Aldav:
You could 'seal' the PVC conduit that's in the ground BUT there will still be water/condensation in the conduit. AS the green box is buried, with the cover at grade, it will collect water/condensation.
The splices have to be made with connectors that are suitable/listed for the location. That said, over time corrosion will WIN, no matter what you, or me, or anyone else does.
I serviced parking lot lighting for 30+ years and that's the first place we would check. I avoided (as much as possibel) quazite boxes, tried only to use them as a pull box, or when I had to, dug 5' & backfilled with gravel; not that doing so would guarantee drainage.
Silicone loaded wirenuts (Ideal) help; splices 'high' in the box help; but there is no 'cure' for long term.
