Copper will patina in the presence of moisture, just look at the statue of liberty. In wiring, this patina/corrosion is accelerated due to current flow. Bad thing is it happens especially around crimped ends that can hold moisture. I build all my own wiring using stranded copper wiring with proper insulation rating for the task. I use bare crimp on ends, and solder the fitting just at the connector. If you allow solder to travel down the wire, it make a brittle point. It is possible to strip the wire back and put on new ends, but water generally has a capillary action, and the green runs down the wire some distance. When a dash board is finished I use an rtv sealant for windshields(no salt)to seal the connections that are exposed keeping the water out. Whether the whole harness needs replacement is a difficult decision on larger boats, they get complex. Some folks are against splicing, but a good soldered and taped Western Union splice will Outlast the life of the boat. Acceptable on low current items, don't splice main battery cables and such. Hope this helps.
