Why Does Stainless Steel "Rust"?

ktmrfs wrote:
Its called Stainless, not rust free, it is just that "Stain LESS". and depending on the type, some stainless will get a good surface rust. Look at stainless exhaust systems.

Stainless used in exhaust systems is a LOWER GRADE hence LOWER COST, it has LESS Chromium, possibly less Nickel and a few other expensive metals..

Top quality Stainless is in use on many fancy buildings as an exterior cover or even part of the exterior structure.. Has a MUCH higher Chromium, Nickel concentration..

Many if not all of those buildings have never exhibited a bit of rust since they were built.

It has nothing to do with how "good" or "bad" the stainless is or how "cheap" or not the stainless is, it has to do with what properties and applications the steel has been designed for.

Whether Stainless rusts or not DEPENDS HIGHLY on the concentration of Chromium, lower concentration and it will most likely develop rust, higher concentration and most likely will never develop rust.

The concentration of Chromium can affect the PRICE of stainless, more = higher cost and less = lower cost..

What do you think standard flatware will be made from?

Yeah, the LOWEST COST STAINLESS STEEL WHICH HAPPENS TO HAVE A MUCH LOWER CHROMIUM CONTENT.

There is flatware that does have a good quantity of Chromium, but it ISN'T sold at Walmart. We have a bunch of RADA cutlery, not a speck of rust on that stuff..

Where looks and esthetics are important, the stainless puts that first, other qualities like, strength, edge holding, sharpness on the back burner. Where longevity and heat resistance is priority, surface rusting takes a hit.

Acutally not totally true, Chromium is what provides the "Stainless" part, there ARE other metals that are mixed to provide strength, edge holding, sharpness..

Now heat resistance is another story, overheat ANY version of Stainless steel and you will damage the protective Chromium layer..

And, not all stainless is non magnetic either.

That is because there are many variations of metals that can be mixed to make Stainless steel.. Typically more expensive (hence higher concentrations or Chromium) versions are pretty much all non magnetic..

Some cheaper versions (like the stuff used on my "stainless" gas grill) WILL easily attract a magnet..

I bet a auto exhaust system will attract a magnet do to the very low concentration of Chromium, nickel and other metals (I HAVE successfully welded standard steel to stainless steel exhaust systems with regular steel MIG wire).