Spray some enamel paint on them if you suspect that, i can assure you the naala is not corroding it, otherwise you would be in a hospital on ventilator too.
The seaside locality of karachi has severe corrosive environment, new suzuki bolan has 2 year life only before it rusts and breaks apart everywhere, ac units of houses rust out at the 3 year mark.
I think your installer is not purging and vacuuming the lineset properly causing corrosive buildup inside the unit.