At the risk of sounding sarcastic, it rusted because it got wet. Even if you only used it once in four years. Once the rust started, it picked up enough moisture from the air to continue. Then you didnít catch it soon enough.
On the other hand, you might well ask why they built a marine
air conditioner with steel parts
. But youíll probably get the same answer that I got when I asked why the mild steel
frame on my scuba diving compressor
rusted away. I didnít catch it either. After all, the frame was suitable for a basement in Chicago on a dry day. Or the aluminum
base on my refrigerator compressor
after a "minor" leak in the cockpit
. Or the exhaust
on my engine
after 300 hours. Or the steel screws holding the "marine water
pump" together. Or my marine stove
which is all SS on the outside, while whatís left of the inside isnít. Or ....
Manufacturers cut corners, or just donít really pay attention to details. Stuff corrodes whether you use it or not. Scraping, sanding
and painting are a continual job. I just figure it keeps me from breakfasting in the local bar.