Correlation between hardness of gray cast iron and graphitic corrosion

Abstract

Gray cast iron has been used as a piping material for cooling water system of nuclear power plants. In that system, graphitic corrosion is frequently occurred in long-term operation. Graphitic corrosion is occurred by galvanic corrosion mechanism between graphite flakes and iron matrix. Iron atoms are leached from metal and porous, weak graphite structure only remains. It is difficult to detecting because it retains its original dimension. So it can cause sudden failure of cooling water system pipes. To detect graphitic corrosion in advance, hardness measurement was suggested for inspection method. But there is no standard for estimate the degree of graphitic corrosion from hardness decrease. So it must be confirmed that there is some correlation between hardness decrease and degree of graphitic corrosion. In this study, correlation between degree of graphitic corrosion and hardness change of gray cast iron was explored by accelerated graphitic corrosion and hardness measurement. First, artificial graphitic corrosion test was conducted to make various depth of graphitic corrosion specimen. Specimens were prepared by static immersion test in the aqueous solutions containing chemicals like FeS, HCl, and Na2SO4 at 75oC. And then, surface hardness was measured by portable Leeb hardness tester. After that, specimens were cut perpendicular to the corroded surface, and depth of graphitic corrosion was measured at cross-section using optical microscopy. Using scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS), analysis on graphitic corrosion layer was conducted. In addition, indentation force was obtained when indenter goes down along the depth of graphitic corrosion using micro-Vickers hardness tester. Measured hardness data were correlated with depth of graphitic corrosion layer, and it shows only 10㎛ of graphitic corrosion layer makes a half of decrease in surface hardness. Since then, hardness decrease gradually and reaches minimum valu...