‘As sturdy and reliable as they’ve been, by the time they’re put out to pasture, they’re pretty well used up,’ Andy Burns, Naval Flight Officer.
American aircraft carriers at their peak are the queens of the high seas, outclassing even America’s nearest peer competitors. They’re the anchors of US seapower, and have a commensurate price tag, costing billions of dollars to build and thousands of sailors to man.
But even the proudest US Navy ships reach the end of their service life and must be decommissioned.

The question is: would it be possible to “mothball” aircraft carriers instead of scrapping them to allow the Navy to expand quickly if the US needs more flattops?
‘No, it wouldn’t,’ Andy Burns, former US Navy Surface Warfare & Naval Flight Officer, says on Quora.
‘For one thing, mothballing something the size of a carrier is really, really pricey. Ships can’t simply be “put on the shelf,” but have to be extensively corrosion-proofed, preserved, and maintained to prevent deterioration.

‘The bigger problem is that it’s cost prohibitive to mothball a nuclear plant. Defueling and preserving a plant is extremely expensive and time consuming, and bringing one back online is even more so. Carrier plants aren’t really designed to be refueled more than once in their fifty-year service life. It’s a very lengthy and invasive process, that involves cutting giant holes in several decks to get at the reactor. The mid-life refueling overhaul carriers get takes almost five years typically.’
Burns concludes;

‘And then there’s the point that carriers are old by the time they’re finally decommed. Fifty+ years of salt-water sailing throwing planes off the roof and catching them is a hard life for a ship. Nimitz was commissioned during the Ford administration. Even the newest boat in the [Nimitz] class, GHW Bush, is already 14 years old. As sturdy and reliable as they’ve been, by the time they’re put out to pasture, they’re pretty well used up.’
Photo credit: YouTube via USNI News

Why don’t we sell old aircraft carriers to friendly allied nations in need of protection like South Korea, Taiwan Ukrane etc.
I don’t think they should be”mothballed” or decommissioned because the Navy needs Aircraft carriers in every ocean to intercept any threat towards the United States and NATO and the Navy could also “flex” the “muscles” of U.S. Naval prowess all over the globe. I know they cost a crap ton of money to maintain but it’s probably 10x more to decommission them and mothball them
The other comments: We can’t sell them to other countries or dump money into them endlessly either as the article explains, they’re “used up”. Half a century at sea is brutal. The best tires on your car can’t be used for endless decades, especially if you’re driving hard every day, in every climate, the rubber gets “used up”. Obviously, ships ie carriers aren’t a piece of rubber, but they’re billions of pieces of almost every type material of material you & I could imagine, thousands of miles of cable, pipes, joints etc and after 50 years especially with huge holes cut into the flight deck, they’re used up. They’ve reached the maximum service life of use.
Further to what @DocBull8404 said:
There are national security risks as well as ‘just not a good idea’ issues. Yes even at 50 years old there’s still kit onboard that you don’t want your enemies getting ahold of.
Radar systems, pretty much all of the Warfare Departments equipment, FoF system. The list is long & expansive. Then you’d need to train the donor nation how to use, maintain & operate everything. Even if you pass the entire bill for that on to the other nation you’re still looking at a high impact outcome regarding personel over, most likely, at least a 2 year period. Throughout that period the ship would still incur running costs. Next is the provision of a nuclear fuelled vessel but more importantly the reactors and all the tech that makes them run. Russia has never managed to produce a nuclear powered flat-top and it’s, comparatively speaking, a non-nuclear navy outside of it’s submarine fleet ( which is far from it’s glory days anyway). China is only now making it’s 1st serious foray into nuclear powered, US size comparable, carriers but they’re nowhere near completion and creating their powerplants is a new thing for them likely to be far from efficient ( granted that’s an assumption as they’re a ‘great unknown’ right now). Not getting into the politics or Force Assessment of Ukraine – Russia. Only a fool would assert that if Ukraine were provided a Nimitz carrier Russia wouldn’t throw everything to capture the thing. In Taiwans case. Nothing would tip china into invading quicker than the former taking delivery of one. Ukraine simply doesn’t have the man power to operate one. Taiwan maybe,. probably not though.
Then there’s the legal & geopolitical implications which we’ll just put aside lest we be here a year unwrapping all that.
So in, not that short, short there’s quite literally thousands of reasons why it should never be done.