I am forging 304 stainless steel. It has a hammer textured surface and want to remove the blackened surface but not remove the hammer marks with grinding. My goal is to remove the blackened surface and then wire wheel to brighten the surface. I soaked it overnight in a citric acid bath. The solution is a concentration of 1 pound citric acid/ gallon of water. There was no discernable change in the blackened surface. I did get some cleaning benefit using BarKeepersFriend (Oxalic Acid). Any suggestions? 

I know boiling copper in vinegar does the trick when I am working it not sure if it would have the same effect on stainless.

I would try muriatic acid available at most hardware stores.  Dilute it some (add acid to water) and don't breath the fumes.  Also do it outside to avoids from rusting everything in your shop - it won't freeze.  Neutralize with baking soda.

Greetings Steven,

       All the chemistry is correct. When you wire brush don’t use a steel brush only a stainless brush . Steel will transfer into the metal and will later rust and leave black marks. 

Forge on and make beautiful things 

Jim

look into electro polishing of stainless.

Sorry if I'm going against any grain here;  I just looked up the data sheet on 304L stainless at Crucible online.  There is no mention of any black oxide forming metals on their data.  So my next thought was:  "What in my shop causes black coatings most often?"  Answer:  any organic, carbon-based compounds like acetone, gasoline, linseed oil, etc.  Or any activity that resulted in soot or carbon-black from something burned.  If I wanted to remove anything like that, I'd use an organic solvent, like acetone,  95% rubbing alcohol, high-proofed vodka, etc.  Pure carbon coatings can be really stubborn to dissolve, but I wouldn't heat any aforementioned liquids.  They really love to burn and with a totally colorless flame.

The heated vinegar idea might work too, because it contains some acetic acid, and doesn't burn.  In that case, formic acid might even work better;  don't know where to get that, though.

They don't mention metals in the alloy that will turn black when heated to forging temps? Might I suggest you don't know your metals nor high temp chemistry very well? ANY metal will burn if you get it hot enough in the presence of oxy. Black scale is the ash. So to speak.

Electro polish is the easiest and better method for preserving texture while reverting the oxide layers to clean metals.

Frosty The Lucky.

Please educate me in metals and high temp chemistry, as practiced in small shop forges.  My only experience is in making 400 ton batches of "bottle-top" steel in Open Hearths.  1000lbs of Manganese, 400lbs of silicon, 400lbs of magnesium, 400lbs of chromium, 1000lbs of powdered coke.  No, Please, don't let it be a Sulphur heat!

As you say, metals burn in the presence of oxygen; does that mean that there's also a presence of free carbon?   Do your propane or natural gas forges ever use reducing flames?  I've heard flame adjustment mentioned before.

Have you or anyone else here at IFI had laboratory chemical analysis done on the black scale from forging?  Could this mill-scale be metallic carbides as well, from the forge burner?

Have you or anyone else here seen the lack of significant "mill-scale" coming out of your electric heat-treating ovens?  

why do my posts now show up in technicolor?  they didn't used to when I first joined the forum.  This browser doesn't do that on any of the other 3 forums I belong to.

Your posts show up normal to me. Might check the underlined A in the editor next to size. That lets you select a different color.

or maybe because your posts are being moderated and they show up to you as the poster highlighted until they are approved

15 hours ago, eutrophicated1 said:

metals burn in the presence of oxygen; does that mean that there's also a presence of free carbon? 

Carbon isn't the only thing that burns. Burning is an exothermic reaction between a fuel and an oxidant -- when metal burns (for example, when overheated in the forge), the metal itself is the fuel. 

15 hours ago, eutrophicated1 said:

Have you or anyone else here at IFI had laboratory chemical analysis done on the black scale from forging?

Yes, it's a mixture of iron(II) oxide (FeO), iron(III) oxide (Fe₂O₃), and iron(II,III) oxide (Fe₃O₄, magnetite).

15 hours ago, eutrophicated1 said:

Have you or anyone else here seen the lack of significant "mill-scale" coming out of your electric heat-treating ovens? 

Scale starts to form at (very!) roughly the same temperature as the critical temperature of steel, which is why many bladesmiths wrap their blades in stainless steel heat treating foil to prevent oxidation. However, scale doesn't fall off the workpiece on its own until about 1750F, much higher than the temperatures in a heat-treating oven.

Yes and it seems to be the standard "mill scale" as reported by the ASM Handbook.  Note that you get scale formation using an induction furnace where there is not carbon involved just iron and oxygen.  Even when you are using 4 0 steel  .0000X % C

I've also gone backwards and made metal from scale by smelting it---hard to do if there isn't iron in it in considerable amounts!

Thank you all for your good and detailed responses.    I just had one more question about this issue(typing furiously to get this all down):  as a welder in a former life I sometimes used a carburizing or reducing acetyene torch mixture when brazing pieces together; has anyone here done that with their forging furnaces while heating up various steels to press or pound on?

Edited February 12, 20197 yr by eutrophicated1
More clarity of speaking

putting the stainless in muriatic resulted in a grey discoloration over the whole surface. I was able to remove it mechanically. Anybody understand what is going on? Not clear if the muriatic is creating new staining or is that the result of iron particles being etched by the muriatic?

5 hours ago, Steven Bronstein said:

putting the stainless in muriatic resulted in a grey discoloration over the whole surface. I was able to remove it mechanically. Anybody understand what is going on? Not clear if the muriatic is creating new staining or is that the result of iron particles being etched by the muriatic?

Use Citrus cleaner. A little low voltage DC and you're electro polishing. 

Frosty The Lucky.

18 hours ago, Frosty said:

Use Citrus cleaner. A little low voltage DC and you're electro polishing. 

Frosty The Lucky.

Thanks Frosty, I will be putting together a low voltage unit so i can avoid all of these issues.

 Do you understand what the cause of the darkening in muriatic. I thought it would eat away the free iron leaving the surface brighter , not darker. thanks.

Sorry, no. I've never used anything to passivate SS but chemicals designed for the job or citric acid cleaner.

Frosty The Lucky.

If it was me I would bead blast or wire wheel the scale off, then do a passivation to get the non staining properties back. We sent items out to be passivated to customer specs. Some requested citric acid, others nitric acid.