Heat-produced Patinas on Stainless Steel and Chrome

A. Wow.
Why did the bolts darken?

Most likely an oxidation reaction.

After heating the stainless steel or chrome plated bolts, is the surface just as hard?

probably not, although the underlying bolt strength shouldn't be greatly affected by that temperature.

Can I expect that at some point the color will stop changing and then remain the same (no oxidation)?

It will depend upon the grade of stainless (amount of nickel & chromium in the alloy.)

Is there a medium (the Incas (or Aztecs) used KNO3 (saltpeter) heated to its melting point to produce a beautiful turquoise patina on steel) I should use?

Since neither the Inca nor the Aztecs used iron, much less steel, I'm not sure about this information....

How hot do I need to heat chrome plate and stainless steel to get yellow and blue?

Sorry, don't know.

Is there a better way to have a hard surface with a metal yellow and a blue that won't oxidize or change color?

There is a technique for anodizing stainless--used to be a company in Colorado that used it for making signs, I do not know what the process is--but it must be out there, google is your friend.

Depending upon how "hard" the surface needs to be, you could also use titanium bolts, which can be surface colored to a dozen shades, everything except red.

This process is highly controllable since it can be done using controlled voltages--each voltage will represent a color. You can color different parts of pieces this way--even put pictures on the surfaces.

But while the surface will stand up to handling, it's likely that any burr or scratch on a wrench would scratch through the oxide. That said, it could be done in situ.

These colors are stable unless voltage or heat is applied to the surface. The oxide must be physically removed in order to start the process.

A. Old book on metal colouring by D.Fishlock (D.Fishlock: Metal Colouring, Teddington 1962.) can be very good source of infos.

Try next webpage www.bssa.org.uk/topics.php?article=140 -very good article on heat tinting of stainless steel. Hope it helps and good luck!

A. Color can be as much from if the torch was an oxidizing flame or a reducing flame. If you are using a furnace, the color will vary with the amount of oxygen present and the rate of cooling.
In short, trial and error on scrap.

A. You can get anywhere from a light tan to a light blue to a dark blue by heating panes in an open air furnace. Not sure of what would be the best temp for your desires. Not many places can take a 4 x 8 sheet, but you could tack weld or pin the ends to form a cylinder that would be 4' high by less than 3' in diameter.
A "dirty" part will have stronger colors. This would take some trials to sort out what would be best. You might use 1' panels and piggy back them with someone else parts in the furnace.
The color is mostly oxides of nickel and or other elements in the alloy. It takes a harsh acid treatment to remove it on aircraft parts, so I will guess that it should be long lasting outside.

Torch work could produce some wild patterns.

A. Download free booklet on coloring and plating of metals, including heat tinting of stainless steel :
attachments.wetpaintserv.us/TfuRNJ_GLYiy-a_0jvIMxA483937

Ed. note Jan 2017: Although wetpaintserv.us was a legitimate site when Goran posted this, it is currently a malware site. DON'T GO THERE!! Goran has been kind enough to upload his booklet to https://www.finishing.com/library/budija/budija.pdf.

Q. Did you ever get a suitable answer to this? I want to do a heat patina on a piece of public art and have the exact same question. Furthermore, once the patina is on there, does it need some sort of top coat to keep the color...

A. Hi Chukk. You can google for info and case studies of buildings made of colored stainless steel, and I think you will find that if the coloration is done electrochemically and done well it can be rather durable.

But my suspicion is that some, and maybe a lot, of the coloration applied by a torch is not a true change in the structure of the stainless, or even a durable translucent oxide coating, but oil residues which won't prove very permanent.

I doubt that you'll find any info in finishing or architectural journals. So, unless you can find some info in artists' magazines, I would suggest doing some scrap pieces and then trying to remove the coloration with detergent and very mild abrasives, and trying to acquire a feel for its durability. Best of luck.

Regards,

A. Hi kris,

From what I know about titanium, these colorations that you see are due to thin film optics.

As you flame the surface, a thin (COLORLESS) oxide layer is formed, now due to the interference of light (because of this layer) it shows a color.

Different film thickness will give different colors, hence the longer you blow the torch the color changes accordingly.

Since its an oxide layer, it basically becomes heat anodizing, so it should help prevent corrosion.

regards,
vedant.

Hello VEDANT

Thank you so much! Titanium thin-film optics?, that's fascinating and - dare I say it? - Cool, in my opinion ;) I do know that, in the area of non-precious jewelry, numerous items, such as hematite, are often given a Titanium coat which gives the item a "rainbow" coloration, and I *had* wondered about that as well. And yet, as a pigment, Titanium White is about the purest, brightest white available.

I remember seeing something about using *chromium* oxide thin films as nano-resistors and using the principle of interference to measure the thickness (in nm) - I just think those little intersections of info are fascinating =:D

Well, I'll stop before I go off on who knows what tangents, but anyway, Thanks again! =:D

A. Kris,
Discoloring stainless with heat is similar to the blue/brown Heat Affected Zone (HAZ) discoloration seen on either side of a weld on stainless, where the alloy reached almost-but-not-quite the melting point. The HAZ is certainly more susceptible to corrosion than the areas away from the weld; however, it can be passivated to restore the corrosion resistance. Passivation with citric acid will not remove the color.

Thank you for that info, Ray! It's interesting that citric acid won't remove the color. I'll look into that some more, and give it a try.

A. Very good article about heat tinting stainless steel:
https://www.bssa.org.uk/topics.php?article=140

According to Russian sources, electrochemical and heat-obtained colours on stainless steel can by treated in potassium dichromate solution too (5-15 minutes, 5-10% solution, 70-90 °C). Cathodic treatment in 250 gms chromic acid/ 0,75 gms sulfuric acid can be used too (increases abrasion resistance - treatment duration 20 minutes).

Abrasion resistance can be improved with diluted 'water glass' ⇦ this on eBay or Amazon [affil links] (according to one expired patent).

Hope it helps and good luck./p> Goran Budija
- Zagreb,Croatia
May 18, 2020