Start with the model of the 30 Amp contactor you're using. Go to the catalog pages, look under the accessories and select the appropriate surge suppressor for whichever coil you have.
The contact rating for your model loop controller is 5 Amps resistive, but you are switching an inductive load. I suspect the inrush current is what is killing the contacts.
Also, once every 18 seconds is a bit on the high side of normal for a contact output. If the system is running 24/7 you're cycling on and off 4800 times per day. If the contact is rated for 1 million cycles under ideal conditions, that's less than one year.
You might be better served by a controller with solid state (no moving parts) output, with proper surge suppression. Or a solid state contactor with an analog front end and use a loop controller with 4 - 20 mA output.