Unexpected Corrosion Problems by Organic Fat Chargedin Loading Tanks of a Cargo Ship

A failure analysis has been conducted on board a cargo ship to assess the extent of corrosion damage. The damage was classified as unusual by the ship owner since the material, an organic fat(namely tallow),was usually loaded into stainless steels on board tanks without any noticeable corrosion problems. The corrosion phenomenon was classified, by the technical crew, as pitting on the bottom of two tanks, particularly the 6p (port tank) and the 6s (starboard tank). The pitting was defined as anomalous since only two tanks were affected and a total of five tanks were made of same stainless steels and loaded with same organic fat mixture. Corrosion pits were found during final inspection by the captain, following load discharge and tanks cleaning operations. The initial inspection was carried out while the cargo ship was harboured at Algeciras (Spain). Because of the observations, the ship remained in harbour for 3 days awaiting inspection by qualified technicians to evaluate the possibility of resuming navigation. The cargo ship was then inspected by one of the authors in collaboration with two metallurgists acting as consultants to the ship owner. The purpose of the inspection was to investigate on damage phenomena, to evaluate the integrity of cargo structures and to decide on two possible options: resuming navigation or immediately ordering a very expensive stop for urgent maintenance. This paper was structured to show documentation (some details have been omitted for propriety reasons) and visual inspection results used to establish failure mechanisms andprobable failure-root causes.These results enabled a decision for the cargo ship to resume travel to next dockyard for maintenance operations.