Corrosion ultrasonic inspection method

In general, the most widely used field technologies for inspection are ultrasonic thickness measurement, while for online methods they are corrosion test specimens, electrical resistance, and linear polarization probes. Both of the inspection methods and the first two online methods measure metal loss. The last method measures corrosion rate, but only in a sufficiendy conductive process environment, normally water. 

Measurement sensitivity divides the metal loss methods. Ultrasonics and radiography are usually considered as inspection methods. Typically, ultrasonics has a measurement resolution of around 50 pm (0.002 in.), and radiography 250 p,m (0.010 in.). Consequently, these types of measurements are typically made annually. Corrosion test specimens (coupons) assess metal loss typically over a one to three month interval. Electrical resistance probes, as an automatic coupon, assess metal loss typically over a few hours to a few weeks. 

Designing in a system a corrosion allowance based on the possible loss of a material thickness is one of the simplest methods for dealing with uniform attack. Ultrasonic inspection has been used for decades to measure the thickness of solid objects. A piezoelectric crystal serves as a transducer to oscillate at high frequencies, coupled directly or indirectly to one surface of the object whose thickness is to be measured. The time the wave of known velocity takes to travel through the material is used to determine its thickness. Since the late 1970s,... 

Projection radiography is widely used for pipe inspection and corrosion monitoring. Film digitisation allows a direct access to the local density variations by computer software. Following to a calibration step an interactive estimation of local wall thickness change based on the obtained density variation is possible. The theoretical model is discussed, the limitations of the application range are shown and examples of the practical use are given. The accuracy of this method is compared to results from wall thickness measurements with ultrasonic devices. 

Direct assessment gives a possibility for direct mechanical or ultrasonic measurement of pit depth and wall thickness change due to the corrosion. X-ray radiography is used to inspect welds immediately after they are made. This technique can also show stress corrosion cracks and corrosion pits in the weld area, where the coating has failed to seal the metal structure. A promising method of mapping corrosion on the metal surface is the use of laser scanning equipment. 

Inspection techniques for the detection and measurement of corrosion range from simple visual examination to nondestructive evaluation. Significant technological advances have been made in the last decade. For example, the combined use of acoustic emission (AE) and ultrasonics (UT) can, in principle, allow an entire structure to be inspected and growing defects to be quantified in terms of length and depth. Advanced corrosion monitoring methods have been developed that have both online capabihty and the abihty to detect problems at an early stage. The oil and gas production and petrochemical industries have assumed a... 

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