Does chrome rust?


PDA






mister_wilburn
June 26, 2007, 11:04 PM
I have read a few posts that made statements to the effect that chrome doesn't rust (my paraphrase.) Is it that chrome is rust-proof, or that it just takes alot more to rust. Is there a iron content to chrome, i am assuming that without iron you dont get any iron-oxide. I could be wrong, just wondering? Thanks

If you enjoyed reading about "Does chrome rust?" here in TheHighRoad.org archive, you'll LOVE our community. Come join TheHighRoad.org today for the full version!
Neo-Luddite
June 26, 2007, 11:29 PM
A metalurgist could explain better--but 'chrome' doesn't rust--the ferrous metal it's bonded to will. Poor example: I have a thirty year old chevy w/ chrome bumpers. Now, surface rust on top of the chrome forms and I polish it away. But this isn't, I don't think, caused by the chrome but by other factors (actually disolved iron from the car that 'runs off' onto the bumber top is my bet). Old chrome bumpers always rust from the 'inside out'--from the back where many were bare steel.

Why are you curious? What ARE you thinking about doing?

Cannonball888
June 27, 2007, 08:42 AM
Chrome itself doesn't rust rather the metal underneath. Chrome-plating IMO is superior to bluing but inferior to parkerizing.

Onmilo
June 27, 2007, 09:46 AM
Once a chromium finish is applied oxidation cannot form or continue as active because there is no oxygen to feed the reaction.
One scratch or surface blemish that penetrates the chromium surface and allows oxygen to the surface metal and rust can and will form not only at the surface break but it can continue to consume the base metal under the chrome finish.

Some of the newer chromium finishes such as 'Metalloy' molecularly bond to the surface steel and make it much harder for oxygen to get to the base metal.

Chrome is more similar to silver in that it corrodes in a different fashion and at much much slower rate than steel.
Chrome itself is not affected by the oxydizers that cause rust.

mister_wilburn
June 27, 2007, 11:17 AM
Just curious, thanks for the responses. I was mostly interested in alternatives to parkerization (which rusts when i sweat on it), melonite, and other finishes to firearms. Not for anything I have now, but for future purchases. I am assuming that they dont make firearms out of chrome, and only use it as a coating...but I am no metalugist.

fletcher
June 27, 2007, 12:05 PM
Once a chromium finish is applied oxidation cannot form or continue as active because there is no oxygen to feed the reaction.
One scratch or surface blemish that penetrates the chromium surface and allows oxygen to the surface metal and rust can and will form not only at the surface break but it can continue to consume the base metal under the chrome finish.



Right on. Chromium is an element, and of itself will have no iron content. It may form a clear oxide layer in some situations (which is protective), but it will not "rust". Penetrating this layer will expose the iron, allowing it to rust - perhaps more quickly than plain iron if a corrosion cell is created due to the difference in potential.


You are correct in thinking that firearms are not made out of chromium. That would be an expensive and poor choice of material for the job.

Aries-
June 27, 2007, 12:40 PM
a good chroming job should last for quite a long long long time before the metal under it starts rusting. as the company that does it should polish and clean the steel to remove any thing that could possably become rust. car bumpers are a bad example as they are cheap and mass produced lol. (also thin steel)

RPCVYemen
June 27, 2007, 12:41 PM
It may form a clear oxide layer in some situations (which is protective), but it will not "rust".

Interesting. So when chrome appears shiny, is it the chromiuj itself that is reflective, or is the chrome a clear layer that preserves the reflectivity of the underlying substrate?

Mike

fletcher
June 27, 2007, 12:58 PM
Chromium is a metal, and is opaque - that's what you see, and it must be polished to the proper finish, of course. The chrome's oxide layer is clear. So, you have this (crude diagram):

************** Air **************
--------- Chrome Oxide (Clear) ---------
------- Chrome (Metallic, Shiny) -------
-- Base Metal (Won't see this anymore) --


The best example is stainless steel, where chromium is alloyed in with nickel and iron. The steel is protected from rust because of the protective oxide that forms on the surface.

358minus1
June 28, 2007, 09:10 AM
Been a ferrous metallurgist for 20 years. Previous posters have explained it well. Chrome plating is essentially really, really, really tenacious paint. Rust is a generic term used to generally describe the oxides of iron, which form at the temperatures, pressures, and humidity levels found on earth. But really, rusting is the same as oxidizing. Just about all metals "rust", including chromium (gold is one of the few that really doesn't rust). The neat things about chromium oxide or chromium "rust" (which is actually a greenish color), is that (1) it is chemically very stable; not much reacts with it. Once it forms it does not change over time. (2) It is very thin. So thin that its greenish color cannot be detected with your eyes. The chromium plating may be a variety of thicknesses, but relative to fletcher's diagram, the thickness of the oxide is totally insignificant. (3) Chromium alloyed in steels can give the same basic effect as chromium plating, and thus we have stainless steels. The more chromium that is alloyed in the steel, the more stainless the steel becomes. Any chromium content in steel above 12%, makes that steel a stainless steel. Above 18% chromium, the corrosion resistance is very, very good, but the mechanical properties of the steel suffer. Stainless steel with 12-18% chromium (used for gun components) can still have good corrosion resistance and they can have very good mechanical properties. This is why you'll sometime hear stainless steels used for guns are "not 'real' stainless".... This is really a matter of degrees. The stainless steels used for gun components are a trade off between corrosion resistance and mechanical properties.

