This chain plate has suffered crevice corrosion and is in jeopardy of complete failure.  This damage, unnoticed by the vessel owner compromised the attachment of the split backstay on a rig that stands over 75 feet to the transom.

Often this type of staining in the gelcoat is indicative of crevice corrosion that is attacking the oxygen deprived stainless steel that is out of sight. Stainless steel is by nature corrosion resistant due to the addition of chromium although when immersed in salt water and deprived of oxygen it loses this characteristic.  If you look closely you can see the longitudinal fracture in this chain plate.