Eventos Anais de eventos
COBEM 2021
26th International Congress of Mechanical Engineering
Corrosion fatigue analysis of grade R4 Steel in simulated seawater.
Submission Author:
Felipe Azevedo Canut , DF
Co-Authors:
Felipe Azevedo Canut, Alda Simoes, Luis Reis, Ivan Bastos, Edgar Mamiya
Presenter: Felipe Azevedo Canut
doi://10.26678/ABCM.COBEM2021.COB2021-1130
Abstract
Offshore structures are often submitted to severe field conditions, combining fatigue loads and corrosive marine environments, which may result in early failure and safety issues. The study is focused on the combined effect of corrosion and mechanical cyclic stresses observed on mooring chains of Floating Production Storage and Offloading (FPSO) units. Grade R4 steel, commonly used in the production of mooring chains, was studied. Corrosion fatigue tests were performed in artificial seawater (3.5 wt% NaCl) under stress-controlled loading using a corrosion cell developed for this purpose. Fully reversed (R = -1) and tension-tension (R = 0.1) loads were applied. In-air fatigue and no-load immersion tests were also carried out for comparative purposes. The results have shown an acceleration of the failure caused by the corrosive environment when compared to the in-air fatigue. The corrosive environment also seems to have ever-increasing importance towards lower stress amplitudes on the observed corrosion-fatigue life. After the failure, the specimens were examined in a scanning electron microscope (SEM) and a confocal microscope. The corrosion attack on the specimens' lateral surface was evaluated using the corroded area fraction as an index. The analysis has shown that the application of mechanical loads causes a greater corrosion degradation level, highlighting the synergistic nature of the corrosion-fatigue phenomenon. The morphology of the corroded surfaces also varied for different levels of fatigue loads. A high density of microcracks and pits were observed for the lower fatigue loads while large secondary cracks and few pits were identified for the higher amplitudes. A closer examination of the corroded surface revealed the formation of micro notches at the bottom of corroded sites for specific load conditions. Therefore, the failure of grade R4 steel in the seawater environment could be divided into two regimes: one dominated by electrochemical effects, observed in lower stress amplitude, and the other dominated by mechanical effects, for higher stress amplitude.
Keywords
Corrosion-Fatigue, Grade R4 Steel, FPSO, Mooring Chains
