Why are my copper PCB solder pads oxidizing?
If you are using an acetic acid cured (acetoxy) silicone that would explain the corrosion.  It should never be used with electronics.
Only use a non-corrosive type of sealant designed for use with electronics, such as ASI 338. They tend to be a bit more more expensive and slower curing but they won't rot your circuitry and they don't release ionic (conductive) moisture.