There is a kind of steel which oxidizes (rusts) to leave a coating which does not further deteriorate. The term Core 10 popped into my brain, but may not be it. Advantages to the steel wall. It is hard to sneak up behind it and plant explosives, or build a ramp, or raise a ladder without someone seeing. I suspect that visual surveillance could be set up at some distance to scan a fairly long section of fence regularly over time. Also, patrols driving along a fence could see what could not be seen driving along a wall. Of course terrorists are coming in from Canada, and by plane and boat, not from Mexico. I wouldn’t be surprised to find out that there was a bounty payable to coyotes to turn in Muslim terrorists. Also, I doubt that the drug cartels would appreciate the attention and they play even dirtier than the terrorists.
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.