If it's old, real brass hardware you can polish it up. My parents house was built in the 50's and still has original or period correct hardware on the outside doors. It all has to be polished every couple of months to stay looking nice.
If it's modern residential **** (Schlage, Kwikset, Defiant, Weiser, Brinks, etc) even if you replace it, the finish will not hold up on the exterior but a year or two depending on weather and usage. Yes, they all have a 5-10-Lifetime warranty on the hardware and finish. Save you're receipt and the boxes with the model numbers because you will need it when you call the mfg to file a warranty claim when they tarnish.
Some of the nicer residential hardware (Baldwin, Emtek, etc) do offer a lifetime warranty on their plating and are very easy to deal with. Their hardware will tarnish like everyone elses, but it will take a lot longer for it to happen.
That's just part of living with bright brass finished hardware that is exposed to the elements and touched by human hands.