The main research object of this article was the corrosion behavior of cast steel (G20Mn5QT) and butt welds between cast steel and Q345D in the seawater environments, in addition, this article also studied the corrosion fatigue behavior of cast steel. The immersion corrosion test of cast steel and the butt welds have been done. Using scanning electron microscopy (SEM) techniques to observe the corrosion surface, the results show the variation tendency of corrosion pits. In addition, the corrosion rate formula was deduced by contrast the weight of specimens before and after the corrosion test. Through the axial tension-compression corrosion fatigue test and the analyses of materials and fracture appearance, the morphologies of the pit could be determined which lead to fatigue crack initiation. Through simplification of the opening width of the pit and the crack tip depth, a simplified method for corrosion pit was obtained.
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