Detecting Internal Pitting Corrosion: A Smart Pigging Case Study

A European refinery approached us with a challenge: assess the extent of internal pitting corrosion in three sections of 6-inch furnace tubing that had been removed from service due to severe degradation.

The objective? Evaluate the ability of our Mark IV Smart Pig to detect, map and characterise internal pitting – one of the most insidious forms of tube wall degradation in fired heaters.

Risk of Pitting in Process Tubing

Internal pitting is a localised form of corrosion that results in small cavities or “pits” on the internal wall of a pipe. While these pits may appear minor, they can lead to sudden failures due to their depth and concentrated stress points. Accurate detection and mapping are critical to inform maintenance decisions and prevent unplanned shutdowns.

The Smart Pigging Solution

The inspection was conducted at our UK Technology Centre using our Mark IV Smart Pig – an un-tethered, single-bodied device equipped with ultrasonic transducers designed to measure both wall thickness (WT) and internal radius (IR) circumferentially.

Data is stored on-board and later uploaded for post-processing and analysis using advanced software that generates 3D C-scans, tabulated datasets and linear graphical plots.

“The goal wasn’t just to identify pitting, but to verify whether our intelligent pigging system could resolve the severity, density and morphology of such defects in real-world conditions.” – Technical Director, Nick Bettley

Trial Configuration

Three tube sections – designated C28, C29 and A27 – were outfitted with temporary flanges to facilitate closed-loop pigging. Each section underwent:

• Four bi-directional Smart Pig runs for data validation

• Additional comparative inspections using different pig configurations

Initial visual inspection already indicated internal pitting, with Section C29 appearing to be the most severely affected.

Initial Results

The Smart Pig successfully identified and resolved internal pitting across all three sections. Key findings included:

• Section C29 exhibited the most severe pitting, with damage distributed along the full tube length.

• Minimum wall thickness measured at just 3.0 mm, implying pit depths of up to 8 mm.

• C-scan imagery and corresponding graphical plots revealed high variation in wall thickness – consistent with pitting corrosion.

In areas of dense pitting, ultrasonic scatter occurred due to surface irregularity, creating greyed-out regions on the C-scans. These were interpreted as high-density pitting zones rather than data loss.

How pitting impacts Ultrasonic signal

Ultrasound-based inspection tools rely on consistent internal surface profiles to return distinguishable back-wall echoes, this allowing accurate wall thickness measurements to be calculated. However, when pitting is present on the internal surface of the tube, the surface becomes irregular. This irregularity increases the attenuation of the ultrasonic signal due to scattering, ultimately degrading the quality of the A-scans collected.

In this case, knowledge of pit density allowed Cokebusters analysts to interpret these grey areas reliably, ensuring that critical insights were not lost despite signal disruption.

Mapping Every Defect

The Smart Pig not only detected internal pits – it accurately mapped them along both the axial length and circumferential position of each tube section. This level of resolution is essential for planning targeted repairs and for follow-up inspections with handheld UT or alternative NDE methods.

Technology in Evolution: Mark IV to Mark V

Originally launched in 2012, the Mark IV Smart Pig has completed over 700 inspection runs worldwide – from 500m lengths to systems stretching up to 20km.

Building on this success, the Mark V Smart Pig was released in 2020, offering:

• Double the radial measurement capability

• Expanded tooling range for greater application flexibility

Conclusion

This case study reinforces the power of intelligent pigging in identifying and resolving internal pitting corrosion:

✅ Accurate detection of pit locations and severities
✅ Reliable mapping of defects in 3D space
✅ Valuable pre-screening for targeted manual UT follow-ups
✅ Cost-effective insight for maintenance planning and risk mitigation

The Cokebusters Smart Pig delivers a powerful first line of defence in the battle against internal corrosion.

Don’t let hidden corrosion compromise your operations.

Discover how intelligent pigging can offer you a clearer picture of your furnace health—before costly problems arise.

Get in touch with us today: Contact Cokebusters