PSI - Issue 60

Vivek Srivastava et al. / Procedia Structural Integrity 60 (2024) 233–244 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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1. Introduction 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 lead to premature failure of ship hull structures. Corrosion fatigue is the primary damage mechanism for ship hull structures, fabricated using high strength steel and welded steel joints(Cheng, 1985; Soares and Garbatov, 1999; Abbas and Shafiee, 2020). Historical instances of fatigue failures called for the corrosion fatigue-resistant design of ship structures by major ship classification societies, where corrosion fatigue strength assessment of shipbuilding steels is of primary concern. Cathodic protection, often in combination with organic coatings, is widely used for corrosion protection of ship hull structures. Two types of cathodic protection methods can used for ship hull protection, namely sacrificial anode cathodic protection (SACP) and impressed current cathodic protection (ICCP). In SACP, the sacrificial anodes are carefully selected based on the electromotive force (e.m.f) series so that it is less noble than the material of the ship structure and hence act as an anode. This anode sacrificially dissolves into the electrolyte, makes the structure to be protected as cathodic, and thereby retards its corrosion. In ICCP, a direct current is impressed between an inert anode and the structure which forces the electrode potential down to an immune region or below a protection potential(P. Schweitzer, 2010). In general, SACP is deployed for protecting small boats, whereas ICCP is preferred for large ships(Xu et al. , 2021a). ICCP is one of the most reliable and cost-effective way to protect against corrosion due to its unique advantages, such as the ability to discharge large current output, automated control of hull potential under varying conditions, produces little drag force, hull weight reduction, and has long service life(Kramar et al. , 2015; Xu et al. , 2021a). Lindley et al(Lindley and Rudd, 2001) have studied the corrosion fatigue behaviour of quench and tempered 500 MPa YS grade and normalized 350 MPa YS grade steels under freely corroding and cathodic protected conditions. They found that the effectiveness of cathodic protection varied between the two steels with both applied stress and level of potential having an influence. Similar effect of ICCP potential levels on the corrosion fatigue performance was reported for AISI 4140 steel by Genel et al(Genel and Demirkol, 2002) and E690 steel by Zhao et al(Zhao et al. , 2018). It has been widely accepted that the protective potential of mild steel or low alloy steel with yield strength less than 550 MPa should be in the range of (-) 800 mV to (-) 1100 mV measured with respect to Ag/AgCl reference electrode. For high strength steels with yield strength higher than 550 MPa, the protection potential should be maintained in range (-) 830 mV to (-) 950 mV(Xu et al. , 2021a). Frequency of loading has also been reported to affect the cathodic protection efficiency by Knop et al(Knop et al. , 2010) who have found an order of magnitude increase in CFCGRs of API-2H and AISI 4130 steels, when frequency level was reduced from 1 Hz to 0.1 Hz or below. This was attributed to higher exposure time of aggressive chloride ions on the crack front at lower frequencies. Ship hulls also typically operate in sea wave loading conditions, having similar detrimental frequency as 0.1 Hz and their corrosion fatigue crack growth behaviour should be categorically studied at such frequency. Above literature survey establishes the significance and unique aspects of ICCP protection governing the corrosion fatigue behaviour of shipbuilding steels. To the best of authors’ knowledge, limited literature is available on the ICCP effect on the corrosion fatigue behaviour, especially the damage mechanisms, of specially designed XS-grade shipbuilding steel having 390-MPa yield strength. With this motivation, the aim of this study was to understand the effect of ICCP on the corrosion fatigue behavior, fracture mechanisms and fatigue life of XS-grade steel. CFCGR (CFCGR) behavior of this steel was investigated in unprotected freely corroding (FC) and ICCP protected (at – 800 mV) conditions using compact-tension specimens at 0.1 Hz frequency in artificial seawater (3.5 wt.% NaCl solution). Cathodic protection potentials for aerobic seawater is recommended in the range of - 800 mV to -1100 mV for low alloy steel used ship steels having yield strength less than 550 MPa. This range for steels having yield strength more than 550 MPa is recommended to be maintained within -830 mV to -950 mV(Genel and Demirkol, 2002; Xu et al. , 2021b). Decreasing the protection potential below the minimum limit, also termed as hydrogen overpotential, is likely to increase the risk of hydrogen evolution and hydrogen occlusion at crack tips, thereby causing accelerated crack growth rates(Kim, Okido and Moon, 2003; Cabrini et al. , 2020; Xu et al. , 2021b).