In an effort to improve corrosion monitoring and control at the Hanford Site, an eight-channel electrochemical noise (EN) based corrosion monitoring system was designed, fabricated and installed into double-shell tank 241-AN-107 in September 1997. This system is a larger scale version of a prototype system installed in double-shell tank 241-AZ-101 in August 1996and monitors eight three-electrode channels positioned at different elevations in the tank. The system is capable of detecting the onset of pitting and stress corrosion cracking should tank waste conditions change to allow these mechanisms to occur. No finding was provided for troubleshooting and data analysis for nearly a year following installation. When these activities were finally permitted, it was found that most of the data collected was corrupted to some degree by an unknown source of electrical interference. The source of this disturbance has not yet been located. Data collected between 10/97and 1/98do not appear to have been affected by the disturbances and signal interference that plague much of the rest of the data. Both summary statistical data and raw data collected during this time period indicate that uniform corrosion is the dominant active corrosion mechanism intank241 -AN-107. Uniform corrosion rates were calculated form EN data. Despite correction for differences in surface are% uniform corrosion rates calculated from noise data collected on small pin-type electrodes do not agree well with rates measured on larger C-ring type electrodes. Between 10/97and 1/98,C-rings in the supemate showed an average uniform corrosion rate of approximately 7 roils per year (mpy). Pin electrodes in the supemate over the same time period showed a uniform corrosion rate of less than 1mpy. Which value is correct and the source of the difference have not yet been determined.
INTRODUCTION AND BACKGROUND
The Hanford Site has 177underground waste tanks that store approximately 253 million liters of radioactive waste from 50years of plutonium production [1]. Twenty-eight (28) tanks have a double shell and are constructed of welded ASTM A537- Class 1(UNS K02400), ASTM A515-Grade 60 (UNS K02401), or ASTM AS16-Grade 60 (UNS K02100)material. The inner tanks of the double-shell tanks (DSTS)were stress relieved following fabrication. One hundred forty-nine (149) 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 constituents resulting from plutonium separation processes [1-4]. Leaks began to appear in the single- shell tanks shortly after the introduction of nitrate-based wastes in the 1950s. Leaks are now confined or suspected to be present in a significant number of single-shell tanks [1]. The probable modes of corrosion failures are reported as nitrate stress corrosion cracking (SCC) and pitting [2].
Corrosion monitoring of DSTS is currently provided at the Hanford Site by process knowledge and tank waste chemistry sampling [1-4]. Tanks found to be within chemistry specification limits are considered to be safe from excessive corrosion damage. The recent discovery of five low hydroxide (out of corrosion specification) tanks at Hanford indicates that the current system of chemistry sampling is inadequate to support corrosion control. Tank samples are infrequent and analyses are difficult and expensive. Furthermore, waste streams that are exempt from the corrosion control specifica
