, along the same line. My mother lived in a seaside house which had a galvanized gate. Where people handled the gate to open and close it the oils on peoples hands gave the galvanization longer life. (In non-salty environments, the opposite is true, as the salt from peoples hands accelerates the corrosion.)

tom_rochester
Tom Rochester
Plating Systems & Technologies, Inc.  

Jackson, Michigan, USA


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