@colmstiofan That black sticker looks similar to what I've seen on iFixIt/3rd party iMac display adhesives. I would not worry about it unless it impedes airflow or something. [br]
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I don't spot any corrosion from your photos, like @danj said , the color is a bit off, but that is normal from heat after some time, depending the conditions the machine was kept in.[br]
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Thermal pads on RAM is never required, and is really only common on laptops and small form factor machines, I would not worry about it on your iMac.[br]
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