BigBendFishing.Net

Ever wonder why you see all those rust stains around screw heads and the mounting surfaces of hardware? You most often see this around rail stanchion bases. It's because there is water in the screw hole or under the hardware. This causes closed cell corrosion which will rust even the best stainless steel. In fact, some metallurgists say that it is the highest grades of stainless that are the most vulnerable to closed cell corrosion. What many blame as low grade stainless is often simply the result of failure to bed the hardware properly.
If your boat is of foam core construction, that means water is leeching into the foam. The red rust trails are your red flag.

Stainless steel rusts because it contains carbon. The more carbon in a steel the less stainless it is, but the less carbon there is in a steel the weaker it is also. That is why a knife made out of high carbon steel holds an edge longer and is stronger than stainless knives.

Nathan

Stainless steel rusts because it contains iron!
Carbon does not rust! (rust = iron oxide)
Stainless steel rusts due to the uncommon breakdown of the protective coating of chromium oxide on the SS surface (SS = steel + chromium, and steel = iron + carbon). Chromium oxide at the surface of the SS forms a very thin, but tight molecular film which protects the iron in the steel underneath from air and water borne oxygen thereby normally preventing the iron from combining with oxygen and forming rust (mostly considered the stain associated with steel). Other metals are alloyed in SS for added characteristics, also. Under certain conditions, the chromium oxide coating on SS breaks down, and admits oxygen to get into the SS and form rust. Prolonged contact with certain salts (like those found in saltwater) allow oxygen to get beyond the chromium oxide protective coating of SS (usually at microscopic surface pits, that are rather common on SS surfaces and rust forms). This rusting can sometimes progress rather severely into SS and cause serious rusting and weakening of the SS. SS is rust resistant, not rust proof, although close to rust proof with the exception as mentioned.