Non-contact, nondestructive hydrogen and microstructural assessment of steel welds

Nondestructive low-frequency impedance has been developed to determine hydrogen content in operating pipeline steel and weldments through a structural coating. A low frequency impedance measurement is similar to a resistivity measurement with a depth function due to the sensor coil reactance. Resistivity introduces variability in impedance measurements because resistivity is a function of the conductivity of the material, the depth of the measurement, and the alloy content. The conductivity, based on the free electron model, is a function of the electronic effective mass, the electron concentration, and the dominating scattering mechanisms, which is altered by such factors as inclusions, microstructure, temperature, and strain. Each of these variables must be separated out to obtain a hydrogen content measurement in operating pipelines (with a structural coating) using low frequency impedance. Techniques used to separate out the variables associated with operating pipeline steels are presented. The use of real-time low frequency impedance measurements to monitor hydrogen content as it diffuses out of a steel weldment is presented and discussed.