Alloying Effects of Molybdenum and Niobium on Chromium Bearing Low Alloy Steel for Water Ballast Tank

抄録

This study examines the effects of chromium (Cr), molybdenum (Mo) and niobium (Nb) alloying on the corrosion properties of low alloy steel for water ballast tank using electrochemical tests for 15 days in artificial seawater and surface analyses. The EIS and LPR tests revealed the outstanding corrosion resistance Cr+Mo+Nb bearing steel. An SEM analysis after 15 days immersion in the artificial seawater indicated that, among the tested specimens, the Cr+Mo+Nb bearing steel formed the smoothest rust layer with the least cracks. An EPMA analysis for the cross section of the rust layer confirmed that the Nb elements of the Cr, Cr+Nb, and Cr+Mo+Nb bearing steels were enriched at the rust layer; however, the Mo element was enriched only at the rust layer of the Cr+Mo+Nb bearing steel, and not at that of the Cr+Mo bearing steel. This finding is because the small grain size of the Cr+Mo+Nb bearing steel results in the formation of more active sites, where a dissolution and precipitation of the alloying elements occurs. For this reason, the Cr, Mo, and Nb elements are concentrated at the rust layer of the Cr+Mo+Nb bearing steel, thereby improving the corrosion resistance of low alloy steel.