Above ground storage tanks are commonly used in industry to store liquids and chemicals. Since many of these substances are corrosive, regulations require routine inspections to ensure the structural integrity of the tanks. Detecting defects is essential for preventing leaks and environmental damage.

The accepted practice for surveying tanks necessitates removing it from service and emptying it so inspectors can enter and assess the interior. This process is hazardous due to residual chemicals and the demand for working in confined spaces. Methods that allow tanks to be inspected from the outside could offer significant cost and time benefits, while also providing a large reduction in the risks faced by inspection personnel.

A new study published in the Open Access journal Sensors presents a novel inspection method for detecting storage tank defects using guided waves generated by magnetostrictive sensors mounted on the exterior surface of storage tanks. The researchers evaluated different data collection approaches and tested the probes on structures with naturally occurring corrosion to confirm their success.

Overall, the method enables non-invasive mapping of corrosion on the interior of storage tanks and provides estimates of defect width. The authors note that it is primarily intended for initial corrosion screening, rather than quantifying the severity of metal loss. Dr. Sergey Vinogradov, author of the study, hopes the technology can provide some practical use to the industry:

“By allowing inspection without emptying the tank, our probe reduces expensive down time and improves inspection safety, by avoiding work in hazardous, confined spaces.”

How do magentostrictive sensors work?

The magnetostrictive effect is a physical property of ferromagnetic materials that causes them to slightly change shape when subjected to a magnetic field. By applying a static magnetic field and a perpendicular time-varying magnetic field, tiny vibrations can be generated in the material, producing guided waves that travel along the surface of the structure.

Guided waves appear as a promising strategy for screening storage tank walls due to their ability to propagate over large distances from a fixed location. As the name implies, their propagation is constrained by structural boundaries, which guide waves along the surface. This behaviour is analogous to water travelling through a canal, the channel walls direct the wave motion along a defined path, unlike waves in the open sea that spread freely in all directions.

The guided waves are reflected by any features along the surface, and these reflected waves are detected and analysed. However, structural features such as welds, corners, and other attachments also reflect waves which can interfere with those from defects. This makes interpreting guided wave signals in storage tanks challenging. Reflected waves from structural features can appear similar to defect signals, increasing the risk of misinterpretation.

This research introduces a new generation of magentostrictive sensors designed to generate guided waves over a wide range of frequencies while improving the ability to distinguish useful defect-related signals from unwanted background interference.

Designing a magnetostrictive sensor for long distance defect detection

The researchers designed an 8 × 8 magnetostrictive array probe consisting of 64 sensor elements arranged in a grid. Flexible joints between the segments allowed the probe to bend, enabling installation on both tank walls and tank bottom extensions.

The long-distance detection performance was evaluated using a test plate containing known artificial anomalies, with the probe positioned six metres away. Different signal processing approaches were tested to determine which provided the most reliable defect detection. Using all possible segment combinations as pulsing and receiving pairs delivered the most accurate localisation and the best detection of small defects.

Overall, these experiments successfully demonstrated the ability of the probe and processing algorithm to detect and locate anomalies at a long distance from the sensor. Being able to locate defects further away from the sensor has been a common weakness in guided wave inspection due to interference from reflected waves caused by random structural features, which complicates data analysis.

Challenges for tank bottom inspections

Non-invasive inspection of tank bottoms has many challenges, including:

• Uneven and heavily rusted tank extension surfaces.
• Small tank extension area available to mount the probe.
• Guided wave energy leaking into the vertical wall of the tank.
• High loss of signal amplitude in the presence of generalised corrosion.
• Variety of tank bottom geometries, without any documentation of these geometries.
• Blind spots along the tank bottom that are difficult to access.

Laboratory models were used to simulate a tank bottom and evaluate the probe’s performance. The results revealed that reflections caused by edges and corners produced relatively small signals compared with those caused by defects.

Additionally, lap welds running perpendicular to the wave direction could be used as a reference indicator allowing anomalies to be assigned a percentage reflection. Understanding these reflection patterns helps distinguish structural features from actual defects and reduces the risks of false detections during inspection.

Field testing on storage tanks

The technology was applied to a 40-year-old water storage tank with significant corrosion damage.

A conventional phased-array (PA) ultrasonic testing probe was used to confirm the results obtained using the magnetostrictive 8 x 8 guided wave probe. Some key findings were highlighted in table 1.

Table 1: Comparison of features between the developed guided wave probe and commonly used PA probes

Overall, both probes were necessary for the accurate assignment and quantification of tank wall loss. The generated guided waves were particularly useful for the rapid screening of large or difficult-to-access areas, such as tank bottoms, while conventional ultrasonic methods remain important for detailed follow-up inspections and precise thickness measurement. Dr Vinogradov highlights this:

“Data from the probe is processed […] it can create a high-resolution map of the structure, showing areas with potential corrosion. This helps users assess the extent of damage to decide when to schedule expensive, time-consuming tank repairs.”

Novel technology for detecting tank corrosion

The study demonstrates a new magnetostrictive sensor capable of screening internal anomalies in storage tanks using guided waves generated from outside the structure.

The 8 × 8 probe can survey several metres of tank wall or bottom from a single location, helping inspectors quickly identify areas that may contain corrosion. While the technique does not directly measure wall thickness, it is well suited for rapid screening before detailed inspection using methods such as phased-array ultrasonic testing.

This approach could significantly reduce inspection time, cost and safety risks associated with storage tank inspections.

More studies on research exploring magnetostrictive materials can be found across the Open Access journals Materials and Sensors. Alternatively, you can access the full MDPI journal list here.