Southwest Research Institute^® (SwRI^®) News

San Antonio, Texas -- September 11, 2000 -- Scientists at Southwest Research Institute^® (SwRI^®) have modified the cylindrically guided wave technique to detect and characterize borated water corrosion in the all-thread bolts used by the nuclear power industry in heat exchanger flanges.

The only technique for detecting corrosion on these bolts before now was to visually inspect for discoloration. Suspect bolts were then physically removed for a closer inspection -- at great expense.

"In the nuclear power industry, heat exchanger flanges are clamped together by using a nut on each end of the all-thread bolt and tightening the nuts to the proper tension," says Dr. Glenn M. Light, director of the SwRI Nondestructive Evaluation Science and Technology Department. "When used this way, most of the bolt is hidden by the heat exchanger flange, and it is difficult to inspect for corrosion."

The cylindrically guided wave technique (CGWT), a zero-degree longitudinal wave method that allows ultrasonic energy to be injected into metal samples, measures changes in the characteristics of the ultrasonic energy propagation that signal the presence of corrosion and cracks.

Most types of corrosion leave rough, jagged surfaces which destroy the mode conversion signals that make CGWT effective. In this application, however, the water running through the heat exchanger tubes is borated (mixed with borax or boric acid) to adjust the pH level which, in turn, minimizes corrosion. In the event of a flange leak, this borated water can flow over the threads of the bolt and corrosion can occur over time, resulting in a loss of bolt material. The corrosion that results is unique because as the borated water corrodes the all-thread material it leaves a very smooth, almost polished surface. This smooth surface allows the ultrasonic mode-converted signals to form and produces the information needed to assess damage.

Under sponsorship from the Southern Nuclear Operating Company of Alabama, SwRI scientists evaluated CGWT on 22 bolts with varying degrees of simulated symmetric and asymmetric corrosion.

This method can detect and characterize borated water corrosion levels on the order of 10 percent of the bolt diameter, in bolts approximately 20 inches long and 1.5 inches in diameter. The ultrasonic data is monitored using an oscilloscope. By comparing the location of the end of the bolt signal and the time differences between the subsequent signals observed on the oscilloscope, the remaining diameter and amount of corrosion damage can be determined.

"We developed this technique about 20 years ago for inspecting any bolt except all-thread," says Light. "Borated water corrosion presents unique circumstances that enable inspectors to use CGWT in a slightly different manner than has been used previously."

SwRI is an international leader in the development of new nondestructive evaluation technologies. The Institute provides clients from industry and government with inspection techniques that use such methods as cylindrically guided waves, eddy current technology, and magnetostrictive sensors.

For more information on detecting borated water corrosion contact Maria Martinez, Communications Department, Southwest Research Institute, P.O. Drawer 28510, San Antonio, Texas, 78228-0510, Phone (210) 522-3305, Fax (210) 522-3547. 

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