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topic 8114

What Exactly is the Heat Stain on Stainless?



A discussion started in 2001 and continuing through 2020 so far.
Adding your Q. / A. or Comment will restore it to our busy Current Topics page

2001

Q. Curious ... when SS is ground, what is the brownish stain left? "Heat stain"? What is the composition, and does it need to be removed to have successful passivation? Thank you.

Tom Greifenkamp
- Cincinnati, Ohio


simultaneous 2001

A. The surface discoloration on stainless steel after grinding or welding is surface oxidation. At elevated temperature, the metal comprising the stainless steel will combine with oxygen to form an oxide layer on the metal surface. The color of the "stain" is a function of the layer thickness. Oxidation of stainless steel indicates that the metal temperature at the oxidized surface was in excess of 1000 °F.

This surface oxide must be removed for the surface to be passivated and reach optimum corrosion resistance. An oxide layer that you can see by color change is not a protective oxide layer that is obtained by chemical passivation.

In addition, if you are causing a brown stain by grinding, you may be doing more damage to the material than forming a little surface oxide. Heating the stainless steel to temperatures in excess 800 °F can cause metallurgical changes and reduce corrosion resistance in a manner that is not corrected by surface cleaning or chemical passivation.

larry hanke
Larry Hanke
Minneapolis, Minnesota


2001

A. I suspect the gold/brown you are seeing is iron oxide--you will see the same color if you roast the metal in air, with the oxidation increasing proportionate to the temperature and the time at temperature. Stainless normally has an atomic layer of iron and chromium oxide on the surface which protects the steel from corrosive attack--hence, it becomes "stainless"; the layer is thinner than the visible light wavelengths, and therefore appears clear. [to all of the purists: there is no color because the oxide layer is not thick enough for there to be an interaction between reflected and refracted light on and through the oxide layer] Heating the metal in air converts more of the surface to the oxide, the oxide layer becomes thicker, and the characteristic red-brown color of iron oxide is able to be detected visually.

Technically, this process which is "browning" the stainless is passivating the metal by ensuring an oxide layer at the surface. However, a quick dip in an HCl solution followed by a thorough rinse should remove the oxide "stain", providing it only has a slight "heat" discoloration. You will need to re-passivate after removing the surface oxide layer.

Dale Woika
- Bellefonte, Pennsylvania, USA



Sharpening stainless surgical instruments and the effect on passivation

2006

Q. I need insight on the effects of surface grinding as in sharpening surgical osteotomes on the passivation layer on the steel, and I know when scratched the layer will return, is this true for surface grinding. Also does blending out the grind marks with a less aggressive wheel and polishing with a buffing wheel help the passivation layer to return. Any help will be appreciated.

Clifford Russell
surgical instrument repair - Lake Elsinore, California, USA


2006

A. The passivation coating on stainless steel is due to the formation of chromium oxide and this will whenever the chromium is exposed to air. Using a fine honing wheel will give you a sharper edge as you will be removing the edge fluctuations caused by a coarse wheel, but, in theory, it will not affect the formation of the oxide. In practice it will give a better looking and sharper edge. Depending in the type of stainless steel you are using, you may start to work harden the edge and change its magnetic properties as you grind it.

trevor crichton
Trevor Crichton
R&D practical scientist
Chesham, Bucks, UK



To minimize search efforts and to offer multiple viewpoints, we combined previously separate threads onto this page. Please forgive any resultant repetition, failures of chronological order, or what may look like readers disrespecting previous responses -- those other responses may not have been on the page at the time :-)



2007

Q. Removal of heat stain on stainless steel and chrome. Once a heat stain is produced on a chrome surface, what it the best method for removal?

Donald Dobrow
golf club fitter - Huntington Beach, California, USA


January 21, 2011

A. There are several metal polishes out there that you can use to remove the heat bluing or stain. Brasso [affil. link to product info on Amazon], SimiChrome [affil. link to product info on Amazon]. Check your local hardware store, or motorcycle accessory shop. They typically carry products to get rid of bluing on exhaust pipes.

Good Luck,
Steve

Steve Sullivan
- St. Charles, Missouri, USA



April 21, 2012

Q. I found your information on stainless steel rusting. I have a question and need a simple answer if one exists. I have an outdoor kitchen with a grill that costs $5 thousand to replace. My wife left it on with the lid closed (it has searing plates that get VERY hot). Needless to say one end actually got red hot. We slowly cooled it putting the fire out over a period of 15 minutes using low volume of water since it was a grease fire. We went out to begin the process to try to clean it (the lid is warped and toast, but is available to buy from the USA made vendor). We found rust all over the inside? Why is this and can it be resolved? We were able to remove all the rust and brownish gold color with a mild liquid abrasive and lot of rubbing ... will it rust again? Will it need to be treated? Will it have to be replaced? I would guess it was 1100-1400 °F since a google search said melting was 1510 °F. And yes ... it was my wife that did it ... lol. God Bless her, she felt bad. Any advice would be awesome.

