anymore condensors are never bare copper but usually galvanized or coated with something. Being that it is only 5 years old what does the condensor look like? If he knows the condensor is leaking have him show you where it is. I assume it is under warranty I would find out if he will warranty it and if not I would suggest a second opinion or go to trane themselves and see what they reccomend.
I have problems seeing a manufacturer covering a condenser sprayed with sulfur on a daily basis.
Same as one sprayed with dog piss.
"Better tell the sandman to stay away, because we're gonna be workin on this one all night."
"Dude, you need more than 2 wires to a condenser to run a 2 stage heatpump."
Clearly, you haven't seen a rotted condenser coil.
If you have high sulfur and or salt concentration in your water, and that chemical/salt is being sprayed on copper on a dailey basis, I would say it's a posssibility.
Hard water areas will cause all sorts of bad things to porous metals.
"I am for doing good to the poor, but I differ in opinion of the means. I think the best way of doing good to the poor, is not making them easy in poverty, but leading or driving them out of it."
Benjamin Franklin, 1766
Granted, it does sound like there is a leak somewhere in the system. However, just to cover all the bases, make sure the evaporator coil is clean and free of debris. Also, is the ductwork properly sized for the system and the return filters clean? I have seen situations where other companies were dumping refrigerant into a system that had airflow restrictions which were causing it to freeze up. They didn't check the entire system and had added several pounds of refrigerant to a system that wasn't leaking and only needed either new filters or to have the evap coil thoroughly cleaned. Once you have verified that airflow is not a problem then I would say it is time to go on a leak hunt.