How does a relatively small amount of chromium prevent steel from rusting?
In: 573
Because it rusts instead.
Stainless steel is about 10% chromium. And when exposed to the air the chromium reacts with the oxygen much faster.
This forms a layer of chromium oxide on the surface. And that layer of chromium oxide stops any more oxygen from reaching the steel (specifically the iron atoms). And since the oxygen can’t reach the iron, it cannot rust.
If you are talking about chrome plating, the oxidization of chromium produces Cr203 which is really hard and prevents oxygen molecules from water from seeping below and oxidizing the iron or steel.
Iron oxide is flaky and oxygen molecules easily get to deeper layers of metal lattice. The deeper that goes the more weak the metal gets. If you have mixed iron and chromium (among others) into iron to make a steel alloy, then the same principle happens but it is distributed amongst the metals bonds. The chromium oxidizes and provides protective attributes. If you have enough chromium in the alloy it will be enough to prevent the iron from oxidizing to the point where it weakens.
Stainless steel will oxidize, and eventually it will oxidize to nothing, but it takes forever and the oxidization on the surface will be far less troubling than other steels or irons. Stainless steel now has some competition, there are iron / steel alloys now that will oxidize a hard external layer into a soft brown color which is hard like chromium but less expensive than chrome plating.
Different metals react with oxygen in different ways. Metals like chrome and aluminum form a protective coating when they react with oxygen. Iron turns into rust and falls apart when it reacts with oxygen.
Practical Engineering on youtube has a great series of videos on rust and corrosion.
This what is known as a sacrificial element. As stated elsewhere, the chromium reacts to oxygen at a faster rate than steel or iron. The chromium bond doesn’t force an expansion like oxygen to iron does. This creates a shield that prevents the oxygen molecules reaching the iron and creating rust.
Several other elements are used in such a manner including tin.
Simplest answer I can think of is that it coats it, like any other waterproof coating. Paint, rubber, or grease would also coat iron, and block moisture. But metals form a bond that outlasts all of those.
All the answers miss an important step taken with stainless steel…Passivation
Passivation uses an acid wash to remove any iron on the surface leaving behind only the chromium (and nickel). The chromium then oxides and you left with a layer of only chromium on the outside that does not rust.
Otherwise you will get stainless steel that actually rusts.
As a point of perspective, though it’s only tangentially related to your question, it’s a huge no-no to do anything with carbon steel adjacent to stainless. If you’re grinding on carbon, and that lands on stainless, it will rust. If you scrape a stainless piece with a forklift, it’s gonna rust. Sometimes it happens over the course of an hour or two (flash rust on wet stainless exposed to carbon).
Oxide layer, but here is another explanation why using a little amount still works.
Imagine that the chromium is distributed evenly across the entire alloy, so that means in any arbitrary volume, you will roughly find one Cr for every 9 Fe (in reality, there are lattice defects and other things but ignore that for now). Now, corrosion of steel is mostly a surface phenomenon, so you only really need to consider the surface layer atoms. Examining the 2D surface layer, you will find a similar distribution of atoms, i.e. 1 Cr for every 9 Fe atoms. For simplification, let’s say that it only takes one layer of atoms to passivate the metal (i.e protect). So, when the surface atom is Cr, it oxidizes, which then protects the test of the metal underneath. What if the atom was Fe instead? It would rust and flake off to reveal the next layer underneath.
Now, when that Fe atom flakes off, the next atom can either be another Fe atom, or a Cr atom. If it’s Cr, then all is good. But if it is an Fe atom (it is also more likely), the process can now just be repeated again, and eventually you will hit a Cr atom (because encounter chance is roughly 10%, because the atoms are evenly distributed), which will then stop the oxidation process.
So, although the steel looks smooth from the top, microscopically you have a lot of irregularities across the surface where there are “valleys” and “peaks” until the chromium atom(s) were reached. In reality, the oxide layer is several atoms thick, the oxide itself has a slightly larger volume than the base metal, there may be other atoms, the alloy lattice structure also needs to be considered etc. But since atoms are so small, you don’t notice these sub-microscopic imperfections anyway.
As several have mentioned, the chromium oxidizes forming a protective layer. The key here is that unlike iron oxide, chromium oxide has a similar molar volume to chromium metal. That is, unlike rust that is more expansive than iron, chromium oxide takes up the same space as chromium metal. Same Delia with aluminum. All those aluminum thingies out there have a nice coating of aluminum oxide protecting the metal underneath
18/8 Stainless will not rust. The combination does not have a galvanic reaction between H2O and O-2.
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