Corrosion Behavior of Rusted 550 MPa Grade Offshore Platform Steel

 The corrosion behavior of a rusted 550 MPa grade offshore platform steel in Cl- containing environment was investigated. The results revealed that the corrosion process can be divided into initial stage in which corrosion rate increased with accumulation of corrosion products and later stage in which homogeneous and compact rust layer started to protect steel substrate out of corrosion mediums. On the contrary, structural analysis of rust layers by X-ray diffraction showed that α-FeOOH increased from 1. 3% to 3. 6% and the Fe3O4 increased from 1. 0% to 1. 5% while γ-FeOOH reduced slightly according to corrosion time increased from 30 cycles to 73 cycles. The results of electron probe microanalysis indicated that Cr concentrated mainly in the inner region of the rust, inner/outer interface especially, whereas Ni and Cu were uniformly distributed all over the rust after 73 corrosion cycles. According to electrochemical measurements, it was found that the corrosion rate of rusted steel reduced from 0. 61 mm/a after 45 cycles to 0. 34 mm/a after 85 cycles, 44. 3% reduction approximately, and Rrust values increased with increment of corrosion time. Therefore, formation of compact inner rust layer and enrichment of Cr are important to improve corrosion resistance of offshore platform steel.