This is for the non techs out there. Next time a tech suggests that you replace a pitted contactor you may want to listen. Most times they are doing a thorough job and not just trting to sell you something.
For the most part, when you see contacts that look like this, The relay is undersized for the load or the load is to much for the relay. Sometimes look for a capacitor in the system to prevent the arcing and ease the contact termination load. Think like the old point ignition systems in cars. If the points pitted it was from the condenser failing. A form of capacitor.
I used to have flexible stones just for dressing them. Snap-On/Blue Point.
I see this a lot in air conditioners- and when you go back a week later because the customer was too cheap to spend 50$ extra at time you were there for other issues- – and a week later they called because now its failed- its frustrating when you are super busy- they are very impatient and critical- “you were just here and its broken again”—and you go back and its the contactor. Then its no longer 50$- its like 150 for service call and part and I am cheap. Most charge about 200-500 for contactors- i charge 50.. you really dont need to change them if like that- if you clean them up-unless its 115 outside and you dont want to spend time fixing it but just get off the roof asap. I take them and clean them up later- and give them to people who are not working or who cant afford—single moms, unemployed etc. you cant not have ac here in arizona its not an option. And I dont have the heart to leave them with nothing because they have not worked due to covid. But when or if the contacts either pit thru to the base material or they warp, crack, split etc from heat you have to change them. But after doing this for 30 plus years, I think probably 50% or more of contactors are changed when they could be cleaned up with a fingernail file. If you do replace- make sure you get the right contactor–correct coil voltage, and size- FOR EXAMPLE—in air conditioners—–If it is a larger system 3.5-5 ton get 40A contactor (coil) and if smaller get 30. But this applies to everything. Otherwise- you have these problems (I think as fixbear was saying). All of these contactors look the same- and some are 24V coil- some are 120, some are 208/230. Some are 30A, some 40, some more. Some are 1.5 pole, some 2, some 3—-So be careful and make sure you get the right one.