Chimney caps near the beach tend to rust out very fast due to the moist salt air that is prevalent there in the evenings. These types of caps are on most houses built since the early 1980’s..  They are called zero-clearance or factory built chimneys or laughingly called temporary fireplaces, because they tend to rust out after a few years, and are not as durable as masonry chimneys.  The rusting you can see in the pictures poses quite a problem for homeowners.  If and when it ever rains, the holes in the chimney caps caused by rusting will now come down inside the chimney and often will get the walls above and around the fireplace wet.  I get calls, regularly, where people hear water dripping on metal.  That is a sure sign of something wrong.  Notice in the second picture that the neighbor’s chimney cap is severely rusted.  These caps are custom caps and can be manufactured in different materials.  Copper will last the longest and will product that lovely patination.  Also, available are stainless steel and a paint grade “galvanized” material.