Do steel objects have a maximum lifespan? I read that "316 stainless steel is estimated to last for 1200 years in a “rule“ environment before heavy pitting. In the marine environment this is reduced to a mere 260 years." Are they referring to steel exposed to the elements and not cared for? If a stainless object was periodically electroplated or shielded from the elements, would it remain otherwise stable and not degrade?
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2$\begingroup$ Considering the Iron pillar of Delhi exists, why not? I don't see much point in electroplating stainless steel though, especially periodically. That defeats the purpose of being stainless. $\endgroup$– DKNguyenJul 17, 2021 at 20:48
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1$\begingroup$ Did not know about the pillar of Delhi. Fascinating! $\endgroup$– JustifyJul 18, 2021 at 0:16
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3 Answers
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Strictly speaking, very few metals are "stable" in terms of the laws of thermodynamics. True chemical stability is when the atoms are in their lowest energy state. For most metallic elements, various oxides, sulfides, and chlorides are lower energy states than the pure or alloyed metal. This is why corrosion occurs in the first place--the atoms will tend to form compounds that reduce their overall free energy--iron becomes iron oxide, aluminum becomes aluminum oxide, etc.
Most alloys, including stainless steel, are 'metastable', meaning that they will--eventually--move toward a lower energy chemical state (i.e. they will eventually corrode). However the kinetics, or time-dependent processes of corrosion can be slowed down to the point that they appear stable over the normal human lifespan. As others have pointed out, various catalysts and corroding agents can accelerate this process. But even just sitting 'in a museum', degradation will proceed at some vanishingly slow rate.
So the short answer is, "yes, there is a lifespan for all alloys, but sometimes that lifespan is so long that it doesn't matter; and the service environment plays a huge role in how long that takes."
Edit: As an aside, this also means that some of the useful microstructures of steels (pearlite, austenite, martensite, etc.) will also tend to degrade over time since most of these are metastable. Even if the chemical identity of the material doesn't change, the atoms will gradually rearrange themselves over time to reach their most stable configuration. Again, this is usually (but not always) on time scales far beyond what you'll need to plan for. Nevertheless, this is one reason why you should always be aware of the temperature environment of your metals--even if the heated alloy doesn't outright corrode away, a few hundred degrees can drastically reduce the time required for microstructural degradation, which can dangerously weaken hardened steels.
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It depends on the environment , relative to corrosion. Sea water is relatively corrosive to most metals, but again ,it depends. Splash zone is vey aggressive, low oxygen at great depth is relatively benign. What do you want the answer to be ?
answered Jul 17, 2021 at 22:05
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1$\begingroup$ "Splash zone is vey aggressive, low oxygen at great depth is relatively benign. What do you want the answer to be ?" I don't have a preferred answer. I was just concerned I misunderstood whether metal was stable. I read the original statement as though steel just destabilizes at some point. What if someone was trying to preserve some kind of little artifact in a museum, like a little stainless steel ruler or pen? Would that be possible? $\endgroup$– JustifyJul 18, 2021 at 0:14
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1$\begingroup$ @Justify You mean something weird like migration of the carbon in the steel? Something like that does exist in other materials but I don't know about steel. $\endgroup$– DKNguyenJul 18, 2021 at 2:02
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1$\begingroup$ @DKNguyen Yeah, I wondered if they were referring to something like that! I didn't think that occurred with steel or cast iron. Is it safe to say that if someone put something basic made out of stainless steel, i.e. a little machinist square or simple device away from the elements, in a museum it could still exist like it does today thousands of years from now? (I know that is assuming a lot about logistics, but the question is just theoretical). $\endgroup$– JustifyJul 18, 2021 at 2:38
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1$\begingroup$ Pick an alloy , a thickness and an environment. Some thing that can be called stainless steel like Incoloy 825 will probably be around when people are only fossils. $\endgroup$ Jul 18, 2021 at 19:58
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1$\begingroup$ @Justify Stainless steels in tools tend to be selected for mechanical rather than chemical properties. A good 316 fork would last longer. $\endgroup$– DKNguyenJul 19, 2021 at 2:53
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Stainless steel has an average life expectancy of about 50-100 years. Look no further than the two most famous stainless steel monuments, the St. Louis arch and Chrysler building. While Chrysler is made of a high nickel/chrome alloy than most. The Arch is a great example, Upclose it's kinda dingy after 50 years. Inspections show it to be structurally sound but it's stainless veneer is rather ragged.
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$\begingroup$ Chrysler bldg trim is essentially 304 (" 18 -8"). St Louis arch has no corrosion; it is dirt , vandalism and some contamination with carbon steel which leaves rust spots, a common situation with austenitic stainless. $\endgroup$ Jul 22, 2021 at 14:59