Degradation of steels in the hulls of river ships
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DEGRADATION OF STEELS IN THE HULLS OF RIVER SHIPS O. I. Balyts’kyi,1 J. Chmiel,2 and J. Trojanowski2
UDC 621.181:669.018
The surface corrosion of skin plates of the hulls of river ships is caused by the interaction of oxygen and hydrogen with the iron surface at the sites of damage to the protective lacquer coatings on the outer surface of the hull. The inner surface of the hull is subjected to the action of the condensates of water vapor with different degrees of acidity. Furthermore, near the fuel tanks, hydrogen-containing media (mainly, the hydrocarbon fuel) also affect the inner surface of the hull. As a result of hydrogenation, the plasticity of the skin plates of the fuel tanks sharply decreases, which is confirmed by the data of measurements of their impact toughness and microhardness, as well as by the results of microfractographic examinations. The contemporary hull plates subjected to thermomechanical treatment reveal a significant improvement of their plastic characteristics as compared with the requirements of the existing standard specifications.
The process of operation of river ships includes a fairly long period of demurrage caused by the navigation conditions in the waterways (low or high levels of water, winter interruptions, etc.). A significant part of staying ships is concentrated in shallow waters characterized by the repeated and direct contacts with bottom sediments. Quite often the ships are not preliminary prepared for the period of demurrage (e.g., the underwater part of the hull is not conserved by protective coatings). Hence, the risk of damage to the protective coatings in the underwater part of the river ships is much higher than for the sea-going ships [1, 2]. Unlike the sea-going ships whose hulls must have elevated operating characteristics due to the permanent contact with more aggressive media and higher mechanical loads [3], the hulls of the river ships are designed for operation under extremely variable navigation conditions but in the freshwater relatively shallow basins. Therefore, it is necessary to guarantee the possibility relatively small depths of immersion of the hull (but under fairly high loads). Thus, large surfaces of the flat-bottomed hulls of the river ships with low curvature are located under the water surface [1]. The thickness of the skin of the hulls of river ships is, as a rule, much smaller than for the sea-going ships. Indeed, it is necessary to minimize the mass of the hull in view of much lower operating stresses caused by the waves than in the case of sea-going ships. Since the thickness of the skin is smaller, it is more susceptible to the influence of harmful mechanical and corrosion factors [2]. Materials and Experimental Procedure The skin of the hulls is made of killed and semikilled steels of category A with the following chemical composition (recommended by the standard specifications [4]) : Cmax ≈ 0.21%, Mnmin = 2.5 × C, Simax = 0.50%, Pmax = 0.040%, and Smax = 0.040%. The required impact toughness of specimens cut out in the direction of fo
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