Rusty hitch ball vs stainless -- worthwhile?

The only grade of stainless steel worth snot anyway is "316" It contains molybdenum and is significantly stronger than 304 or 18-8. It is still weaker than grade 2 carbon steel and the price of 316 grade anything will make a person weep.

Stainless will rust when in contact with mild steel. Stick with what you have.

To answer a question, it is rusty because the chrome plating has been worn away. The ball is structurally in fine shape. The lube wears off while towing --lots of towing -- and there is moisture from rain while towing. Hence the thin film of rust.

What I am gathering from the comments is that this is no big deal, and there is no reason to run out and buy a new ball, stainless or otherwise.

My balls are rusty, they still work as designed.

How bad is bad? If you want it to look pretty then by all means get a new ball. Not going to matter unless the old is so bad the coupler will lose it's grip. Stainless tends to have a slightly lower rating... just make sure you are covered.

Balls and nuts definitely need some good threads.

"Damned knee kept getting greased. Damned ball kept getting wiped"

Stainless steel is a weak sister compared to "steel" the issue with them is not breakage, it's with the damned exposed threads beneath it. If you go stainless, do yourself a favor and hunt down enough extra nuts to cover all the threads (even if the nut is too thick and sticks below the threads). That way the stud can take a pretty good whack and not destroy the threads. You do NOT want to learn what it takes to remove a stainless hitch ball that has damaged threads. The damaged thread syndrome of course also applies to plain steel balls. I guess plain steel nuts are no big deal since they can be "greased" without endangering a fifty dollar pair of slacks.

How did it rust if kept properly greased?

Use the rusty ball, in a little while it will wear smooth as the new one. On the other hand if you want the SS ball, go buy it, I've wasted more money on other things.

Bill