Corten Steel -- Q&A, Applications, Problems

A. Hi Christian. Corten weathering steel involves a small amount of alloying metals to help the rust be more adherent. There are no toxic off-gasses or corrosion products that I've ever heard of, and the heat shields should not get very hot (don't fasten them tight against a plaster or wood wall, mount them on standoffs an inch away from the wall and an inch or more off the floor to allow airspace between them and the wall so they function as radiators and the heat is transferred mostly to the air rather than the wall).

Regards,

A. A continuous repeated application of a simple acidic solution, lemon juice and water if you like, will cover up the handprints in a matter of weeks ,be sure to allow to dry between apps). However, you might find that the treated area appears differently colored in the long-term. I personally, would simply keep the area damp then dry, damp then dry, and so on until the affected area catches back up with the surrounding. Until then, it will just have to make for good conversation.
Good Luck!

Hello Berkley Choate. I'd really like to dialog with you about Por-15 clear coat since I have a corten project I need to seal. Please reply to this message if you are still active here. Thank you.

A. Hi Limay. Corten lasts longer than other steel because its rust is more stable and less prone to powdering off. Iron makes up 6% of the earth's crust, so a little rust washing off of things sounds pretty harmless.

However, if the bridge is over salt water you might want to investigate Corten's resistance to salt water first.

Regards,

A. Hi Robert. Because weathering steel is similar to other steels (about 1/2% of chrome and copper added though), I don't see any reason it shouldn't be stampable.

By this point in your education, you probably realize that the best way to learn is by trying to teach! So, as a Masters' student working with Corten, please try to answer one of the many unanswered questions on this thread. Thanks!

Regards,

A. I am also looking at using bolts to join Corten and I don't think you can get these made of corten. So I am going to either paint the heads or get some that will rust. They won't last as long as corten but I think they will do the job. By the way I only live up the road from Limerick. What a coincidence!

Weathering Steel

A. Hi Kristina. I'm certainly no expert on weathering steel, but it seems to be 97%+ iron, with about 2.5 to 3% combined manganese, copper, chrome, and nickel. In other words, it's not terribly different from hot rolled low carbon steel, just better at producing a stable corrosion product to slow the continued rusting.

Sheets of steel would not seem to me to be a good public walking surface, as they might be slick, although probably fine as some sort of perforated or expanded metal grating. But if you do a Google image search for "Corten as walking surface", I think you'll find many examples that you can further investigate. Good luck.

Regards,

A. Hi Carran. Life is complicated, and oftentimes we must take our best guess. I doubt that the many people on this page who are complaining of Corten staining are lying, but I'll bet also that your builder has some local and relevant experience with it.

Your town is well away from the ocean, thus minimizing the problem. Your painted siding is a relatively slick surface compared to porous concrete, and it will be exposed to that same "terrible sideways rain", helping to wash down any rust. My guess is that your builder is not talking through his hat, but has found that it is unlikely to be a serious staining problem for you in this particular application.

Regards,

A. Hi Cristi. Many metals can last a long time out in the world, not because the metal itself is corrosion resistant, but because its corrosion products tend to build into a somewhat impervious coating that tends to cover the surface and thus limit water and air from reaching the raw metal, and thereby slow down the corrosion. This is true of aluminum, zinc, copper, stainless steel, etc.

Steel does not enjoy this sort of corrosion resistance benefit because its corrosion product, rust, continues to slough right off, and the surface is hygroscopic, holding on to water and causing continuing corrosion.

But steel is much much cheaper than other construction materials on a strength for strength basis. The intent of Corten steel, as I understand it, was simply to formulate an inexpensive steel that is somewhat more resistant to deterioration than regular steel by virtue of a tighter-adhering, more impervious rusted surface.

Corten is a structural material, it's not a decorative finish, so it can be optimistic to expect a stain-free highly decorative appearance. Sometimes it's just hope for the best. The stains on your sign will probably not go away, and the most consistent results will probably be had by sodablasting [soda blaster on eBay or Amazon [affil links] the surface and then simply allowing it to rust. Good luck.

Regards,

A. Hi Alex. Cost wise, steel painted with stove and BBQ paint ⇦this on eBay or Amazon [affil links] would be the way to go, but if you want the raw steel look, the bowl will probably not have a fire in it 99% of the time, so I think Corten might be worth it.

