Assessment of corrosion resistance, material properties, and weldability of alloyed steel for ballast tanks

Ballast tanks are of great importance in the lifetime of modern merchant ships. Making a ballast tank less susceptible to corrosion can, therefore, prolong the useful life of a ship and, thereby, lower its operational cost. An option to reinforce a ballast tank is to construct it out of a corrosion-resistant steel type. Such steel was recently produced by POSCO Ltd., South Korea. After 6 months of permanent immersion, the average corrosion rate of A and AH steel (31 samples) was 535 g m−2 year−1, while the Korean CRS was corroding with 378 g m−2 year−1. This entails a gain of 29 %. Follow-up measurements after 10, 20, and 24 months confirmed this. The results after 6 months exposure to alternating wet/dry conditions are even more explicit. Furthermore, the physical and metallurgical properties of this steel show a density of 7.646 t/m3, the elasticity modulus 209.3 GPa, the tensile strength 572 MPa, and the hardness 169HV10. Microscopically, the metal consists of equiaxed and recrystallized grains (ferrite and pearlite), with an average size of between 20 and 30 µm (ASTM E 112—12 grain size number between 7 and 8) with a few elongated pearlitic grains. The structure is banded ferrite/pearlite. On the basis of a series of energy dispersive X-ray spectrometer measurements the lower corrosion rate of the steel can be attributed to the interplay of Al, Cr, their oxides, and the corroding steel. In addition, the role of each element in the formation of oxide layers and the mechanisms contributing to the corrosion resistance are discussed.