The Use of ASTM G192 (Tsujikawa-Hisamatsu Electrochemical Method) to Evaluate the Susceptibility of Hanford Tank Steels to Pitting Corrosion

Abstract

The Hanford site contains approximately 55 million gallons (208 million liters) of radioactive and chemically hazardous wastes that are stored in 177 underground storage tanks, of which 150 are single-shell tanks (SSTs) and the remaining are double-shell tanks (DSTs). Traditionally, the cyclic potentiodynamic polarization (CPP) technique (ASTM G61) has been used in the DST integrity program to evaluate the susceptibility of different tank steels to localized corrosion in specific waste chemistries. However, a number of waste chemistry conditions have been encountered where the CPP technique has produced an uncertain or undefined protection potential (E_prot) related to pitting corrosion. The ASTM G192 standard or Tsujikawa-Hisamatsu Electrochemical (THE) technique was originally developed to address similar problems in assessing the crevice corrosion susceptibility of corrosion resistant alloys. The ASTM G192 method basically consists of the sequential combination of three standard techniques (potentiodynamic, galvanostatic, and potentiostatic steps) to allow for a slower and more controlled method of applying anodic polarization to the specimen. This paper investigates the use of the ASTM G192 method to evaluate the pitting behavior of tank steels in various simulated waste chemistries. The protection potentials determined using the G192 method are compared to the CPP technique to highlight the cases where CPP tests could be overly conservative in estimating the protection potential related to pitting.

Introduction

The Hanford site contains approximately 55 million gallons (208 million liters) of radioactive and chemically hazardous wastes arising from weapons production beginning with World War II and continuing through the Cold War. The wastes are stored in 177 carbon steel underground storage tanks, of which 150 are single-shell tanks (SSTs) and the remaining are double-shell tanks (DSTs). The U.S. Department of Energy, Office of River Protection is responsible for retrieving the tank wastes, treating them in order to encapsulate them in glass logs, and then permanently closing the tanks and associated facilities. Current plans call for transferring the wastes from the SSTs into the DSTs over the next 25 years, retrieving wastes from the DSTs and vitrifying them, and closing all tanks by approximately 2048.¹ Such a time line places a great emphasis on maintaining the integrity of both types of tanks. The waste compositions in the storage tanks are grouped according to their main constituents such as nitrite/nitrate-based and carbonate-based chemistries. Most of the wastes are highly alkaline in nature; typically with pH values between 12 and 14. Under alkaline conditions, carbon steels will tend to be passive and undergo relatively slow, uniform corrosion. However, carbon steels can become susceptible to localized corrosion (e.g., pitting) and stress corrosion cracking (SCC) in the presence of certain aggressive constituents, such as chloride and nitrate, even in these passive conditions. ² Cyclic potentiodynamic polarization (CPP) tests have traditionally been used to determine the pitting susceptibility of tanks steels to the various waste chemistries that exist at the Hanford site. However, a number of waste chemistry conditions have been encountered where the CPP technique has produced ambiguous or undefined protection potentials (E_prot) related to pitting corrosion. The difficulty in accurately determining E_prot using the CPP technique is related to the interpretation of the reverse scan of the polarization curve or the hysteresis loops that are attributed to pitting or crevice corrosion repassivation.