Luck & Regards,


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

A. To the people questioning about streaking, mottled or inconsistent oxidizing patterns. It is in my opinion (whatever that's worth) that it is due to not blasting and/or solvent cleaning the raw plate surface before letting the surface oxide to form. During the sheet manufacturing (rolling at the mill) the surface layer may have inconsistent scale and/or oils on the surface. These of course, one would expect, need to be removed consistently to allow the climate (or any patina enhancer/accelerant)to effect the surface uniformly.

To those questioning about finger prints/hand prints etc. These of course are likely due to acids and oils from ones hands again interrupting a uniform condition on the surface. I doubt there is an easy answer other than blasting or otherwise etching the surface. If it is going to be touched, a protective coating of some type of oil or wax (as mentioned by others) should be applied.

Thanks everyone who has shared their experiences and problems with Corten weathering steel. I am currently researching the use of the material for a farm gate sign and some landscape pieces.

Rust patina kits

on eBay or

Amazon

(affil links)

A. Hi Karen. Sorry for your disappointment! Unfortunately I think you will get the same result no matter what clearcoat you apply -- because the problem isn't that the clearcoat was tinted and lacking in clarity. The problem is that the lighter, diffuse, color you had achieved before clearcoating was the result of the surface topography: a field packed with miniature/microscopic rust boulders scattering the light in every direction. When you submerge those boulders in a clearcoat the look is different (if you've ever collected sea glass or beach pebbles you'll notice that they are dull and matte-looking when dry but bright and shiny as long as they're in water).

You might try a paint specially made to look like Corten, or see if a rust colored patinating solution ⇨
gives you the look you seek (those approaches have at least two parts: first, apply the metallic surfacer with the iron particles in it , then follow that with the rust activator, but usually an acid-blocking primer first, and sometimes a clearcoat after.

Baking soda [in bulk on eBay or Amazon [affil links] is a neutralizing solution for acids and it doesn't matter how strong or dilute you make it as long as there is enough baking soda in total to neutralize however much acid was left on it … and it was probably very little; I'd be pretty confident that half of a small box in a half gallon of water would be enough.

Regards,

A. Hi Karen. To the degree possible, we try to suggest types of finishes not brand names (why?)

The manufacturers of super-quality chandeliers spray six or even more layers of very highly thinned clearcoats, so the secret to smoothness is multiple highly thinned layers, whether brushed or rolled or sprayed.

The only thing I can suggest at this point is to get another piece of Cor-Ten steel and do your experiments on a small piece, or multiple experiments each in a small area of a big piece. Conjecture gets us only so far, and you don't want to keep instantly ruining your carefully finished piece with experiments, but I have tried to explain why clear-coated rust won't look like bare rust. Best of luck.

Regards,

A. Hi again. It is the baking soda [in bulk on eBay or Amazon [affil links] , not the water it is dissolved in, which reacts with and neutralizes the acid. So how you neutralize the small amount of remaining acid with baking soda probably doesn't matter much. But whichever way, neutralization creates carbon dioxide gas; so if you see fizzing, the acid is not quite neutralized yet. The baking soda should be rinsed off before doing the clear coating, and probably dried.

I don't have any real experience in trying this 'smoothing' idea, and you can see on the thread that others have difficulty getting a uniform finish. I suppose you could try a light steel wooling with a very fine grade (#0000 steel wool ⇦this on eBay or Amazon [affil links] ). As for limiting the streaking when your treatment chemical dries, maybe try a little dish detergent in it to make it wetter.

Regards,

A. If you want to preserve colour of Corten patina you must use very diluted Paraloid B 44 ⇦[paraloid on eBay affil link] clearcoat (1,5 - 2 % = 15 - 20 grams on 1 lit toluene or acetone ⇦this on eBay or Amazon [affil links] Warning! highly Flammable! ). Very diluted 'water glass' ⇦this on eBay or Amazon [affil links] can be an option too. Some testing on small corten plates can be helpful. Hope it helps and good luck!

