yamaha f150

[noah]

I have a 2200 v pathy with a f150 with 680 hours on the engine.  The paint on the engine coweing where it meets the aluminum base is peeling away. The paint on the motor mounting bracket at it very bottom and skeg  is now beginning to peel.  The anodes are replaced periodically with aluminum or zinc.  The boat is kept in a sling when not in use in salt water and rinsed with fresh water after each use. What is causing this?  What can I do to prevent this.  and repair this.