Stainless steel silverware is an example of a stainless steel containing 20-25% chromium. Silverware has excellent corrosion resistance, but at a cost. You can take your average spoon and bend it easily. So the mechanical properties of these types of steels would not work well for guns.

Also mentioned was chrome plating showing signs of iron rust. The only way the iron rust can show through the chrome plating is if there has been some wear on the chrome plate and it has been physically removed (allowing air and humidity to reach the steel surface) or there can be some manufacturing issues such as less than optimal adhesion of the plate to the steel or something like significant porosity in the chrome plate which allows oxygen and humidity to reach the steel surface. This rusting is accelerated by something called "small anode-large cathode", but the best analogy is a small scratch through the paint of your car, exposing the metal underneath. The exposed metal will rust very quickly because the area that can rust (steel exposed by the scratch) is very small relative the area of steel that cannot rust (the whole rest of the painted door).

Sorry for the long post, but metallurgy is my life..... ;^)

Onmilo
June 28, 2007, 09:44 AM
"sorry for the long post but metallurgy is my life."

Welcome to The High Road.
I have to figure out how to machine all the wonderful stuff you guys come up with,,,,,,,,,,,,

geekWithA.45
June 28, 2007, 09:45 AM
A metallurgy: the grandfather of materials engineering, the discipline without which we'd still be making stuff by stacking rocks. :)

anygunanywhere
June 28, 2007, 10:19 AM
Are we talking about chrome plating as in bumpers or chrome alloys as in gun barrels?

Chrome alloys do rust since they contain carbon steel along with the chrome.

Anygun

Thin Black Line
June 28, 2007, 10:32 AM
Are we talking about chrome plating as in bumpers or chrome alloys as in gun barrels?


If it's in reference to a "chrome bore" or "chrome-lined bore" then it's going to
be like plating. This is what we find in mil-spec M16 bores and (non-jugo) AKs.

daniel (australia)
June 28, 2007, 11:30 AM
One point to note is that there are different types of chrome plating, principally decorative and hardchrome. The chrome plating on your typical bumper bar is decorative chrome, and in fact it isn't just chrome at all. Usually there's a thin layer of copper laid down first, mainly to fill any surface flaws, followed by one or more comparatively thick layers of nickel, and then the chrome is an exceedingly thin layer (less than a micron) on top. The chrome is hard and scratch-resistant, and gives a nice shiny finish which lasts well on your car's brightwork and similar applications, but the corrosion resistance essentially depends on the nickel.

Nickel is anodic to the chrome, so is sacrificed electrolytically while protecting the cathodic chrome, and for best results in fact the chrome is either microporous as a result of plating technique or else deliberately riddled with microcracks so as to distribute the electric potential, and thereby to distribute the loss of nickel evenly and thus reduce the overall rate of attack. The nickel layer/s are several times thicker than the chromium too, giving you plenty to work with for reasonable coating life.

Hard chrome, such as might be used inside a rifle bore, or on brake caliper pistons and other high duty parts is a whole different creature. The chromium might be 10 or 100 times as thick - or even more - as the chrome layer in a decorative chrome application. At this sort of thickness the chromium is there for its great hardness and resistance to corrosion, erosion and wear.

fletcher
June 28, 2007, 12:03 PM
It's fun to see the metallurgists emerge from lurking ;)

Chrome alloys do rust since they contain carbon steel along with the chrome.

Mostly correct, but the other way around - the alloy will not rust because chrome is contained along with the steel (it's a low fraction in comparison).

scout26
June 28, 2007, 12:13 PM
I've heard Rust Never Sleeps.

Omaney
June 28, 2007, 12:18 PM
I think my brain just wrinkled again:D

Kentak
June 28, 2007, 12:31 PM
I think what you really are asking is, "Does chrome oxidize?"

**Disclaimer--I'm not a metallurgist, but I believe the following is correct**

Chromium is a very hard and corrosion resistant metal, and this includes resistance to oxidation under normal environmental conditions.

When you see rust on something that has been chromed, it is either rust from the underlaying ferrous metal breaking through or forming through a break of the chrome layer. Although unlikely, I suppose there could be some surface particles of a ferrous metal embedded on the chrome that might give the appearance that the chrome is rusting.

