The Effect of Cathodic Protection on the Corrosion Fatigue Behavior of Carbon Steel In Synthetic Sea Water

Abstract

Certain questions can be asked about the steel used on offshore platforms: What effect do the various properties of the steel have platforms: What effect do the various properties of the steel have on its corrosion fatigue behavior in sea water? What effect does cathodic protection have on this behavior? Is there a combination of properties, cathodic protection, and coatings that will yield optimum properties, cathodic protection, and coatings that will yield optimum fatigue performance? Here are some answers. Introduction Metallurgical study of structural failures in four offshore platforms has shown that the primary mode of failure was brittle fracture and that failure occurred almost exclusively near weld areas. Although some laminations in the steel and some weld failures were found, it has been reported that the primary cause of failure was the extremely high notch brittleness of the steel used in these structures. To prevent recurrence of this type of failure, changes in steel chemistry and microstructure have been recommended that greatly improve the mechanical and metallurgical properties of the steel. It has been further properties of the steel. It has been further recommended that a notch toughness specification be included in the over-all steel specifications. The recommended changes should significantly reduce the occurrence of failures of this type in future platforms. Once brittle failures are eliminated or reduced to an acceptable minimum, it is necessary to consider the problem of fatigue failures. The combination of a corrosive medium (aerated sea water) and cyclic loading due to the wind and wave action makes this type of failure a very real possibility. The critical area will be the welded tubular joints. To evaluate the possible contribution of this failure mode, several questions need to be answered: What effect do the various compositional, microstructural, and mechanical properties of the steel have on its corrosion fatigue behavior in sea water? What effect does cathodic protection have on the corrosion fatigue behavior? Is there a combination of properties, cathodic protection, and coatings for optimum fatigue performance? protection, and coatings for optimum fatigue performance? Corrosion Fatigue Behavior of Carbon Steel It has been conclusively shown that most metal treatments that are of great benefit in extending air fatigue life are usually of very limited help in a comoske environment. This is graphically illustrated in Fig. 1 for fresh water. The results show that the fatigue strength in air (endurance limit at 10(7) cycles) is approximately proportional to tensile strength. The fatigue strength of the carbon and low-alloy steels in water, however, is almost independent of the tensile strength. All the results on carbon steels, covering a range of carbon content from 0.03 to 1.09 percent, both annealed and hardened and tempered, showed corrosion fatigue strengths between 12,000 and 22,000 psi. For the low-alloy steels, with tensile strengths ranging from 80,000 to 224,000 psi, the range was 11,000 to 27,000 psi. JPT P. 283