Lowes and HD sell 3/4 brass or stainless steel - more expensive but that might be the solution to your problem. Also - are you on well water? - high levels of minerals from well might cause this. Install water filter to capture the minerals and other chemicals in your water. I would also use stainless steel braided flex hook up hoses - so if you have to do any repairs later you don't have to cut and redo all the copper. Just unscrew it and screw back.