What Does Linear Scanning Mean?
Linear scanning is a method to test for corrosion and metal loss on the surfaces of equipment. A linear scan is generally performed to determine the extent of corrosion on the bottom plates of storage tanks containing oil, petroleum, chemicals or any other acidic compound. Linear scanning uses guided ultrasonic waves and electromagnetic acoustical transducers (EMAT).
Corrosionpedia Explains Linear Scanning
Water and waste materials often accumulate on the bottom plates of oil and gas tanks, which can create an acidic environment and cause pitting (corrosion) of the metallic surface. Therefore, regular inspections are necessary.
An early method was the ultrasonic straight beam technique using longitudinal wave mode. However, this technique had some disadvantages:
- The surface first needed to be cleaned.
- A high density scan was performed
- Large area scanning was costly and time-consuming.
These limitations have been resolved by linear scanning through ultrasonic techniques using guided plate waves and transducers (EMAT). This technology can linearly scan a large area at once. Here the horizontally polarized shear wave modes are transmitted and received by EMAT units generating SH waves. These SH waves can easily propagate over a distance of one meter and can penetrate several millimeters inside the metal's surface. If any corrosion is found on the surface, then the wave is partially reflected back and this area is noted by the instrument. The guided wave technique can quickly perform the linear scanning of a metal surface as long as 500 mm with a search unit offset of 20-30 mm, depending on the lateral sound field properties of the selected search unit.