Would man-made steel artifacts be able to be preserved like fossils for millions of years, or would corrosion make them dissipate? What about metals such as aluminum that don't corrode as easily?
The short answer is no.
Most metals do not fossilize well, they are too reactive/water soluble. Metals are usually what is doing the fossilizing, dissolved metal in groundwater is being attracted to the electrical properties of organic material and filling in or swapping places with a porous material. You might have a stain left or some of the shape as a natural casting, but not material preserved. The exception might be in amber, but it could also be really bad: amber preservation is tricky chemistry, so I am not comfortable saying yes or no. Metal objects have good short term preservation but horrid long term preservation. there are some low reactivity metals that might survive like gold, but not as fossils. But even gold is water soluble over millions of years, so even that would require some unusual conditions.
When we speak of a fossil, we don't usually mean a preserved object, but rather a preserved trace of an object, like it's impression in other material. Metal objects certainly would leave impressions in the surrounding material, as well as oxides. So in that sense they can easily leave fossils.
Whether the steel artifact its self lasts for eons depends on the specifics of the metal and the chemical and thermal conditions it encounters. But under most conditions, reactive metals like steel and aluminum would react away through multiple different corrosive mechanisms listed here. There's a very nice table here of the rates of corrosion progression for a few types of steel alloy. This includes Hastalloy, which is one of the most corrosion resistant forms of steel and for that reason is commonly used in nuclear power plants. For industrial use, with "Excellent Corrosion Resistance" being less than 0.1 mm of corrosion progression per year. That rate of corrosion will eat through 100 meters of metal in a million years. Zinc-steal in contact with fresh water corrodes at a rate of at least 10 mm/year, which would rapidly destroy it over large time scales.
Another way of looking at it is that basically all iron on Earth is found in the form of iron oxide, so iron compounds do not tend to stay oxidized over large time scales. Similarly, aluminum reacts with oxygen rapidly and is hardly ever (but not never) found in nature as an elemental metal. Some surface treatments such as anodization can extend this time, but will eventually fail. Some elemental metals can be found in nature, which is an indication that under some conditions they could be found as fossils. These "native metals" particularly include copper, gold, silver and platinum-group metals (see link for a full list).
If, however, a reactive metal object is kept well away from oxygen and water it may last much longer. This is the case for satellites, although most satellites orbits are not entirely stable. The longest lasting may be those boosted into graveyard orbits; standard geosynchronous satellite graveyard orbit results in an expected orbital lifetime of millions of years. Objects on Mars stand a good chance of lasting for long time scales, since they are not exposed to air or water, and are kept cold with little thermal cycling. They are also likely to become buried in dust blown by the Martian wind. The Philae comet lander may be similarly preserved, though comets tend to crash into planets after a few million years. Similarly, several of the outer planet probes may last for extreme time scales since they will be in vacuum, cold, and safe from major collisions. In particular: Voyager 1 & 2, Pioneer 10 & 11, New Horizons. But these are continuously moving away from the solar system, so while preserved, won't leave any fossils to find.
In total, the answer appears to be eons, but few objects will find suitable conditions, and the best place to look will be on Mars.
For thick steel like a pressure vessel with 12" thick walls ( not many but there are some) must leave some "fossil" evidence for a very ,very long time if buried. And there are many vessels with walls about 6 " thick. These vessels contain a little Cr and Mo so are somewhat more corrosion resistant than carbon steels. And very tall buildings are made with composite columns have many steel bars ( roughly 3" diameter bars) contained in concrete. There will be evidence of them for a very long time. And deep wells , 4 +/- individual concentric pipes with walls of about one inch thickness encased partially in cement and buried miles into rock. etc, etc, etc.
The best chance if the oxydation process leaves a hollow shell which can become a mold for another mineral. The trouble is that ferrous oxide cakes up so the boundary of the original object is chemically and physically blurry.
If you have alloys and zinc coatings and paint, that can aid against the blurring effect of rust, so a zinc or vanadium steel will give the mud and sand more time to become a fixed barrier against the steel.
There has to be a process which sends dissolved iron away from the steel object while sending a cementing chemical into the steel object. Some micro-organisms can perhaps cement the sediment around the steel fast enough that the steel can completely vanish after a million years and be replaced by iron pyrite/limestone. Ferrihydrite is also good for transporting iron away from the mold.
If there is lots of sulphur so that the bacteria can create iron-pyrite inside a sediment/plastic paint shell, then perhaps there will be a bacterial mat that can strongly photograph the steel.
It all depends on the conditions of fossilisation and the type of steel. Human flesh doesn't usually last long, but the Ice Man found cryogenically preserved in the Austrian Alps had been there for 5,000 years and would have been there for another few thousand if he hadn't been removed. Dinosaur proteins and DNA much older than that have been found.
Iron doesn't last long in damp conditions unless it is alloyed with another metal. The best stainless steel artefacts could fossilise under the right, non-acidic conditions. Copper, silver, gold, platinum and several other metals have been found fossilised in their pure, metallic state, so it follows that artefacts made of these metals could fossilise. Iron-nickel fragments of the famous Barringer meteorite in Arizona have been fossilised under the crater for at least 50,000 years and will probably (apart from the bits that have been removed) stay there for millions of years longer in an almost pristine state.
Aluminium may not corrode as readily as iron, but it does corrode. Nevertheless, aluminium artefacts might fossilise under the right dry, non-acidic conditions, as there are plenty of even more remarkable examples of fossilisation known to science..