Experimental analysis of pitting corrosion in offshore structures

A large portion of the marine and offshore structures in the oil and gas industry use different types of stainless steel for the piping and equipment. High corrosion resistance capacity is the key factor for such popularity of stainless steel in offshore structures. A passive layer on a surface is the determining factor in preventing corrosion and preserving the metal from corrosion damage. However, marine and offshore structures perform ordinarily in contact with sea water containing chloride ion (Cl ) conditions. These chloride (Cl ) ions promote pitting corrosion by damaging the passive layer. In this study, AISI 304 and AISI 316L austenitic stainless steels are used to perform pitting immersion tests to demonstrate pitting susceptibility of marine offshore structure materials. The American Society for Testing and Materials (ASTM G48) guidelines for pitting corrosion tests were modified to create real ocean conditions in the laboratory. Changes were made in the defined temperature, salinity, exposure time, and sample orientation. The experiments were designed by applying a Response Surface Methodology (RSM). A quadratic model was used to identify the influence of temperature, salinity, and exposure time on the corrosion rate of both materials. Among all considered factors, it is temperature that highly influences AISI 304 surface, whereas AISI 316L is affected by salinity. It is revealed that AISI 316L is more pitting resistant than AISI 304.