Will Ferric acid solution take away carbon?

Mark Barone

Mark Barone

Well-Known Member
I probably already know the answer but I’ll ask anyway. So I grind a bevel and the flat is very clean and smooth. I then heat treat and quench creating the carbon on the surface. If I want to acid etch;wash, will blade will the etch eat away the carbon first or do i need to resand the bevel and flat? I could do a test piece but that involves me getting off my @ss. I would resand the bevel but was hoping not to have to do the flats if they are already smooth.
 
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OK, I'm going to assume that you are talking about scale? Scale is a product of carbon (and other elements with the steel) being changed through combustion. I often refer to it as "Steel Ash". Anyway, no matter what you call it, it is a very hard/tough thing, that we strive either not to have....or to get rid of..... which is what I suspect your after?

Again, I'm going to make another assumption..... when you say "acid" I will assume you are referring to Ferric Chloride solution that most of us use to etch blades? If so, yes, Ferric Chloride will "cut" scale on blades, but it will take a LONG time.... as in days, plus you will likely wreck the ferric chloride in the process.

With the cost of ferric, a much wiser, cheaper option is distilled white vinegar. You can buy two gallon paks as Sams Club for less than $5. I keep two containers of it....one for smaller blades, and a full 5 gal bucket for larger items. Depending on the level of scale on a blade, it can take as little as a few hours soaking, to sometimes a few days. It helps to pull the piece out after a few hours....scrub it off with a steel brush and rinse with clean water. Then you can judge about how much longer will be necessary.

And now, my final assumption for this post..... I assume you are hoping to reduce or eliminate some sanding/finishing? If so, that's not really going to happen....unless you want a grainy/globby looking dull gray finish. What it will do is make grinding/sanding/finishing easier, and less costly. Scale is super hard and tough, and the reason that I use vinegar to "cut" it, is to save money on abrasives.
 
The carbon, iron and acid confusion had me reread things a couple of times as well Ed. Don’t worry Mark, these are all very understandable confusions, that I myself would have had when I was getting started with them. I thought I would add a note here because I have a lot of hours logged with a unique perspective on where carbon could go on a knife blade. There are a couple of very specialized circumstances where FeCl (ferric cholride) can be used in micro-etching steel for metallographic (microscopic) examination, and if it could take carbon away I would have spotted it immediately. What it goes after the most is the iron, and it may take the other elements along with the iron.

There is a lot of confusion over scale and decarb when forging, or heating steel. Scale is mostly iron oxide, think of it as superheated rust. The high temperatures greatly accelerate the bonding of oxygen to the iron on the steels surface, and does it very evenly so that you get an actual skin of iron oxide that flakes off the metal. There are other oxides involved with the other metallic elements in steel but they mostly actually slow the process- forge some old style bloomery steel sometime and you will see scale on a level you never imagined, because there is just iron. This is not to say that you don’t lose carbon when it bonds to atmospheric oxygen, because you most definitely do, just it is not in the scale. Decarburization is a much more insidious process that we can as easily see until we find our knife blade has an invisible soft skin that has to be ground off. But, interestingly enough, this invisible decarburized iron can be reveled on the surface of our blades with FeCl. If you sand you blades clean and dip them in FeCl, the steel will etch darker than the decarburized iron with will show up as lighter splotches or dots covering the blade surface.

As Ed pointed out FeCl is not a very strong acid, it is on the lower end of the pH scale, but it is very diluted. What FeCl is is HCL (hydrochloric acid) that has had so much iron dissolved into it that it is supersaturated, and can hold no more despite its acidic nature. This is why you can’t use the FeCl PCB etchant straight out of the bottle without diluting it with some water; it will bite on non-ferrous PCB (printed circuit boards) but it can't take any more iron, diluting it makes room for some more iron in the solution but also makes the HCL part even weaker. And this is why it would not work so well for what you are thinking of, and also why, as Ed pointed out, it will just shorten the life the FeCl.

Vinegar works very well for descaling your blades, it is actually a stronger acid than the diluted HCL in the FeCl, I also like sodium bisulfate. You can find sodium bisulfate in any store, right now, in the pool supply section. Look for the “pH down” or “pH low” chemical, it is pure sodium bisulfate. Sodium bisulfate (NaHSO3 ) is a compound derived by taming down sulfuric acid with sodium, industry uses it for descaling metal because it won’t eat the steel as aggressively as it will the oxides and there many instances where it can be a bad idea to expose steel to hydrogen based compounds. I mix the sodium bisulfate crystals with water in a strong solution and then put my forged blades in it overnight. The next day, whatever scale is still on them can be removed with a scrub brush.
 
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Super good answers from both Ed and Kevin.

Both resonate with me because I've just started using vinegar to descale my blades after a couple years of eating belts up and grinding for what surely amounted to hours extra.

You still have to do a little bit of decarb clean up. But it really takes about 90% of the clean up grinding away. Highly recommend the vinegar soak.
 
Sodium bisulfate (NaHSO3 )
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I'd forgotten.....you mentioned that before Kevin. I went looking for it around here, and got the dog at the TV screen look when I asked for/about it....not any pool outfits around there, but even the hot tub people had no clue.... I never thought about checking Amazon! Time to hunt around some more..... I bet our "everything store" (North 40 Farm and Ranch) will have it. Thanks for reminding me! :)
 
EdCaffreyMS said:
OK, I'm going to assume that you are talking about scale? Scale is a product of carbon (and other elements with the steel) being changed through combustion. I often refer to it as "Steel Ash". Anyway, no matter what you call it, it is a very hard/tough thing, that we strive either not to have....or to get rid of..... which is what I suspect your after?

Again, I'm going to make another assumption..... when you say "acid" I will assume you are referring to Ferric Chloride solution that most of us use to etch blades? If so, yes, Ferric Chloride will "cut" scale on blades, but it will take a LONG time.... as in days, plus you will likely wreck the ferric chloride in the process.

With the cost of ferric, a much wiser, cheaper option is distilled white vinegar. You can buy two gallon paks as Sams Club for less than $5. I keep two containers of it....one for smaller blades, and a full 5 gal bucket for larger items. Depending on the level of scale on a blade, it can take as little as a few hours soaking, to sometimes a few days. It helps to pull the piece out after a few hours....scrub it off with a steel brush and rinse with clean water. Then you can judge about how much longer will be necessary. And now, my final assumption for this post..... I assume you are hoping to reduce or eliminate some sanding/finishing? If so, that's not really going to happen....unless you want a grainy/globby looking dull gray finish. What it will do is make grinding/sanding/finishing easier, and less costly. Scale is super hard and tough, and the reason that I use vinegar to "cut" it, is to save money on abrasives.
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Well yes all your assumptions were correct. I was using the word carbon instead of scale. I thought that it was carbon not iron that came out. Thanks for the knowledge. And you answered my question. I think it is best and more efficient if I sand. I will do the vinigar soak also if that saves some sanding.
 
Even an overnight soak in vinegar is very helpful as Rick mentioned.....in saving you lots of time and money on abrasives versus not.
 
I will add a caution for carbon steel damascus...... make sure you check it every few hours..... I found that damascus can go from no ready yet..... to way over done (the vinegar eats out the layers to a point where the billet/blade is wrecked) in a matter of hours. Straight steels don suffer from this same issue. ;)
 
EdCaffreyMS said:
I will add a caution for carbon steel damascus...... make sure you check it every few hours..... I found that damascus can go from no ready yet..... to way over done (the vinegar eats out the layers to a point where the billet/blade is wrecked) in a matter of hours. Straight steels don suffer from this same issue. ;)
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Good point . Thanks
 
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