if you can afford airplane grade, you won't worry about money.

Aluminum alloy depends on the content and quality it can be brittle. Not easy to weld back.

I did not consider aluminum during my selection, simply because is higher price, if i want to put auto gate arm I worry not many people know how to weld the bracket on the aluminum so my selection and price are limited.

And if normal steel gate can last >10 years why worry about forever? Change it after so many years of usages.

Normal Grill steel won't have high carbon. Unless precast cast iron for grill... which i doubt anyone in right mind will want to spend that much of money.