Sciencemadness Discussion Board - Removing stainless steel lining from copper cookware

Removing stainless steel lining from copper cookware

Quince - 15-3-2005 at 08:40

I have a small set of that French copper cookware, that I bought off eBay for a decent price (nice too, 2.5 mm thick copper), but instead of the common ones which are tinned on the inside, I got the ones that have a stainless steel lining instead, due to availability. Well, stainless steel sucks for cooking! Food sticks on it really bad, especially the frying pan.

There's a place I can take them where they re-tin cookware with worn tinning, but want to get the stainless lining off first with minimal damage to the copper, as the guy said it has to be either bare copper or tin when he's working it. Is there any chemical and/or electrochemical method I can use to do that? (the lining is a fraction of a mm)

[Edited on 15-3-2005 by Quince]

12AX7 - 15-3-2005 at 09:25

Well, theoretically you could use the difference in ionization voltage, but the protective chromium oxide layer is going to thwart any attempts. Maybe an acid that eats CrO2 (Cr2O3?) and not Cu?

Or just go the easier route and get a sheet of heavy copper and bash a frying pan out with a hammer and sack of sand.

Tim

Quince - 15-3-2005 at 10:56

The easier route is no fun, plus, it will look like crap. Yes, looks matter, else it wouldn't have cost that much.

I am curious as to how the stainless steel is bonded to copper ("bonded" is the word they use, not plated or something like that).

So no one can think of a substance that would more readily attack stainless steel than copper?

Maybe I could just monitor the thing carefully and pour the acid out when it starts showing copper, and then apply locally to any steel left, or just use a polishing wheel with fine grit paste. But I need to leave a surface that will allow the tin to take hold.

What about using electrolysis to speed it up?

Edit: in testing, heated HCL+peroxide works (very slowly), but that will destroy the copper too.

[Edited on 15-3-2005 by Quince]

Theoretic - 15-3-2005 at 13:31

You could carefully choose the electrolysis voltage so that it's enough to dissolve iron but not enough to damage the copper.

Quince - 15-3-2005 at 14:54

Interesting. What electrolyte do you recommend? Do you think the fact that there are other things than iron in the steel make a difference in voltage?

neutrino - 15-3-2005 at 16:11

Stainless steel is not entirely iron; it is mostly other metals. If you go by electrolysis, make sure to thoroughly wash the pan before using it, as salts of some of the metals are not good for you. I’m not sure how much of these chemicals you’d get forming, but this is something I’d keep in mind.

Mr. Wizard - 15-3-2005 at 18:45

Since the stainless steel is thick, strong, and obviously the strongest part of the pan, why don't you just overcome the problem of sticking by copper plating the inside of the pan and tinning it? You might even get some silver solder to stick to the stainless, and then you could put on a nice thick tin wipe over the silver solder. Make sure the silver solder you choose doesn't have lead or cadmium in it. You'll have to use an appropriate flux to get the intermediate silver solder to stick to the stainless. These usually contain some fluorides or Zinc Chloride. Any mistakes can be buffed off ;-)

Quince - 15-3-2005 at 18:51

Mr. Wizard, you got it backwards. As I wrote in my first post, the copper is 2.5 mm thick, whereas the stainless steel is merely a lining (acidic foods corrode copper and some ends up in the food, so bare copper can only be used in non-heat non-acidic stuff in the kitchen) -- fraction of a millimeter thick.

Copper has to do with far more than just looks. Its thermal conductivity is around double that of aluminum, and several times that of steel. Putting a stainless steel lining is thus pretty stupid, even though it's thin; tin is not much better than steel, but the layer is thinner. Still, even with the stainless steel, cooking with these definitely makes a difference (I've also used aluminum, cast iron, and stainless steel; I don't use Teflon as the first scratch means shit released into the food).

[Edited on 16-3-2005 by Quince]

12AX7 - 15-3-2005 at 20:48

Quote:

Originally posted by QuinceI don't use Teflon as the first scratch means shit released into the food.

Well, inert shit, that then passes through you to be released stuck in your, uh, shit. (Gee, thanks for bringing up that word and ruining my sentence

)

Cast iron owns you, frying pans, machine tools, what next! :cool:

(Yes, informative post of the year award

)

Tim

Mr. Wizard - 15-3-2005 at 20:59

Sorry about the mix up. I don't know of any way to remove the stainless steel from the copper. Maybe a grinder or a sanding machine. I realize the copper is a much better conductor of heat than stainless, but on thinking about it, wouldn't a slight resistance to the heat flow allow the copper to spread the heat around a little more? How well do the pans work, other than the sticking problem? Just curious.

