(106j) Statistical Decision Making for In-Service Inspection of Underground Waste Storage Tanks

Pitting is a localized form of corrosion that occurs on passivated metal surfaces exposed to aggressive chemical environments.  The pitting process involves the breakdown of the passive oxide film and subsequent anodic dissolution of the metal at a local site, while the remainder of the surface remains protected by the film.

 Deep pits may result in lowering the maximum level of waste that made be stored in the tank and eventually lead to loss of confinement.  Pit depths, established by ultrasonic testing, are subject to variables that cause uncertainty, including the repeatability of metric location, time between measurements, different transducers, and different inspectors. 

 The generalized extreme value (GEV) distribution is fit to the data gathered from ultrasonic testing and statistical projections are made for the full tank wall interior.  A comparison is made with results from other well known statistical distributions. The estimated parameters are used in statistical tolerance limits for bounding pit depth in a waste tank.

 The tolerance limits give an indication of extreme values of pit depth for the current condition of the tank walls.  The upper tolerance limit for pit depth will then be compared to action levels for pitting.  If the upper tolerance limit for pit depth exceeds the action level, more frequent examinations or increased tank surface area inspections are conducted.