I think it is pretty funny people think concrete prevents rust. What causes rust is water. Guess what is inside concrete? Water. Ever seen old bridges falling apart on the highway, with the rusty rebar sticking out and all brown and the concrete cracking and falling away. You know why that is? It is because concrete has water in it, and it rusts the rebar. They put that green crap on the rebar because they think it will stop the water, but it doesn't. My college recently rebuilt an entire 70,000 person stadium at a cost of $50 million because the old one was falling apart. Why? Because all the rebar was rusted. The Roman colisseum has lasted for 2000 freaking years. You know why? Because it has no freaking rebar in it. My college is going to rebuild that goddamn stadium every 50 years for the next 2000 years. I wonder how times they are going to rebuild it before they stop putting freaking rebar in it. Maybe they will never figure it out. They will just rebuild the stadium 40 times, over and over again at $50 million a pop. After they rebuild it 40 times it will cost $2 billion in modern day dollars.
Anyway, sorry for the rant. The way to stop metal from rusting is to keep it dry. This is why I have a dehumidifier in my basement where all my tools are. If I were you, I would dig a 12 inch hole, 24 inches deep and I would put 4 inches of gravel in it. If the location was flat or at a low point I would then bury drainage pipe 20" deep connected to the hole and leading to the nearest drain. Then I would put the light post in the hole and wire it up. Then I would fill in the hole with gravel. It will still rust, but should last about 40 to 50 years assuming it is neglected. If it is painted regularly it will last about 300 years until it rusts inside out. If it is painted regularly inside and out it will last forever.