Metals vulnerable to staining/tarnishing ordered by least to most resistant?

[MythicQuale]

Out of silver, gold, palladium, iridium, platinum, titanium, stainless steel, aluminum, chromium, and rhodium (maybe chuck in brass, tungsten, osmium and copper for fun), can anyone help list these metals in order of susceptibility to corrosion, staining, rusting, and tarnishing for long term storage in 'average' air?

I'd love to know how they all compare against each other and it may influence my own storage of some of these metals.

Here's six to get started. I'd love know where the others fit in. Warning, there may be errors in the ordering as I did very little research to obtain these!

Metals listed by least to most resistant to corrosion:

Iridium. Notes: "Iridium is the most corrosion-resistant material known. " (source)

Platinum: This is apparently lower in reactivity than gold (source). However I don't think "reactivity" necessarily equates to "most likely to rust" etc.

Gold

Silver

Copper

Aluminum

Osmium. Notes: "Osmium is the hardest of the group and has the highest melting point, but its ready oxidation is a limitation." (source)

Edited February 18, 2021 by MythicQuale

[MythicQuale]

Thanks, but do you think palladium, chromium or platinum is even more resistant to corrosion/discolouring/tarnishing than gold? What order would you put them in?

[MythicQuale]

Yeah just sit indoors. I've heard even gold can discolour or tarnish (due to sulfur and even oxygen in the air I think), so I was wondering if platinum or palladium would be even better than gold in that regard.

My interest is just as much academic as it is practical. Just be interested to know the ideal as a kind of benchmark.

Edited February 18, 2021 by MythicQuale

[Robda1986]

Oddly stainless will rust but not in the conventional way ie iron oxide it corodes when no oxygen is present 

[danmc82]

5 hours ago, MythicQuale said:

 I've heard even gold can discolour or tarnish

 

It wont be the Gold that tarnishes, it'll be the other metals still present. Gold is inert.

[MythicQuale]

On 18/02/2021 at 14:22, danmc82 said:

It wont be the Gold that tarnishes, it'll be the other metals still present. Gold is inert.

This site talks about 'toning', and it appears it can happen even with pure gold. Curious if platinum suffers from something similar.

Edited May 17, 2021 by MythicQuale

[HawkHybrid]

On 17/05/2021 at 07:45, MythicQuale said:

This site talks about 'toning', and it appears it can happen even with pure gold. Curious if platinum suffers from something similar.

 

gold turning(toning) into what exactly?

 

HH

[Bimetallic]

On 16/05/2021 at 23:45, MythicQuale said:

This site talks about 'toning', and it appears it can happen even with pure gold. Curious if platinum suffers from something similar.

That article is bogus. All their photo examples are non-pure gold, while talking about pure gold's properties... The American Double Eagle coin they feature is only 90% gold, balance copper. The Krug is 91.7% gold (Crown gold), balance copper. So what are they talking about? It's weird.

Their Roman example might be 99% gold, but it varied and lots of emperors issued those coins for a roughly 400+ year period. I wouldn't expect modern standards of quality control with 2,000 year old coins, even Roman ones...

[Bimetallic]

Silver is the worst by far. Then:

Copper

Aluminum

Gold

Osmium

Note that aluminum is usually made into alloys with magnesium, zinc, silicon, and sometimes copper and other metals. Pure aluminum isn't used as much as the alloys are, and the alloys vary in their corrosion resistance.

Osmium is super rare, hardly used in the world.

Stainless steel's corrosion resistance varies depending on the type of stainless alloy it is. It's like aluminum. There is nothing simply called "stainless steel" – it's always a specific grade/alloy. They all have double-digit percentages of chromium, and one of the usual differentiators is whether they have nickel as well, and how much nickel. Nickel enhances their corrosion resistance, and the common 304 and 316 have quite a bit, especially the latter. 316 also has molybdenum, which helps too. Another approach to better corrosion resistance is to add nitrogen, and you see this in some stainless knife steels. You can achieve good corrosion resistance without any nickel if you have nitrogen and maybe better control over the process.

The cheapest stainless has no nickel, e.g. the 400 series (nickel is super expensive at over $8.00 per pound). Like 430 stainless, which is what Mexico makes all their circulation coins from. It's not as corrosion resistant as the 300 series with all their nickel, but it's good enough for a lot of purposes, like coins evidently. Smart mints use steel for coins, not expensive nickel and copper as base materials, with the big differentiator being whether they plate cheap mild steel with nickel (or layers of nickel, copper, and nickel again, like the Royal Canadian Mint) or whether they use stainless and don't bother with plating. It's a trade-off with the cost of the chromium in stainless steel, vs the lower material cost of plain mild steel but then having to plate it in nickel and copper.