Jason Struble
- Muncie, Indiana, USA


May 4, 2012

A. I'm not a pro, just checking in for answers to my own question, but I have some experience with stainless, so maybe I can help.

First, there are many different alloys of stainless, with differing resistance to corrosion, but they'll all rust under the right conditions. Stainless is iron with other metals, usually nickel and chrome, added.

Anyway, as far as I know, oxides of chrome and nickel aren't brown, they're black. Iron oxide can be brown or red. So I think what you had was surface oxides of the iron in the stainless, and I think you did well in removing it with abrasion, because that won't hurt the metal. What's bad in stainless is black pitting, I know from experience, which can lead to holes all the way through thin sheets. I believe by polishing off the rust, you've exposed fresh metal, which will passivate (form a protective oxide coating) on its own, and be just fine.

The other stainless friendly cleaner is Barkeepers Friend [affil. link to product info on Amazon], which I believe is a mild abrasive and dilute oxalic (sp?) acid.

Hope that helps, and glad that the grill was *outside* when it had the near-meltdown.

Tom

Tom Smith
- Albany, New York


May 12, 2012

A. OUCH! Got lost in all the emotion of the event and really feel for your pain.

adv.
HOWEVER, Scratch-B-Gone has and will quickly rescue your damaged surface of your hood and remove all of the rust and discoloration.

Chromium in the Stainless helps to ward off the rust element of steel but under these extreme heats may have broken down. Another possibility is the manuf used 430SS which has more iron content at the outset and the rust is a result of air/pollution/water. See if it is magnetic and this will help with answer.

adv.
REMEMBER..Scratch-B-Gone is your friend here! Regards

Barry Feinman

Barry Feinman
BarrysRestoreItAll
supporting advertiser
Carlsbad, California
barrysrestoreitall


May 17, 2012

A. In the welding biz, with stainless steel you get something called the "heat affected zone" (HAZ). This is the area on either side of the weld where the metal was almost but not quite heated to its melting point. Actually what is occurring is some of the metal components in the alloy want to melt and others don't, so you have something that is sort of partway between liquid and solid. As this cools, depending on various factors, the metal atoms can move about and segregate themselves differently than the original evenly distributed arrangement they had before. In stainless steel, this means that the corrosion resistance imparted by the presence of chromium is taken away and the HAZ area needs to be passivated. It also tends to leave a blue or brown discoloration, which is composed of carbides that formed during the segregation. This discoloration can be removed using various methods, including abrasive products.

This is no doubt exactly what happened with your grill lid, except over a larger area than is seen with welds.

ray kremer
Ray Kremer
Stellar Solutions, Inc.
supporting advertiser
McHenry, Illinois
stellar solutions banner


June 6, 2016

A. In answer to the coloration after grinding stainless...
... There should be NO DISCOLORATION. Zero. If there is, you're not using the right grinding disks. Don't save money; buy disks made for SS. You'll be AMAZED at the difference. It cuts into it MUCH easier and won't overheat the metal. Overheating can lead to carbon precipitation and RUST.

wikipedia
James Kelsey
(sculptor)

You're Welcome.
James

James Kelsey (Sculptor for over 20 years)
James Kelsey Studios - Centralia, Washington, USA



October 21, 2020 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. Hello, my question is on grinding 403SS ^ 304SS. Can an edge be ground down say, a millimeter without affecting the SS properties? I know welding affects it, but grinding? Thank you for your response. Charles.

Charles Jonker
- Palm Desert California


October 2020

A. Hi Charles. We appended your question to a thread which explains that if you let the grinding cause a significant heat buildup, it will definitely affect the stainless properties. It seems that if you keep that from happening, and use wheels that correctly remove the metal rather than work-hardening it, you should be okay. Good luck.

Luck & Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET
finishing.com - Pine Beach, New Jersey
Aloha -- an idea worth spreading


October 23, 2020

Q. Thank you for your answer. I actually meant to say 304 SS not 403SS. Would there be a difference?

Charles Jonker [returning]
- Palm Desert California


October 2020

A. Hi again. I don't think so.

Luck & Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET
finishing.com - Pine Beach, New Jersey
Aloha -- an idea worth spreading

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