Effects Of Aluminum Coating On The Corrosion Fatigue Performance Of X80 Steel And Its Mechanism Of Action

The primary failure mode of drilling riser used in deep-sea is corrosion fatigue, which is mainly caused by the combination of the corroding effect of sea water and the mechanical effect of the ocean current. In order to strengthen the property of the corrosion fatigue performance of X80 steel and lengthen its service life, the high tensile pipeline-steel has been used to produce drilling riser and some corrosion preventing technologies have been adopted also.In this article, a corrosion fatigue testing machine was designed and manufactured, which could simulate the abyssal environment accurately. The testing machine consists of mechanical system, asynchronous motor control system, temperature control system and counting device. This machine can run stably and record the fatigue life precisely.The X80 pipeline steel was chosen as the research object, and the 3.5% NaCl solution was chosen as the corrosion medium. The corrosion fatigue behavior of the specimens of X80 steel and the ones coated by aluminum at high stress (360MPa) and low stress (140MPa) was tested using the corrosion fatigue testing machine. Optical microscope, scanning electron microscope and electro probe were used to analysis the experiment results. The results indicate that, with the decrease of stress, the corrosion fatigue life of the specimens both coated and uncoated by aluminum coating has a great of increase; At high stress, the mechanical effect has a greater effect on the failure of specimens (coated and uncoated), while the corroding effect dominates the failure of the specimens at low stress; The cathodic protection of aluminum coating has a great effect on the corrosion fatigue life of X80 steel.The mechanism of crack initiation and crack propagation was researched using transmission electron microscope. The results show that, both the crack initiation of the surface and the presence of the secondary cracks of the fracture surface have close connection with the reciprocating slip of the crack tip. Under the action of the corrosion medium, the slip of the crack tip is irreversible. Consequently, the corrosion fatigue damage is accumulated at the crack tip, which can cause the initiation of the corrosion fatigue crack, and accelerate the crack propagation. Because of the cathodic protection of aluminum coating, the slip of the crack tip is reversible. Consequently, the corrosion fatigue damage is not accumulated at the crack tip, so the corrosion fatigue life can be increased greatly.