# Inspection of Coated Hydrogen Transportation Pipelines
## 3 Pipeline Inspection
### Electromagnetic Acoustic Transducer (EMAT)  
An electromagnetic acoustic transducer (EMAT) consists of a coil positioned at the inner pipe wall surface [11]. EMAT is capable of detecting defects such as \"cracks, weld characteristics and wall thickness variations\" [11]. The method does not require any couplant to generate ultrasound as it uses Lorentz forces [11]. However it needs to be less than 1 mm from the specimen and even so, its \"detection ability and efficiency\" are lower compared to UT [11]. Signal processing methods similar to UT can be used for EMAT as well [11]. Eddy Current Inspection (ET)  
Eddy currents--circular currents generated in a conducting specimen by virtue of \"changes of a magnetic field passing orthogonally to the conductor\"--can be used to determine defects in the pipeline walls as well [71]. This magnetic field is typically generated by a coil connected to AC. When there are defects present, the eddy current is disrupted and the presence of the defects can be established [71]. The depth of penetration of the eddy current is influenced by its frequency [71]. At high frequencies, with a low energy setup, the method only allows detection of skin-level defects [71]. Pulsed eddy current (PEC) can be considered for higher depth coverage, which is achieved by transmitting a \"pulse or step function\" through the coil [71]. An extension to typical eddy current inspection is movement-induced eddy current (MECT), where the change in the magnetic field is generated through the relative motion of a magnet with respect to the specimen [71]. This has the added advantage of generating stronger currents, which therefore allows for greater depth of inspection [71]. It has also been suggested that \"Lorentz force eddy current testing\", which uses the magnitude of Lorentz force experienced against the magnet moving relative to the specimen, can increase the depth to which defects can be detected [71]. This is also said of \"magnetic eddy current\", whereby magnetising the ferromagnetic specimen, increases the penetration depth of eddy currents by increasing the permeability of pipe walls [71]. Another extension of the application of eddy-currents is eddy-current induced thermography (ECIT) [77]. In ECIT, the specimen is induction heated and any cracks present cause changes in the flow of the induced eddy currents [77]. The resultant Joule heating is picked up at the specimen surface using an infrared camera and cracks as small as 0.23 mm in steel are claimed to have been discernible by a human operator [77]. However, the results described are obtained after the specimen was painted to improve its optical properties [77]. Radiography Testing (RT)  
Radiography involves generating an X-ray of the specimen being inspected. There are methods available for pipelines, mainly the \"double wall\" method and the \"single wall\" method [71]. The orientation of the pipes differs during each method, such that X-rays pass through both walls before reaching the detector plate in the former, and only one wall before reaching the plate in the latter [71]. The method chosen depends on the accessibility of the area of inspection among other considerations, with the double wall method requiring multiple exposures before a complete picture of the pipe can be generated [71]. Apart from the apparent safety concerns of radiation, other drawbacks involve the need for the results to be interpreted by trained personnel. A possible technique for improving the visibility of results is suggested in [PERSON] et al. [78], whereby a new technique is used to digitise radio-graphs and extract statistical characteristics as a means to measure the \"granularity of film noise\". Electromechanical Impedance (EMI)  
This technique involves the use of piezoelectric transducers and using the difference in mechanical impedance to identify damage [79].