Quince - 16-3-2005 at 04:24

12AX7, you are wrong. It is not inert even at some of the cooking temperatures that the pan can reach (260*C by DuPont's own admission produces polymer fume fever). Also see several comments at http://www.mercola.com/2001/aug/1/teflon.htm and note the last comment by Howard about the toxicity of ultrafine Teflon particulates. If you want a cheap pan that doesn't stick, just use cast iron. The weight will only help you get muscles

(don't forget that cast iron must be primed by oiling and baking first when new, and never stored wet, but reoiled)

Mr. Wizard, as I said, "cooking with these definitely makes a difference". I like my cookware to react fast to any change in the heat (I use a gas stove, of course). Even with the thickness of the copper, I still get that. Many professional chefs use copper, especially the French (but I'm not French)

A warning: some people don't expect it, but copper oxidizes, so there's no way they will maintain the salmon-pink color when they are actually used instead of just sitting around. The copper oxides that form on the surface turn all sorts of funky colors, like brown/green/blue/orange/purple/yellow. You can use ketchup or copper restorer or Barkeeper's Friend etc. to return to the original color, but I don't bother.
Also, depending on thickness, they are heavy, though not as heavy as cast iron. Flipping pancakes is probably out of the question

BTW, they can't be used on induction stove, don't know why, maybe something to do with two metals bonded together.

If you are patient, you can find them new on eBay for as little as half price. Worst place to buy is locally (not online), as far as price. Within about ten years tinned ones will need retinning (if you use them regularly). But I think it's worth it if you are serious about cooking.

[Edited on 16-3-2005 by Quince]

12AX7 - 16-3-2005 at 07:02

"A standard method of producing an aerosol of ultrafine particles is to heat PTFE up to 480°C. This produces some gas-phase products, mainly HF (hydrogen fluoride)."

Even 500°F (260°C, hm the farenheit number is more asthetic, I bet the number has a wide tolerance) is pretty hot for frying. If you have respiritory problems, the small amount of fluorides amongst the plumes of smoking fat are the least of your worries.

But that's off topic...

Fast response is a good point, I get used to it and change the burner before I need it

but I'll have to make a copper pan some day, heck maybe tin it myself, we'll see.

Probably can't be used on induction because the copper is too conductive and doesn't load the stove properly.

Tim

Saerynide - 16-3-2005 at 07:18

That seems like a lot of work for a frying pan (considering how heavy it would be to cook with as well). You must be a really good cook then to appreciate it so much

I, on the other hand, would just stick to aluminum or like 12AX7 said, just hammer out my own

Quince - 16-3-2005 at 12:44

Well, my set of a pan and two pots is worth about $800 Canadian retail price (I bought them far cheaper at auctions). You'd think that's crazy, but I love them!

unionised - 16-3-2005 at 13:43

Last time I checked copper was a noble metal and iron, chromium and nickel were not. In principle any (non oxidising) acid would work.
Stainless resists some acids quite well, but HCl rots it reasonably quickly. You want to put some sort of a lid over it (perhaps clingfilm) to keep the air out. (HCl and air will attack the copper.)

Quince - 16-3-2005 at 18:11

I know the air problem. Is there any oily substance that will resist HCL (lighter than the acid and not miscible with it), so that I can pour a thin air-blocking layer on top of the acid? Can I speed the process up with electrolysis?

chemoleo - 16-3-2005 at 18:42

Common Quince, that really isn't so hard. Paraffin will do the job. Lamp oil. whatever.
Electrolyis will result in dissolution of the copper as well. The surface won't be good, you will have lots of holes etc.
Just do it as he suggests, dump it into HCl, put it on the radiator and leave it there for days/weeks until the steel is gone. It won't be fast. But electrolysis is almost certainly no good. So what options are left? Not many.

So how many questions are you going to ask now?

Twospoons - 16-3-2005 at 19:47

If you heat the whole thing to about 800C, then cool very slowly, most of the chromium ends up as the carbide, and the stainless property is lost. Unless its a welding grade stainless. At any rate, leaving the whole thing submerged in brine would preferentially corrode the iron, thanks to the electrolytic couple formed between the copper and the iron. (or have I got my electrochemical series backwards?)

Quince - 16-3-2005 at 21:17

Quote:

Originally posted by chemoleo
So how many questions are you going to ask now?

Just one: when are you going to stop being such an asshole? Oh wait, I already know the answer to that.

Twospoons, I'd test the brine soaking on a piece of steel and copper, but I don't know how to bond them together so that there isn't a reaction between the contact point of the materials. Will a solder joint (out of solution) between them invalidate the test due to the introduction of a third material?

[Edited on 17-3-2005 by Quince]

Twospoons - 16-3-2005 at 21:39

A solder joint out of the brine will be just fine. This is just a simple battery cell after all.

mick - 17-3-2005 at 11:34

I have a small set of that French copper cookware, that I bought off eBay for a decent price (nice too, 2.5 mm thick copper), but instead of the common ones which are tinned on the inside, I got the ones that have a stainless steel lining instead, due to availability. Well, stainless steel sucks for cooking! Food sticks on it really bad, especially the frying pan.

