Corrosion survey

Just about anything made of common structural metals is subject to corrosion. An important challenge in many industries is measuring remaining wall thickness in pipes, tubes, or tanks and pressure vessels that may be corroded on the inside surface. This corrosion is often undetectable by visual inspection without cutting, disassembling, or accessing the inside of the pipe or tank. Ultrasonic testing is a widely accepted nondestructive method to perform this corrosion measurement, and ultrasonic testing of corroded metal is usually done with dual element transducers and ultrasonic thickness gages designed to measure the remaining wall thickness.

What is a Corrosion Thickness Gauge?

A corrosion thickness gauge is a type of nondestructive corrosion detection equipment used to perform accurate corrosion measurement by determining the remaining wall thickness of metal structures that have been exposed to corrosion, such as pipes, tanks, pressure vessels, or structural components. Instead of relying on visual inspection, which can miss internal corrosion, this instrument typically uses ultrasonic technology to send sound waves into the material and measure the time it takes for echoes to return, allowing it to calculate the actual metal thickness even through coatings or rough surfaces. By providing precise thickness data, corrosion thickness gauges help identify areas of material loss due to corrosion, support ongoing corrosion detection efforts, and ensure the safety and integrity of critical assets without damaging the part being inspected.

Why Is It Important to Inspect Metals for Corrosion?

Structural steel beams, particularly bridge supports and steel pilings, are also subject to corrosion that reduces the original thickness of the metal. If undetected over time, corrosion will weaken walls and can cause dangerous structural failures. Both safety and economic considerations require that metal pipes, tanks, or structures that are susceptible to corrosion are inspected regularly. Ultrasonic thickness gauges are corrosion measurement equipment that enable you to accurately detect potential internal corrosion without damaging the metal and while only having access to one side of the surface.

How is Corrosion Measured with Ultrasonic Thickness Gauges?

All gauges designed for corrosion applications support established corrosion detection methods by measuring the round-trip transit time interval to the first back wall echo. Advanced instruments can also measure the interval between successive multiple echoes, enhancing overall corrosion measurement techniques in challenging environments. They use signal processing techniques that are optimized for detecting the minimum remaining thickness in a rough, corroded test piece. This allows these corrosion thickness gauges to calculate the remaining metal wall thickness without being affected by its coating.

The irregular surfaces that are frequently encountered in corrosion applications give dual element transducers an advantage over single element transducers in many corrosion detection scenarios. Dual element transducers incorporate separate transmitting and receiving elements, mounted on delay lines that are usually cut at an angle to the horizontal plane (the roof angle), so that the transmitting and receiving beam paths cross beneath the surface of the test piece. This crossed-beam design of duals provides a pseudo-focusing effect that optimizes the measurement of minimum wall thickness in corrosion applications.

Duals will be more sensitive than single element transducers to echoes from the base of pits that represent the minimum remaining wall thickness. Also, duals are often more effective on rough outside surfaces. Couplant trapped in pockets on rough sound entry surfaces can produce long, ringing surface echoes that interfere with the thin material resolution of single element transducers. With a dual element transducer the receiver element is unlikely to pick up this false echo from the surface. Finally, most duals can make measurement on material at elevated or high-temperature that would damage single element contact transducers.

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