Thickness gauging for corrosion monitoring

Corrosion control consists of verifying that the thickness of a wall (pipe, tank, hull, boiler) does not fall below the minimum safe value. Ultrasonic thickness gauging obtains this measurement in a non-destructive, fast and repeatable way, accessing the structure from one side only.

Compared with radiographic methods or direct measurement after dismantling, the UT technique is economical, portable and suited both to periodic field inspections and to systematic monitoring of thousands of points.

How ultrasonic thickness measurement works

An ultrasonic thickness gauge emits a short high-frequency acoustic pulse that travels through the material, reflects off the opposite wall and returns to the transducer. The instrument measures the time of flight of the echo and, knowing the speed of sound in the material, calculates the thickness using the relation s = v × t / 2.

For corrosion monitoring, dual element transducers are used (one element transmits, one receives): they are ideal on irregular, rusty or high-temperature surfaces, because they tolerate poor couplant quality and corroded inner walls better. A thin layer of couplant gel ensures the transmission of ultrasound between probe and material.

Typical resolution is 0.01 mm and the measuring range extends from fractions of a millimetre up to several hundred millimetres, depending on the material and the probe frequency.

Why it is ideal for corrosion monitoring

Corrosion reduces thickness from the inside or beneath coatings, often at points that cannot be seen. This is exactly where UT measurement comes in: it detects the remaining wall loss before it becomes a leak or a structural failure.

• Access from one side only: there is no need to reach the inside of the pipe or tank.
• Speed: hundreds of points per shift, with immediate readings.
• Scan mode: automatically shows the minimum thickness along a path, to locate pitting.
• Trends over time: with the data logger, successive inspections are compared and the corrosion rate and remaining life are estimated.

These features make UT thickness gauging the reference tool in preventive maintenance and integrity management programmes for plants and infrastructure.

Measuring through coatings

Many surfaces are painted or coated. Echo-Echo (thru-paint) mode measures the time between two successive echoes coming from the base metal, excluding the thickness of the coating: this gives the true metal thickness without removing the paint. It is a valuable function, as the surface does not have to be prepared at every measuring point.

How to choose a thickness gauge for corrosion

The main criteria are the measuring range and resolution, the presence of a data logger for recording and downloading data, the availability of thru-paint mode and high-speed scanning, and the robustness of the instrument for field use.

The Dakota NDT range distributed by RODER covers these needs, from the ZX series (basic and with data logger) up to the CMX and MX series with A-Scan and B-Scan display: for technical details and models, please refer to the catalogue pages below.

Related products

RODER instruments for this topic. Go to the catalogue pages for models, technical data and documentation.

Frequently asked questions