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Background:

I was a chemistry major and have recently started watching this series of videos by NurdRage.

I learned that Platinum dissolves in Aqua Regia but Ruthenium is only attacked by high temperature halogens and some alkalis.

Question:

Would a Ruthenium and Platinum alloy be resistant to both acid and alkali?

But more importantly, is there any alloy or pure metal that is completely chemically inert to every strong oxidizing reagent? Is there any research about the most chemically inert metal alloys?

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    $\begingroup$ There is no completely inert material. However, in case you work with relatively small temperatures (<200 celsium) teflon is considered (almost) inert. $\endgroup$
    – permeakra
    Jan 3, 2013 at 18:27
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    $\begingroup$ Not an alloy, but this came up in a question about noble gas fluorides - fluorine forms a passivated layer on nickel which is tremendously corrosion resistant. As such, fluorine-passivated nickel vessels are used for fluorine photolysis experiments. $\endgroup$ Jan 4, 2013 at 2:58
  • $\begingroup$ My best guess would be a combination Pt, Ru and Ni. $\endgroup$ Apr 14, 2013 at 4:42
  • $\begingroup$ Why should some alloy be more resistant to oxidation than the "best" of its constituents? And do not mix up passivation (like aluminium) and chemical inertness! $\endgroup$
    – Georg
    Apr 16, 2013 at 10:44
  • $\begingroup$ Alloys have special properties that can exceed those of their constituents. $\endgroup$ Apr 16, 2013 at 12:08

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The problem is that in different media different metals / alloys are so corrosion resistant than we regard them as being "inert".

It is interesting that aluminium in strong nitric acid is passive as an oxide layer which clings very tight to the aluminium forms. Equally titanium, zirconium and halfmium in nitric acid are very passive. Even while they are from an thermodynamic point of view very reactive metals.

In alkali aluminium dissolves well as it forms soluble aluminium complexes.

Titanium in anhydrous methanol suffers dire corrosion if you scratch it. What happens is that you form soluble titanium methoxide which then removes metal from where the scratch is.

In nitric acid palladium and ruthenium will be very slow to corrode, but if you add chloride then the formation of the chloride compelxes such as PdCl4 2- greatly assists the nitric acid in dissolving the palladium and also the ruthenium. I think that if you want a super corrosion resisting metal or alloy you need to state what medium it will be in.

One good all rounder is tantalum, this is poorly attacked by all the common acids. While it will tolerate aquea regia it is attacked by HF and other flouride containing solutions.

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Due to electronegativities no alloy would be thermodynamically stable in the presence of fluorine (F$_2$). Even in an alloy the constituent metal atoms are either "sharing" electrons or receiving/donating electrons. Fluorine is the element with the highest known electronegativity and would "pry" the electrons from the electron-accepting metal atoms.

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According to the Wikipedia article on the chemical reactivity of fluorine, fluorine (a very strong oxidizing agent) reacts with every element except helium and neon. So it looks like no metal has the property you want. The reason helium and neon are so unreactive is that their outer electron shells are completely filled, with no hidden subshell available to accept an electron. No metal (or any other element, for that matter) has this property.

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  • $\begingroup$ Good answer, but what about alloys? $\endgroup$ Jan 3, 2013 at 16:10
  • $\begingroup$ I'm less certain here, but I don't think alloying would change the chemical properties very much. It definitely changes mechanical properties (like hardness) because of the different crystalline structure of the alloy. For example, stainless steel is more corrosion-resistant because the addition of chromium allows a passivating layer of chromium oxide to form (like with aluminum on its own). So in stainless, iron borrows a chemical property from chromium, but the elements are still as they were chemically. $\endgroup$
    – user467
    Jan 3, 2013 at 17:21
  • $\begingroup$ Okay. But anyone else can back this up or refute it? $\endgroup$ Jan 3, 2013 at 17:32
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Not the best source but Wikipedia says that Iridium is one of the most corrosive resistant metal elements (I believe I have seen it referred to as "the most" elsewhere). Only some molten salts and halogens are corrosive to it. However, this does not mean that an alloy could not be more resistant, although comments above suggest alloying doesn't change the chemical properties of the constituents, which makes sense to me. One critical point for what I have mentioned though, is: are corrosion resistance and inertness the same thing?

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