Comment: I am a sheet metal fabricator who has only done a couple of jobs with Weathering Steel to date so I am not an authority. I have spent much time looking at jobs done with this steel, however, and I will say that if you weld it you need to sand your welds up to a good finish if you want a flat face without gouges in it. Finish them with an orbital sander. It is also critical that you use a compatible welding electrode of the same composition as the parent metal. After that sandblast it back to a matte finish all over. This is very important as you need to start with a uniform matt finish right across the job or piece you are making. All of the mill-scale needs to be removed otherwise it will form a patina with black patches in it. These will vary depending on the thickness of the layer of mill-scale. Ultimately it will get a uniform colouring after about 2 to 3 years if you don't do this. But you can achieve that uniformity of finish sooner if you start with a sandblasted matte finish at the beginning.

As to bleeding or leaching it is very very minor. I have my garden fence on white gravel and there is little to no colouration on the stones. It is nothing like you will get with Mild Steel.

It will get marks on it from bird excrement and if you try to wash this off it will leave scrub marks, where you cleaned it, which are very discernible. Leave it alone.

I have seen examples of people trying to speed the patination process up with chemicals but they generally look hideous. Especially if you are not a professional with much experience. They always have streak marks to varying degrees. My comment would be to just leave it alone and let it colour up at its own rate. Period!

A. Hi Ryan. On a drive just this week I was noting that a disadvantage of Corten guardrails is that bad rust stains were running down the curbs and concrete roads even without friction and rubbing. The Corten itself looks okay, and if the guard rails are just over earth, probably okay; but the rust stains on the concrete are ugly. This shouldn't have been a big surprise to me as the brown coloration is, of course, rust. Some people experience very little rust staining, and I'm not sure why those guardrails seemed so prone to it.

There are paints which are designed to look like Corten and I think they'd be a better bet for a bench. But you can try a thin single-component clearcoat on your Corten, perhaps as Goran suggests, although I think it will be at best only partially satisfactory. Good luck.

Regards,

A. Hi Alex. I may not be understanding what you are trying to do when you say 'fireplace wall'. If they are strictly decorative pieces mounted on a wall near a brick fireplace and intended as artwork, you can probably attach them either with glue or screws.

But if you are using them as some sort of heat shield, such as behind a wood burning stove, you want them to return the radiant heat which they absorb back into the room, not pass it on to the plasterboard behind it. To do that you need to use some sort of 'standoffs' to create an inch or so gap between the sheet & the wall, and the sheet & the floor so air will cool both sides of the sheet, and the wall.

Luck & Regards,

A. www.richard-brink.de/fileadmin/user_upload/download/pdfs/Materialhinweise/RB_Materialhinweise_Corten_E.pdf
Hope it helps and good luck!

A. Cleaning and caring for COR-TEN steel products:

"Once the patina layer is formed, COR-TEN steel is extremely low maintenance. Nevertheless, we recommend cleaning the surface regularly and checking it for any changes (once or twice a year).

• Do not using cleaning agents to clean the material. It is generally sufficient to lightly brush the surface in order to remove soiling.
• Never use cleaning agents containing chlorine.
• Use non-ferrous cleaning tools; never touch the surfaces of the material with non-rust-proof tools (scraper, file, etc.), ferrous sponges or non-rust-proof steel wool.
• Remove protective films early, as otherwise the film and steel surface may bond. Remove adhesive residues with spirit. Warming it up slightly makes removal easier.
• Dissolve scale and grout spots with phosphoric acid and rinse with clear water. Never use cement film remover or diluted hydrochloric acid.
• Avoid strong heat transfer to the material (e.g. from working with an angle grinder, drilling without emulsion, or from plasma cutting, the effects of fire, etc.)
• Do not allow sparks to touch COR-TEN steel surfaces.
• Freshly welded areas must be suitably cured.
• Ferrous particles must be removed immediately, as they can penetrate the passive layer of the COR-TEN steel and cause corrosion. Ferrite-free cleaning sponges or similar can be used for this purpose. If corrosion has already occurred, a pickling treatment may be necessary.
• Remove surface rust immediately.
• Avoid contact with salt.
• Oil and grease can be removed with organic cleaning agents. All cleaning agent residues must be removed.
• Residual paint can be removed with an alkaline or solvent-based cleaning agent. All cleaning agent residues must be removed.
• Avoid extraneous rust:
- Work on steel and COR-TEN steel in physically separate areas.
- Use separate tools for COR-TEN steel or ensure that tools are completely free of ferrous particles.
- Do not use transport equipment or hoisting equipment made of steel (forklift etc.), or if such equipment is used, protect the COR-TEN steel products accordingly using protective covers."
(According to previous link!)