Effect of cathodic protection on the corrosion fatigue crack growth behavior and fatigue life of XS-grade shipbuilding steel

Abstract

Shipbuilding steels are subjected to corrosion fatigue damage phenomena due to conjoint effects of cyclic loading by sea waves and corrosive seawater environment, which often leads to premature failure of ship hull structures. In practice, the impressed current cathodic protection (ICCP) technique is applied to mitigate ship hull corrosion by electrochemically making it cathodic. It is implemented by impressing a direct current to minimize the electrode potential to a predefined protection potential. It is significant to study the effect of ICCP on the corrosion fatigue crack growth rate (CFCGR) behavior and fatigue life assessment of ship hull structures. In this study, CFCGR behavior of high-strength XS-grade shipbuilding steel was investigated in unprotected freely corroding (FC) and protected ICCP (at – 800 mV) conditions using compact-tension specimens at 0.1 Hz frequency in artificial seawater (3.5 wt.% NaCl solution). After crack-closure corrections, it was found that the ICCP-protected specimen (∆KTH = 18 MPa √m) exhibited higher sub-critical or threshold corrosion fatigue crack propagation resistance as compared to the FC specimen (∆KTH = 14 MPa √m). Fractography of the FC specimen revealed the debonding of non-metallic inclusions and formation of micro-pits/secondary cracks as dominant damage mechanisms. ICCP-protected specimen exhibited a signature pattern of parallel secondary cracks and a proportional increase in their density with increasing stress intensity levels during primary crack growth. This fracture morphology indicated the primary-crack branching and further arrest, leading to retarded crack growth rates under ICCP-protection. Fatigue life analysis also confirmed this finding, where the beneficial ICCP-protection effect resulted in at least two times enhancement in fatigue lives from the unprotected FC condition.