I have a nice set of cast iron cooking equipment. A good quality SS frying pan is OK. The one I had was non-stick untill some one cleaned it with a scouring pad.

mick

Chloride seems to corrode stainless steel

mick

[Edited on 17-3-2005 by mick]

Theoretic - 17-3-2005 at 13:38

neutrino, stainless steel is mostly iron, which is why it's called steel.

chemoleo, read my post carefully, I said a voltage low enough not to dissolve copper while still dissolving the iron. I recommend... well, most electrolytes will do, a salt solution too.

neutrino - 17-3-2005 at 14:00

If you’ll notice, most of the steels on your site have a lot of nickel in them. Nickel salts being carcinogenic, it would be a good idea to be careful with any chemical process.

unionised - 17-3-2005 at 14:15

" Is there any oily substance that will resist HCL "
Most oils would do. Given that this is for cooking I would try cooking oil, it might hydrolyse a bit but you can be reasonably sure that you won't get any toxic products from it.
OTOH, by the time you have washed away the chrome and nickel salts, you will have got rid of most things.

chemoleo - 17-3-2005 at 15:04

Quote:

Just one: when are you going to stop being such an asshole?

If I wasn't being an 'asshole' you'd flood the forum even more with trivial/repetitive/post-whoring and attention-seeking questions, laced with avoidable inaccuracies here and there (remember the xenon thread?). It all serves a purpose. Just in case you failed to realise that. You weren't warned numerous times for no reasons.

Theoretic - about the low voltage - sounds like a good idea in principle, but I doubt it would work well in practise. The solution would have to conduct very well to enable a decent current density for decent iron dissolution. But it's worth a try I guess.
The potentials are
Iron= -0.44 V
Nickel = -0.24 V
Tin = -0.14 V
Lead = -0.13 V
Hydrogen = 0.00 V
Copper = +0.34 V

At i.e 0.5 V & 20 W power (which isn't much), the solution cell would have to conduct 40 Amps, at a resistance of 0.0125 ohms. Rather hard to achieve I should think. Might be just easier to dissolve the damn thing in HCl. But I am not Mr electrochemical here, so correct me if I am wrong.

Quince - 17-3-2005 at 15:14

The three-turn heater winding on a microwave oven transformer plus rectifier/filter voltage drop should do it.

Also, I thought you were vulture instead of chemoleo, that's why I posted that.

[Edited on 17-3-2005 by Quince]

Mumbles - 17-3-2005 at 16:05

I mentioned this on the other board you posted this on. I'll throw it out here and see what people think. Use a soluble copper salt solution. It will dissolve the iron, and most of the metal in the steel, and replace it with copper which will more than likely fall to the bottom of the solution or be able to flake it off the pan. Digest all the steel off and scour or sand the copper. It will remove all the pits, and carbon, any glue used, and any undissolved metal. It may be worth it to give it a final washing with HCl to get as much off as you can.

I don't think copper metal will be attacked by copper solution. Someone better at inorganic chem should know.

[Edited on 3-18-2005 by Mumbles]

chemoleo - 17-3-2005 at 16:11

That's a good idea mumbles. I am just not sure how the CuSO4 (i.e.) will deal with stainless steel. Many a time I have dealt with spatulas/spoons made of SS and hot CuSO4 solution, and they didn't corrode. I guess it depends on the type of SS. Still, that is def. a good idea!
CuCl2 can react with copper, forming 2 CuCl. It's a way to prepare CuCl. But under those conditions it's unlikely to happen. As long as you use the sulphate rather than chloride.

[Edited on 18-3-2005 by chemoleo]

Quince - 17-3-2005 at 20:15

Perhaps I'll use HCL until some copper starts showing off, and finish off with the CuSO4. This way I also save on the sulfate, as I haven't found a source here; the one I have I brought with me from Europe over a decade ago.

The_Davster - 17-3-2005 at 20:20

You can find copper sulfate in hardware stores in the cleaning products section as a drain opener of sorts. I think I have seen it at either rona or home depot.

neutrino - 18-3-2005 at 03:16

Drain opener? Around here, it is used to keep roots out of your pipes.

kryss - 18-3-2005 at 11:25

Best idea is the Copper Sulphate one - add some HCl as well that will def corrode the stainless steel all right. Especially if its a cheap coating.

uber luminal - 18-3-2005 at 16:50

Stainless comes in many different combinations. Some are rich in Ni and Chrome, some are not. The best thing to kill stainless is heating it over 750ishC to diffuse the chrome. It looses its luster and becomes dull. Its easily etched or reduced at that point. the problem with that is that the Cu would suck away the heat that your putting on the SS. and once the Cu became saturated it would melt.

