I am sure there is some chemical reason for this, but I can’t think of anything that I know of that would make sense. Also, are there other types of materials along the same lines?
From here
SS does’nt rust because it has chromium in it which oxidize to chromium oxide on the surface and forms an ipenetrable layer. If you scratch (stain) the surface - more chromium diffuses from the bulk to the surface and forms the protective layer again - hence the name stainless.
But SS is prone to corrosion too. SS gets pitting and crevice corrosion in environments with Chlorides. Its the worst thing u can use in Sea water 
Is there a point at which the Chromium runs out? In other words, if I keep scratching the surface of the stainless steel, will the chromium always come to the surface for protection?
Never knew that! Good to know. Thanks.
moe: I wouldn’t suppose so… the chromium is homogenously mixed throughout… scratching it to remove some would result in a (slightly smaller) piece of stainless steel. Eventually, you’d just scratch the entire thing away.
Even if you could seperate everything (melting, then centrifuging might work…), if you took the chromium out, it wouldn’t be stainless steel anymore.
Incidentally, stainless steel can rust, it just takes much longer to do so than regular steel. I have a stainless steel diving knife that has rust stains in small patches from the times I neglected to oil it after dives.
mecaenas: wouldn’t that be due in part to the salt water, which apparently isn’t good for SS?
It’s still better than other types of steel, isn’t it? Or is there a preferred material for use in sea water?
iron?
Iron? Not really.
Hmm. didn’t know it corroded that easily… It was a joke referring to the density of iron.
I work with different kinds of stainless all the time, and
I can tell you that some stainless 440 in particular rusts
REALLY easily, especially before it is heat treated. Also
stainless steels don’t corrode in acid, which is why we
passivate(acid bath) 'em to remove the “free iron” which
is susposed to increase the corrosion resistance 3-4 times.
In response to the original poster, I bet you can’t get
titanium to rust, however a pencil mark(graphite) can
actually cause a crack in it.
Yeah. When I bought my knife I assumed that stainless steel wasn’t supposed to rust at all, even with contact with sea water. I didn’t realise “stainless” is just that, it stains less than regular steel but it isn’t rust-proof steel. Not really much of a point to my post outside of the fact that I only just found out recently so I thought I’d throw it in for those who may not have known.
Corrosion consultant checking in!
The mechanism described is correct - stainless steels are passivated by a self-healing layer of chromium oxide. The quantity of chromium in the steel varies - low grade stainless is about 13% Cr, the “high grade” austenitics such as 304, 316 and 321 are about 17-18% Cr. (There are a great many more, with variable mechanical properties as well as corrosion resistance.)
Chloride (and also bromide and iodide) breaks down the passive film and allows stainless steel to rust. In some situations this can be worse than using a non-stainless steel, since it tends to occur in tight gaps (e.g. under bolt heads) where you can’t see it until the thing gives up on you. The corrosion also tends to be in the form of pit-formation, which can penetrate faster than general corrosion (a big problem in pipes, storage tanks, pressurised vessels etc. because you can end up with “pinholes.”)
For seawater service, many people use 316 stainless steel in the mistaken belief that it is fit for marine service. (It contains some molybdenum, and technically is seawater resistant. But let a bit of sea spray get on it and concentrate up two or three times by evaporation and it’ll pit very nicely.) For critical seawater applications, copper-nickel alloys such as the monels are preferable to stainless steels. Other options are bronzes and brasses, gunmetal etc. I’d just about trust 316 to make the railings, and I’d still worry about the bolts.
Haven’t heard the one about the pencil mark on titanium before but I can believe it - graphitised gaskets in stainless steel piping have caused problems. Incidentally, stainless steels will corrode like crazy in halide acids (HCl, HBr, HF). I presume you’re pre-passivating in sulphuric, phosphoric or something like that?
Nitric acid, 45% 130 degrees for 30 minutes or materials
call for 20% nitric acid and some % Sodium Bromide (I think
thats what it is its a red powder). Its all mil-spec stuff, so
we just look it up and go.
As for tht Ti, That came from a very knowledgable old
guy in our shop, he worked at Rolls Royce in England and
they were having turbine failures, they traced it back
to guys in the shop writing with pencil on the material,
after that the pencils were banned from the shop.
God, tell me about it.
When I was in the Air Force, the hospital on base was renovated. Amongst other things, they replaced all of the old water pipes with stainless.
The contractor had all of the piping in the access ways in the basement finished, and did a pressure test to make sure everything was tight before the started switching over to the new piping.
Things got balled up, and after the pressure test, it was few months before the new piping was actually used. Long before then, however, the new pipes started leaking. We had water dripping out of brand new pipes. Not an the joints or along the welded seams of the pipe, but just at random places on the pipe.
The head of engineering got all hot, and blamed it on the contractor. She claimed they’d welded the joints wrong and installed the pipes with the seams down - and that we had leaks from the joints and from the seams.
Being the only one in the engineering squadron who had a head for technical stuff, I got stuck with the job digging up the data to back up the Captain’s accusations. Being the thorough little twit I was, however, I ended up showing that the contractor was in the clear - and that if anything, the managerial screw up that delayed completion of the project for several months was to blame.
If memory serves me of events from 13 years ago, letting drinking water stand in a stainless pipe was the cause of the trouble. The crevice corrosion mentioned above takes place quite rapidly when the water isn’t running. I’m guessing that chlorides in the water settle out onto the steel, and they then proceed to eat through the pipe wall along the chromium oxide “veins” in the steel. I’ll further guess that the chlorides function more as a catalyst, causing the chromium oxide to breakdown without consuming the chlorides - which go merrily along riddling your water pipes with hairline cracks.
Anyway, I spent a couple of weeks digging up info on stainless pipes and water, and finally found a mention somewhere that letting water stand in stainless pipes would cause leaks.
The Captain even had me trying to make sure that the contractor hadn’t cheated us on the pipe and installed pipes of the wrong kind of metal. It seems she’d found that you could stick a magnet to the pipes - and everybody knows that stainless isn’t magnetic.
Sort of. It depends on the particular alloy you’ve got. It even turned out that the stainless alloys most resistant to corrosion from water were some of the more strongly magnetic types.
Read the above with caution. What I’ve written is from memory of things long past, and I may have screwed up at some point.
At any rate, lots of things aren’t as simple as they appear. When in doubt, ask an expert.
One time I used some of that quick silver cleaning dip & got some on the steel sink & it that SS rusted in a few days…
So it does rust if you put the right stuff on it…
Ford actually made a few cars out of SS too:
http://www.alleghenyludlum.com/pages/companyinfo/stainlesscars.asp
I believe IIRC - the point when SS is not stainless anymore is when the percent of Chromium falls below 9%. It has to do with the internal structure of the Steel which changes. You don’t want to get into austenitic, and all that jargon.
Chromium has a higher affinity for halides (chlorides, bromides, iodides, etc.) so effectively when you put SS in seawater you leach chromium out making SS even worse than CS.
For seawater cupro nickel or titanium is the best option.
There are other forms of corrosion too - which may not be dependent on the metal itself. For example Stress corrosion or galvanic corriosion.
Also, Stainless Steel (or any other alloy for that) is most prone to corrosion at the joints (welded etc.). Ever try to weld SS without inert gas, and you’ll see the rainbow colors due to the oxidized film. Also, SS needs elaborate welding procedure (TIG usually for pressure parts) - and PWHT - Post Weld Heat Treatment to relieve thermall stresses and restore the crystal structe in the HAZ (Heat Affected Zone).