In 2006, the Integrated Multi-function Corrosion Probe (IMCP), a new multi-function corrosion monitoring system, was designed for double-shell tank 241-AN-107 at the Department of Energy's Hanford Site. As part of the design work, laboratory testing using the IMCP data collection hardware, software, electrodes, and non-radioactive 241-AN-107 waste simulant solutions was conducted to establish baseline IMCP corrosion monitoring instrumentation performance and to characterize data produced by the major modes of corrosion that could be expected in Tank 241-AN-107 supernate simulants, particularly those forms of corrosion that could initiate during off-normal or upset conditions such as pitting and/or stress corrosion cracking (SCC). Testing demonstrated that standard tank waste simulants produce tow rates of uniform corrosion on tank steet as monitored with linear polarization resistance (LPR) and electrical resistance (ER) instrumentation. Additionally, as monitored with electrochemical noise (EN) instrumentation, uniform corrosion is characterized by random fluctuations of electrochemical current noise (ECN) and electrochemical potential noise (EPN) and low Pitting Percentage and Instability values (statistical parameters related to the degree of localized corrosion); pitting corrosion is characterized by the formation of numerous, distinct, relatively short-lived ECN and EPN transients over time, and moderately elevated Pitting Percentage and Instability values; and stress corrosion cracking (SCC), during active crack propagation, is characterized by negative mean currentlevels punctuated by occasional large, long-lived ECN and EPN transients. During SCC, the Pitting Percentage and Instability values are also highly elevated.
The Hanford Site has 177 underground waste tanks that store approximately 253 million liters of radioactive waste from 50 years of plutonium production. Twenty-eight tanks have a double shell and are constructed of welded American Society for Testing and Materials (ASTM) A537 CL1 [Unified Numbering System (UNS) K02400J, ASTM A515 Gr 60 (UNS K02401), or ASTM A516 Gr 60 (UNS K02100) material. The inner tanks of the double-shell tanks (DSTs) were stress relieved following fabrication. One hundred and forty-nine tanks have a single shell, also constructed of welded mild steel, but not stress relieved following fabrication. Tank waste is in liquid, solid, and sludge forms. Tanks also contain a vapor space above the solid and liquid waste regions. The composition of the waste varies from tank to tank but generally has a high pH (>12) and contains sodium nitrate, sodium hydroxide, sodium nitrite, and other minor radioactive and non-radioactive constituents resulting from plutonium separation processes. Leaks began to appear in the single-shell tanks (SSTs) shortly after the introduction of nitrate-based wastes in the 1950s. Leaks are now confirmed or suspected to be present in a number of SSTs.1 The probable modes of corrosion failures for the SSTs are nitrate induced stress corrosion cracking (SCC) and pitting. 2 No leaks have been confirmed in the DSTs, but a study in 1996 identified SCC and pitting as the greatest potential threats to the long-term integrity of the DSTs' Corrosion monitoring and control of the DSTs at Hanford has historically been provided through a waste chemistry sampling and analysis program. In this program, waste tank corrosion is inferred by comparing waste chemistry samples taken periodically from the DSTs with the results from a series of laboratory tests done on tank steels immersed in a wide range of normal and off-normal waste chemistries.4
