[AndersHoveland]

Is there an alloy that is completely resistant to chemical attack of any kind?
Some combination of metallic elements whose combined chemical resistance together makes the alloy invulnerable to any possible combination of base, acid, oxidizer, or cyanide?

Iridium is supposedly the most chemically resistant metallic element. It is not attacked by aqua regia.
(rhodium has similar chemical resistance and is less expensive)

Aqua regia will dissolve most alloys "...so long as group IVB, VB, and VIB elements do not exceed a critical level. In particular, as chromium and molybdenum levels rise in unison, aqua regia loses effectiveness"
"A Manual for the Chemical Analysis of Metals", Thomas R. Dulski

If the metals are alloyed with some metalloid such as silicon, it would make them very resistant to oxidizing acids.

High-silicon cast irons offer excellent resistance to nitric acid at all temperatures and concentrations, with the exception of dilute hot acid. Aluminum is actually resistant to nitric acid if the acid concentration is over 95%, but if the concentration is below 80%, or if the nitric acid is heated above 40°C, the corrosion rate is much faster.
Handbook of corrosion data. Bruce D. Craig, David S. Anderson, ASM International

The presence of nickel prevents attack by hydrofluoric acid. If the alloy contains a substantial presence of platinum or gold, it cannot even burn in elemental fluorine.

The presence of molybdenum can make alloys resistant against hydrochloric acid.

[curiouscat]

Hastealloy C might be one of the more resistant ones. Though for sure not inert to all. I can still think of many chemicals that will corrode it.

23% Cr, 16% Mo, 2% Co, 3% Fe & rest Ni is one composition I see.

[Enthalpy]

Hi Anders, nice to see you back, we missed you!

Corrosion resistance comes through two (or are there more?) different mechanisms:
- The redox potential of noble metals. Nickel to iridium, the price follows more or less the chemical resistance;
- The formation of a passivation oxide layer: on Al, Cr, Si, Ti, Nb, Ta, approximately in this sequence and if I didn't forget any.

The corrosion resistent alloys tend to combine both, with the limits that
- The redox potential isn't very meaningful when an alloy is heterogeneous, and this is usually sought
- Miscibility limits the elements and proportions.

So starting from Fe-Cr stainless steel, you can switch to Ni-Cr (the suggested Hastelloy, still half-way affordable) then Co-Cr. Or go the Ti way. Or the Ta way, which is the best among the technological alloys.

Cr and Si can't be used pure because they're brittle. Si and Al aren't very soluble in most metals. And so on and so forth, metallurgy is nasty.

I worry when I read "any" kind of chemical. HF, even dilute, atacks SiO₂ and makes Si useless as an alloying element - much so the other oxide-making elements. Mo protects stainless steel's cromium oxide layer against chlorine ions, but does it work for other alloys? My impression is that all oxide layer building elements fail under some condition, HF and Cl^- being the more common ones, and that only noble metals resist these conditions.

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Must it be a metal? I'd rather try plastics and ceramics. Just PVC has impressive resistance to both acids and bases, oxidizing or not. Or PP. Silicon oxide won't survive HF, but maybe BN, Si₃N₄, B₄C...?