Is stainless steel really stainless

[Fat Guy]

What's the deal with "stainless" steel? What does this really mean?

[Sandra Levine]

"stains less"

[g.johnson]

Doesn't rust. Except when it does.

[mamster]

You started this topic just so you could say "metallurgy".

My chemist friend tells me that by adding metals with different potentials, you can make the iron oxidation reaction less electrically favorable. I asked why you wouldn't get oxidation of chromium instead, and he said, "'Cause, when they're alloyed, they don't react like individual elements. You have to look at the whole Fe + Cr + whatever + O2 reaction, and it's disfavored."

Does that help?

[vengroff]

The chromium in stainless steel oxidizes in a very thin layer at the surface. The layer of chromium oxide protects the metal below it, which consists of a combination of chromium, iron, and varying amounts of nickel and/or molybdenum depending on the type of steel.

Gold does the same thin oxide coating thing, but it costs a lot more than chromium. Other metals don't produce this kind of self-protecting layer when they oxidize; they just keep ozidizing all the way through. This is why gold and chromium are so shiny and don't tarnish or rust, but silver, copper, and iron do.

The reason we use stainless steel instead of pure chromium has to do with cost, ductility, and heat conductivity. You can play with all kinds of percentages of different metals in the alloy, to get one stainless steel that is good for knives, one that is good for pots, and one that is good for decorative trim.

[g.johnson]

This site confirms vengroff's explanation.

[Anna N]

If stainless steel is stainless why am I advised to buy a "stainless steel cleaner"? Why so many hints on how to keep stainless steel stain free? I have always thought it to be a misnomer. Why wait until water boils to add salt? Why do I keep a half-lemon on hand to re-shine my stainless? Why do I have to take baking soda to my daughter's tea-stained stainless steel teaspoons? The plot thickens.....

Anna N

[g.johnson]

Why do I keep a half-lemon on hand to re-shine my stainless?

From the site I linked:

There is some dispute over whether the corrosion resistance of stainless steel can be enhanced by the process of passivation. Essentially, passivation is the removal of free iron from the surface of the steel. This is performed by immersing the steel in an oxidant, such as nitric acid or citric acid solution. Since the top layer of iron is removed, passivation diminishes surface discoloration. While passivation does not affect the thickness or effectiveness of the passive layer, it is useful in producing a clean surface for a further treatment, such as plating or painting.

I think you're right that 'stainless' is a misnomer. Rust-proof would be more accurate.

[Dave the Cook]

I think you're right that 'stainless' is a misnomer. Rust-proof would be more accurate.

"Rust-resistant" would be even more accurate. Drop your s/s pan into salt water for a while and see what happens.

[Coop]

I think there are two key factors in why does stainless steel rust. The first is the alloy and the second is the finish. Bear with me I am not a chemist or metalurgist (check spelling), I do however manage a company that builds high polish stainless steel yacht hardware.(www.starlinebyirion.com)

The first factor is the alloy. We use only 316L, and in a briny marine environment it will not rust or corrode.

The second is the degree of polish. If the product is finished to high degree of polish it will inhibit oxodization. What I am talking about here is mirror polish.

These are what I believe to be the key in why does stainless steel rust? I can guarantee you that 316L highpolished product will not rust in any environment. However once you heat it your on your own. I also would say that anything with a brushed finish will oxidize because of the rough surface.

Why are stainless steel knives undesirable? I think it is because of the hardness of the blade makes it almost impossible to sharpen. In the process of making a blade the metal becomes work hardened. Thus ruling it out for future sharpening.

I can assure you that any cookware made from 316L polished stainless steel would be priced beyond the reach of many of us. I find my All Clad Stainless Steel cookware to be acceptable but tarnished. They do need to be polished from time to time with a mild abrasive. Be careful not to use any polish or tool that has been contaminated with other metals as it will then leave surface rust.

As for Passivation I do believe it works but is not suited for home use. We use a highly corrosive nitric acid solution. A lemon just won't be acidic enough.

