If a ferrous material is magnetized using a solenoid, would the magnetic strength of the material fluctuates over the next few hours? How long would it take for the magnetic strength of the material be stabilized?
It may be better to post this question in the "Magnetic engineering" forum. Where is the fluctuating come from? the ferrous metal may have a residual magnetism depends on the metal but I have no knowledge of fluctuations.
I assume that you are not doing anything to the material, it is just sitting there.
The field levels around it should stabilize very quickly. Maybe a few seconds at the most.
Sometimes you can measure changes over the course of a few minutes, but I believe that most of those are related to temperature effects. If you warm the metal during magnetization then it will take a while to cool.
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Corrosion never sleeps, but it can be managed.