Corrosion Analysis of Carbon Steel Transfer Line Encasement Pipe Sections Removed from Nuclear Waste Transfer Service at the Hanford Site

Abstract Borescope visual inspections of buried pipe-in-pipe slurry transfer lines in the tank farms at the Hanford Site revealed corrosion and standing water in the annulus space between the encasement and primary pipes. Both the primary pipe and encasement were constructed from schedule 40 seamless carbon steel (ASTM A106 Grade B). Three encasement sections were cut from the transfer lines, and corrosion analysis was performed to assess the level and possible causes of encasement interior surface corrosion. The solid debris/material collected from the pipe interior surface was predominantly corrosion products, but it also contained a small amount of soluble fluoride, chloride, nitrate, acetate, and sulfate that could have facilitated the corrosion process and increased the corrosivity of the water. One of the three encasement sections had the deepest and most severe pitting corrosion induced by the standing water and the presence of O2 for possibly over a decade. The maximum pit depth was about 23% of the wall thickness on the interior surface. Residual water used for hydrotesting the lines may have remained in the annulus since the beginning of transfer line service, initiating corrosion. Water and corrosion product accumulation at the later stage of corrosion and the presence of bacteria at the bottom of the transfer line exacerbated corrosion. The other two of the three encasement sections had less corrosion.