[Coop]

Dave the cook. I beg to differ. If your pans were completly submerged in the brine they would not rust. You need the presence of oxygen thus the term oxidation. Even after hundreds of years ships made from mild steel survive as wrecks under the sea.

[Dave the Cook]

Dave the cook. I beg to differ. If your pans were completly submerged in the brine they would not rust. You need the presence of oxygen thus the term oxidation. Even after hundreds of years ships made from mild steel survive as wrecks under the sea.

Sorry, I didn't explain fully.

If you take a piece of stainless steel and submerge it in brine for a while, chlorides will destroy the surface layer that keeps stainless stainless. (This doesn't happen in salty air because oxygen in the air continuously renews the surface layer, and the mirror polish doesn't give rust a place to take hold.)

If you then bring it back into the air, I am certain that it will rust -- unless the surface layer is restored. Maybe this restoration happens more easily than I think, and that may be related to properties of the alloy, as you suggest. I guess it would depend on which oxidized first, the chromium or the iron.

Thanks for clarifying.

Edited February 19, 2003 by Dave the Cook (log)

[Rail Paul]

Be careful not to use any polish or tool that has been contaminated with other metals as it will then leave surface rust.

Thanks for the explanation, coop.

Would you elaborate on the "tool" statement? Would that include a Brillo pad or scrubby which has been used on an aluminum or cast iron cooking utensil? If so, that transference might explain some of the tarnishing issues noted earlier in the thread.

[=Mark]

Dave the cook. I beg to differ. If your pans were completly submerged in the brine they would not rust. You need the presence of oxygen thus the term oxidation. Even after hundreds of years ships made from mild steel survive as wrecks under the sea.

But there is dissolved oxygen in water, or fish would not survive. And sunken steel ships do rust, the Titanic is rusting away quite merrily. I'd hesitate to state that they have lasted hundreds of years as steel ships simply have not been around that long.

[Coop]

Yes any scotch brite pad, scouring pad etc that has had contact with cast iron or mild steel could cause surface rust. In our shop environment we do not work on anything but 316L stainless steel to prevent any type of crosover between sandpapers or scotchbrite. Also cleaners specific to stainless steel pots are not really necassary, any mildly abrasive cleaner can buff out scratches and mild oxidation.

Yes I may have overstated my case for submerged steel. it will eventually rust but not as fast as it would at the surface. I also believe they have been building steel ships since at least 1863 when the Star of India was commisioned. So I will change 100s of years to approx. 151.

[oraklet]

Why are stainless steel knives undesirable? I think it is because of the hardness of the blade makes it almost impossible to sharpen. In the process of making a blade the metal becomes work hardened. Thus ruling it out for future sharpening.

depends on the tempering, i think. the japanese make some very hard carbon steel knives - and on the other hand, my s.s. sabatiers are quite easy to sharpen. (not as easy as my two french carbon steel knives, though.)

by the way, most manufacturers of s.s. knives advice against leaving their knives wet for long, saying that they WILL stain.

[elyse]

I just tried a search, but didn't really come up with anything. I've found that when I put something aluminum near something else in the dishwasher, the plate or whatever gets rubbed with aluminum, and marked. Does anyone know how to get rid of this stain? Regular scrubbing hasn't done it for me.

[Coop]

elyse. Why put aluminum in a dishwasher? The surface area if unanodized will oxidize very quickly. That oxidation will then leach onto everything it touchs.

[Coop]

BTW. The biggest problem with stainless steel is the name. It will always be a compromise of price point, hardness(as in knives) and visual appeal.

I believe cookware is like life, Who umongst us is unblemished? The tarnish shows the life experience we have all been through.

[elyse]

elyse. Why put aluminum in a dishwasher? The surface area if unanodized will oxidize very quickly. That oxidation will then leach onto everything it touchs.

I'm talking about "disposable" aluminum from take out etc.. I reheat food in them, and reuse them. Is there nothing that will take off the stains?