What Exactly is the Heat Stain on Stainless?
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 a successful passivation? Thank you.Tom Greifenkamp
- Cincinnati, Ohio
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.
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
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
[linked by editor to product info at Amazon], "Blue Away" [linked by editor to product info at Amazon], SimiChrome
[linked by editor to product info at Amazon]. Check your local hardware store, or motorcycle accessory shop. They typically carry products to get rid of bluing on exhaust pipes.
- 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 degrees since a google search said melting was 1510 degrees. 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
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 [linked by editor to product info at 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.
- Albany, New York
May 12, 2012
A. OUCH! Got lost in all the emotion of the event and really feel for your pain.
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.
REMEMBER..Scratch-B-Gone is your friend here! Regards
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.
Stellar Solutions, Inc.
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.
James Kelsey Studios - Centralia, Washington, USA
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