Preparatory Notes for ASNT NDT Level III Examination - Ultrasonic Testing, UT

6.App-1.2 Application Examples
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TOFD for Weld Root Corrosion and Erosion
For piping and other flow systems, certain conditions exist that lead to
corrosion and erosion in the weld root and the heat-affected zone (HAZ) of
the weld. The contributing factors are often metallurgical, chemical, or flow
related, and the resulting metal loss can lead to failure of the weld/base metal.
The shape of the corroded or eroded weld or base metal can make ultrasonic
inspection extremely difficult to apply, thus impeding accurate detection and
measurement of anomalies.
The time-of-flight diffraction (TOFD) technique proves to be a valid option for
evaluating weld root corrosion and erosion, as well as similar conditions such
as FAC (flow-accelerated corrosion). The goal of any of these inspections is
to accurately measure the wall thickness, the weld, and the HAZ. The
unpredictable shape of the remaining material often makes pulse-echo
ultrasonic inspection ineffective.
http://www.olympus-ims.com/en/applications/tofd-for-weld-root-corrosion-and-erosion/