PSI - Issue 42

Darko Pastorcic et al. / Procedia Structural Integrity 42 (2022) 374–381 Darko Pastorcic et al./ Structural Integrity Procedia 00 (2019) 000 – 000

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1. Introduction Marine structures, particularly made of steel, are susceptible to corrosion as a consequence of the aggressive surroundings they are meant to operate within, be it outer (atmosphere, water, sea) or inner (ballast water, oil, aggressive cargo), Ivosevic et al. (2019). In general, one of the most damaging forms of corrosion is pitting corrosion. A threat to the structural integrity is posed by the inconsistency of pit distribution on the corroded surface of affected structures. As for the effect of pitting corrosion on marine structures specifically, in a recent numerical study it was revealed that the pit depth and location have a serious impact on the ultimate strength of stiffened ship panels, while the type of collapse is changed by the random distribution of pits compared to the regular distribution, Feng et al. (2020). Numerically studying the effect of pitting corrosion on the hull structural stiffened plate showed the ultimate strength reduction equation of such plates based on corroded volume loss, Zhang et al. (2017). Also, a comprehensive study aimed at developing an advanced technique for prediction of the time-dependent corrosion damage of marine structures was developed using corrosion depth expressed as a function of time, Kim et al. (2020a) and technique was then used in order to determine the damage of ship ballast tanks, Kim et al. (2020b). Shipbuilding steels, with specifications in accordance with the ASTM A131 standard are extensively implemented in the marine industry. The numerical study on ultimate strength of ship steel stiffened panels has been performed in relation to pitting distribution across panels, Cui et al. (2019), showing how the ultimate strength decreases depending on the stochastically distributed pitting depths and densities. In the research presented in this paper, the butt-welded specimens, made out of ASTM A131 AH36 shipbuilding steel, were exposed to natural corrosive environments (tap water, sea water, sea slush) for prolonged time periods (6, 12, 24, 36 months). Once extracted, to obtain hardness, impact energy and engineering stress-strain diagrams, standardized destructive procedures and uniaxial tensile tests were performed. The inspection of the corroded specimen surfaces was done using optical and scanning electron microscopy. The results are given corresponding to the exposure time as well as the environment type. Based on experimental results, numerical analysis comprises model of corroded surfaces and simulation of tensile tests. 2. Experimental procedure 2.1. Material The ASTM A131 standard covers structural steel shapes such as plates, rivets and bars meant to be used in ship construction. The steels in accordance with the ASTM A131 standard with higher strength are graded as either AH, DH or EH. A prevalent choice for commercial ship, bulk carrier, ferry and offshore structure construction is grade AH36, a low-alloy high-strength steel. In Table 1 is the chemical composition and the equivalent carbon content (CE), according to Dueren (1990) : 2.2. Specimens, Exposure to Marine Environment, Mechanical Tests Welding two blanks of 12mm thick AH36 steel, primed with a single V-groove joint, with complete joint penetration, was done by the metal inert gas (MIG) welding technique and using ER50-6 welding wire as filler material. The butt-welded specimens were prepared by cutting them out of the plates with the dimensions determined in accordance with the uniaxial tensile test standard (EN 100002-01), Fig.1. The specimens were exposed to three corrosive environments: tap water (in laboratory), sea water (specimens submerged at 10m below sea level in the north Adriatic), sea slush (same location, specimens were exposed to atmosphere and daily tides) for a period of 6, 12, 24 and 36 months, according to ASTM G52-20 standard. As a control in subsequent testing, a number of specimens were kept corrosion-free. After the exposure, the products of corrosion were removed from the specimen surfaces mechanically, using a soft brush under running water, after which the specimens were rinsed with Table 1. Chemical composition of the AH36 (wt%). C Si Mn P S Cr Cu Al Ti V Mo 0.08 Ni CEq 0.157 0.392 1.501 0.014 0.003 0.03 0.015 0.042 0.003 0.003 0.01 0.2763