I agree. Black oxide is not a very durable coating, and if you live in a high humidity environment, you can expect to see them rusting way quicker than you might expect. Clear or black paint will go a long way to prevent this, but I would also put anti-seize on the threads too. Since you (apparently) live in Arizona, rust may not be a problem, but fading from the sun may be.
After a 4200 mile trip, with lots of rain, the clear coated black oxide is starting to turn a bit red.
Time to pop them off and Cerakote them
