Greetings, 'show me' guy;-) Not to be too picky, but words matter. I don't 'think' this, I know it;-) The short answer would be 'experience', with 50 yrs in the electronics industry, and the last 6+ of those in the aluminum industry specifically....and driving an aluminum car for ~30 yrs;-)
Your question is a good one though. Many are not aware of these issues, and many who are still fail to adequately / fully appreciate the extent of them and their potential effects on their electronic/electrical equipment; including their flashlights. I'm just an electronics guy and a chemistry dummy, but here's how the smart guys in the room, (of which I don't claim to be one), might answer your question.
Another very short answer might be 'aluminum oxide'. Al, having 3 valence electrons, is (relatively) a
very good electrical conductor. It is also a fairly '
reactive' metal, and has a
particular affinity for
oxygen. As soon as raw Al is exposed to Earth air, it begins oxidizing. It doesn't slow down until it forms a
very thin covering of Al oxide on its surface (which doesn't take long). This oxidation is generally
not particurlarly noticeable visually in the earlier stages, unless the part is polished / shiny, in which case it will make the finish look a bit dull. Two Al atoms transfer a total of 6 valence electrons to three oxygen atoms, resulting in Al oxide (I'm not a math guy either, but I think that's right;-). Al oxide is a
very good electrical
insulator. This is why Al can be a nightmare mixed into electrical environments unless understood and properly dealt with / its negative properties mitigated. While it's true that high moisture content in the air makes matters worse, it's plenty bad here in the dry desert, I assure you. As an aside, if dissimilar metals are involved, particularly in DC applications (like flashlights), this also opens the door for electrolysis to occur, which further compounds the issue(s), and is definitely worse when moisture is involved.
Stray
resistance results in a
voltage drop (I²R) across it when
current passes through it, that voltage drop subtracts directly from from the voltage being delivered to the load (like your light engine modules) under operating conditions, and the resulting power loss produces and is dissipated in heat. The connection failures often tend to be not only progressive, but intermittent, and the user may just notice an intermittent output from their flashlight as the primary symptom (what Bykfixer would call a "whack-a-palm" flashlight
).
Just to tie this back in with flashlights: If you ever buy high grade hard alloy Al round bar / rod extrusions for machining into flashlight bodies, if they came from a certain 5-letter household word Al supplier I'm familiar with, you will notice that the cut pieces arrive looking very pretty and shiny. They come out of the extrusion press that way, and I'm told that is enhanced by the timed injection of liquid nitrogen into the right part of the (very hot) die stack at the right time.
After going through a dozen or so fairly complex processes over a period of days, just as the rods pass down the last conveyor prior to being cut/sawed to length and packed, they pass through an enclosure which produces a very thick cloud of well circulated fine mist of aerosolized oil of a special type inside it. This leaves them with a
very fine coating of that special oil, and that's why they're so pretty and shiny when they arrive to you. I don't remember the name of that oil, but it specifically inhibits that rapid oxidation.
Cheers!
