Corrosion behavior of steel in Chengdao offshore oil exploitation area

Abstract

AbstractFrom September 1992 to September 1995, the corrosion behavior of three marine steels was studied in Chengdao offshore area by means of individually and electro‐connectedly hanging plates in order to provide data for anticorrosion design of oil‐exploitation structures there.The variation trend of corrosion rate with exposure time showed that the variation curves and degrees of three steels were generally identical and had nothing to do with the exposure time. The whole change trend of corrosion rates in three years showed that the corrosion rates of 20# steel were nearly identical in different positions, each of which was a little lower than that of 16 Mn steel specimens in corresponding positions, also was lower than that of A3 steel specimens in tidal zone and immersion zone. Although 20# steel corroded rapidly at first, its corrosion rate has a decreasing trend in the long‐term service. Therefore, 20# steel is feasible to be applied for a long time in various marine zones, especially nearby the low tidal line and immersion zone. The results of A3 steel, 16 Mn steel and 20# steel by means of electrically connected hanging plate in three years indicated that the corrosion of the steel specimens in the splash zone was the most severe as the result of dry‐wet cycle and macrocell effect. Specimens in the other positions corroded mildly, and that in tidal zone corroded most slowly. The corrosion rate of 20# steel was the highest among the three steels in the test, and the corrosion rates of A3 steel and 16 Mn steel were identical on the whole. When the specimens were hanged electro‐connectedly, the macrocell effect generally tended to increase with exposure time. The highest corrosion rate of A3 steel in the splash zone in the three years was about 0.278 mm/a, while that of 16 Mn steel was about 0.321 mm/a, and that of 20# steel was 0.330 mm/a or so. In contrast, the corrosion rate of the steels in the tidal zone was the lowest in three years, which were 0.052 mm/a (A3 steel), 0.048 mm/a (16 Mn steel) and 0.040 mm/a (20# steel).