COR-TEN (U.S. STEEL TRADEMARK)is to what you refer. True, it is an alloy that oxidizes on the surface, but needs no painting. It is stable after several years of weathering. It has both corrosion resistance and tensile strength.
Additionally, a steel wall can be electrified - say with 20,000 volts - sufficient to kill on contact. And, no - I really don’t give a damn if we electrocute these trespassing bastards by the thousands.
Wow, my 80 year old brain can still kick out something I read about 40 years ago. The may have to electrify it, or wire it in some way that signals that tampering is taking place. Tonight I saw a photo of one of these fences where 2 chunks of steel had been taken out of those square vertical fence pieces. On the subject of walls, the Great Wall of China comes to mind. I read that it was not all that effective in keeping “barbarians” from crossing it. It was very effective in some very rough country as a roadway for horses and troops to march to trouble spots. There were towers along the way for surveillance, and for housing troops and horses. Something like that is certainly needed along the border if it does not already exist to service the wall/fence when it is being attacked by crossers. However, the road can be built and serviced on the ground, not built into a wall.