[Vicegrip]

Replace ASAP/ They like to split open on the weld seam when they reach the end of service life. A pinhole is simply the place where the inside rust has made it all the way through first there is plenty more unseen rusted metal inside.

If you can't replace it today remove it and install a threaded plug until you can. It has not been working as an expansion tank for a while I bet. Yes, it has a bladder to make an air chamber. The bladders tend to fail, the air gets absorbed and they become future flood generators.

Take a sharpy and write the in service date and in big numbersreplace by date on the new one. IIRC most say right on the data sticker the service life is 10 years. Better to go 80% of rated service life. Cheap insurance.

Bitch slap the lazy ass plumber that mounted it right above the gas vent too. I might be inclined to move it away from the service life shortening extra heat it is getting.