It is *my* understanding that chromium does not depend on a protective layer of oxidation for its protection properties. In fact, I believe, all oxides of chromium have a color that would be apparent on the surface. It is the bare, naked chromium that provides the protection. This is different from the properties of aluminum, which is a very reactive metal. Exposed to the environment, a bare aluminum surface quickly forms a protective layer of oxide. If this layer were not able to stabilize on the surface, the aluminum would quickly oxidize away.

A *good* quality chrome plating on a gun will give it extreme resistance to corrosion, oxidation, and wear and scratches.

K

Neo-Luddite
June 28, 2007, 12:37 PM
Rust is the great heartache of the iron age.

fletcher
June 28, 2007, 12:49 PM
^^ You are correct in that chromium is corrosion resistant in itself, but the oxide provides a "seal" of sorts to protect it, same as you mentioned with aluminum. Once the oxide reaches some stable thickness, it will protect the metal from additional oxidation.

Those reactions are part of the protection, just how some large beams used in bridges are designed to rust over so that no additional oxidation can occur underneath (thus protecting the base metal).

358minus1
June 28, 2007, 03:26 PM
We're now moving into the world of semantics.....

All metals (save a couple) are instantly coated with an oxide layer if they are exposed to air. There is no such thing as a metallic surface if oxygen is available. Freshly ground steel, even though it looks nice, shiney, and metallic is instantaneously coated by iron oxide. Iron's oxide does not block further diffusion of oxygen and over time the oxide changes, grows and continues to attack the underlying iron. As the layer of rust gets thicker and thicker, the rate at which oxygen penetrates down to the iron base slows, so iron rust does eventually slow the oxygen attack to a crawl, but this is after years.... not seconds. Chromium oxide forms instantaneously and immediately blocks further oxygen diffusion through it. Chrome plating is protective not because chromium is stable and unreactive, it is the exact opposite. Chromium is "violently" reactive with oxygen coupled with the fact that oxygen and water cannot diffuse through the oxide to further fuel the fire.

Good thread, more interesting than "Is 38Spl enough for HD?..." or "Which is better 38Spl or 9mm?...."

Bobo
June 28, 2007, 04:31 PM
I believe your original question was asked because parts of your gun are Parkerized and are rusting. You want to refinish the parts with something that won’t rust or won’t rust as easily. If this is the case, then…

The best finishes are generally also the most expensive.

Of the most common gun finishes…
Hard chrome plating is probably the best finish for rust prevention and also the most durable (scratch and chip resistant).
Nickel plating is second.
Parkerizing is next.
Guncoat.
Blueing.

AJ Dual
June 28, 2007, 04:38 PM
Aluminum has similar oxidization properties as chrome. (and I did not know until now chromiums mechanisim was simliar) A near instantaneous, very hard oxide layer forms, and re-forms just as quickly in response to any abrasions.

I don't know about chromium, but an application of mercury will destroy aluminum very quickly, as it allows for the oxidization, but constantly allows the oxygen access to fresh aluminum. An aluminum machining several inches thick will corrode into powder within a matter of a day or two if exposed to mercury.

Saboteurs and spies carried tubes of mercury paste to apply to aluminum war materials (like airplanes) during WWII.

Kentak
June 28, 2007, 07:02 PM
358minus1

Thanks for the info. I didn't intentionally misinform, I thought chromium was non-reactive with oxygen at normal environmental conditions. I always welcome a chance to learn more about the way this wacky world works.

Do you have a link to these properties of chromium and other metals to react with oxygen? I'm nerdy enough to enjoy reading stuff like that.

K

Stevie-Ray
June 28, 2007, 08:47 PM
^^ You are correct in that chromium is corrosion resistant in itself, but the oxide provides a "seal" of sorts to protect it, same as you mentioned with aluminum. Once the oxide reaches some stable thickness, it will protect the metal from additional oxidation.Hmmm....similar to sulphuric acid in black steel pipe. While that sounds like a disaster waiting to happen, the acid corrodes the pipe just enough to form a seal against further corrosion. It was always one of the more fascinating things in chemistry.

358minus1
June 29, 2007, 05:29 PM
http://en.wikipedia.org/wiki/Stainless_steel

Google stuff like "passive oxide layers", "chromium oxide", "corrosion resistance in steels". You'll get more stuff to read than you probably want.

Stevie-Ray is correct about the acid in steel pipe being passivated. You must be very careful though, it is only certain strong concentrations of certain acids that result in a passive layer that protects it from that specific strong acid... Dilute the acid down and the passive layer is then removed and corrosion can begin again!

Essentially, corrosion and corrosion resistance is all about the specific relationship between the metal and the environment its in. Some combinations product little corrosion, some produce rapid corrosion.

Titanium is another metal that has a very "passive" oxide film which gives it very good general corrosion resistance. The real advantage of chromium is that its passive oxide film protects it against some of the widest ranges of environmental possibilities here on earth. Lucky for us!

If you enjoyed reading about "Does chrome rust?" here in TheHighRoad.org archive, you'll LOVE our community. Come join TheHighRoad.org today for the full version!