The other methods people have talked about would be difficult since there are so many different grades of stainless, and its hard to tell what composition you have. If these pans are so dang pricey, they might just have the 18% Chrome, 32% Ni. in any case, im not sure why you want to use nothing but copper to cook anyway?

Copper works great as a heat sink, giving up a lot of heat to the air... Stainless keeps heat where you put it, and it takes quite a bit of it. I would think the added SS layer would be helpful to cooking. Unless you like wasting heat or something.

(btw, quince, Cu doesnt have a better tc. than Al. Al is almost 2x better) This might be the reason the world uses Aluminum for heat sinks on everything important, rather than copper. However copper does do better at higher temps. The copper arc melter hearths that I use take the 5K C temps just fine where Al would have melted or reacted with the metal being melted haha)

Since the stainless is so thin, and exposing Chrome and Nickle to your FOOD cookware is a really dumb idea, I think your best option is to just grind it out. or try cutting it and peeling it out.

Saerynide - 19-3-2005 at 05:42

Quote:

Stainless comes in many different combinations. Some are rich in Ni and Chrome, some are not. The best thing to kill stainless is heating it over 750ishC to diffuse the chrome. It looses its luster and becomes dull. Its easily etched or reduced at that point. the problem with that is that the Cu would suck away the heat that your putting on the SS. and once the Cu became saturated it would melt.

Well, if you can keep the temp around 800-900C it should be fine. Copper doesnt melt til 1083C. As for how you're supposed to keep the temp around 900C? That, I have no idea.

[Edit]: typo

[Edited on 19-3-2005 by Saerynide]

12AX7 - 19-3-2005 at 08:40

Don't forget to sit the copper part in charcoal or coat it with flux (glass) otherwise it'll burn through too. (Well not really, but it will oxidize to all hell, oxygen diffuses into it nicely both as gas and oxides.)

Tim

vulture - 20-3-2005 at 04:31

Copper will get mighty soft at temperatures around 800-900C, so I wouldn't go that high.

unionised - 20-3-2005 at 08:17

"(btw, quince, Cu doesnt have a better tc. than Al. Al is almost 2x better) This might be the reason the world uses Aluminum for heat sinks on everything important, rather than copper."
Er, wrong, copper is a better conductor*. A plausible reason for using Al is cost, another is ease of extrusion.
There are plenty of high-end heatsinks made from copper.

Anyway, if you put copper sulphate solution in the pan you will cover the steel with a layer of copper. Then the reaction will stop.
You will convert the relatively easy problem of removing steel from copper into the more difficult problem of "removing copper from copper, but only in the right places"

BTW, if heating stainless above 700C kills it, how do they cast it?

* CRC handbook, Cu 4.01 W/cm/K vs Al 2.37 W/cm/K

[Edited on 20-3-2005 by unionised]

12AX7 - 20-3-2005 at 11:20

Quote:

Originally posted by unionised
BTW, if heating stainless above 700C kills it, how do they cast it?

Quickly.

Obviously only repeated heating, cooling, oxidation and whatnot will kill the chromium content (leaving a porous (as I recall) nickel iron which is not corrosion-resistant).

Tim

uber luminal - 21-3-2005 at 09:13

Quote:

Originally posted by unionised
Er, wrong, copper is a better conductor*.
* CRC handbook, Cu 4.01 W/cm/K vs Al 2.37 W/cm/K

No, your correct. I misread the alloy I was looking at (I read C26000 series instead. The point I was getting at was neither pure Al or pure copper are used commercialy since they are both rather soft. (especialy when heated) Pure Al tears like butter, and pure copper is warped very easily. I compared Al1100 and Al6061 to C26000... and in that case Al is much better

but since we are looking at a 90%+ copper alloy and not cartridge brass... im wrong.

Twospoons - 21-3-2005 at 14:30

Quote:

Originally posted by unionised

BTW, if heating stainless above 700C kills it, how do they cast it?

Turns out that chromium carbide forms over a reasonably narrow temperature range. I can't remember what the exact temps are, but at much higher temps (as in casting, forging, welding) the chromium redissolves into the alloy. The trick with keeping stainless "stainless" is rapid cooling though the carbide formation zone. There are also 'weldable' alloys designed to minimise this problem.

Quince - 6-6-2005 at 21:45

HCL+H2O2 definitely etches the stuff, but so slowly that I don't see how I can remove a 1/4 mm coating of stainless from the copper without several hours of scrubbing with the solution. Also, the bad thing about this solution is that it destroys the copper as well, so I'd have to find a different method for finishing off the process.

[Edited on 7-6-2005 by Quince]