Caustic Gouging: The Hidden Threat to Fired Heater Tubes

In the energy industry, maintaining the integrity of tubes and pipelines is crucial to efficiency, safety and cost-effectiveness. However, many plant operators and maintenance teams encounter a silent but severe issue known as caustic gouging. This form of corrosion can cause significant damage, leading to costly repairs, operational downtime and even catastrophic failures. In this article, we will explore the causes of caustic gouging, the damage it inflicts and the importance of regular cleaning and inspection to mitigate its effects.

What is Caustic Gouging?

Caustic gouging is a localised form of corrosion which occurs when high concentrations of sodium hydroxide (NaOH) accumulate on the internal surfaces of tubes. It typically happens in areas of high heat flux, such as inside bends where water circulation is restricted. The concentrated caustic solution aggressively attacks the metal, forming deep, irregular grooves or pits on the tube walls.

This phenomenon is often associated with improper water chemistry control, particularly when there is excessive sodium hydroxide in the boiler water. When caustic compounds concentrate in certain areas due to steam blanketing or poor flow conditions, they break down the protective oxide layer on the metal surface, exposing it to accelerated corrosion.

The Consequences of Caustic Gouging

Unchecked caustic gouging can have serious consequences for plant operations, including:

Tube Wall Thinning – The deep gouges reduce the tube’s wall thickness, weakening its structural integrity and increasing the risk of leaks or ruptures.

Reduced Heat Transfer Efficiency – Corroded surfaces and deposits can disrupt heat transfer, leading to reduced boiler efficiency and increased fuel consumption.

Operational Downtime – Unexpected tube failures require emergency shutdowns and repairs, leading to lost production time and higher maintenance costs.

Safety Hazards – A ruptured tube can release high-pressure steam, posing a significant safety risk to personnel and equipment.

Image © Waesseri GmbH

Caustic Gouging Case Study 

Cokebusters was contracted by a client to carry out an in-line inspection of a ten-line Heat Recovery Steam Generator as part of their scheduled maintenance turnaround.

The inspection was executes using the patented Cokebusters smart pigging system for accurate geometric assessment of both the evaporator and economiser coils in ten separate lines, containing two different nominal tube sizes: 3″ and 4″.

The single bodied, neutrally buoyant design allowed the smart pig to travel easily from inlet to outlet and back again, effectively carrying out two inspection runs in one operating procedure. This superior manoeuvrability is a distinct advantage of the Cokebusters design over other inspection tools. Furthermore, the furnace contained a number of short radius bends which the Smart Pig was able to navigate successfully.

During a meeting with the client, it was requested that an attempt be made to improve measurement accuracy and return bend data coverage.

To achieve an increase in measurement accuracy, a complete calibration process review was carried out, resulting in significant improvements in wall thickness and internal radius measurement accuracy.

Moreover, a new prototype alignment brush was designed and developed in an attempt to improve bend data coverage. Having used this new design concept during this inspection, bend coverage was increased to >70% in the 4-inch return bends.

Inspection Results

Six areas of possible localised internal wall loss were observed in the Evaporator 2 & 3 tubes in lines 3, 6, 7, 9 and 10. The possible wall loss was identified by an increase in internal radius measurement from the surrounding mean values.

Unfortunately, it was not possible to obtain wall thickness measurements at these points. This is likely to be due to an irregular interior surface topology resulting in ultrasonic scatter or deflection at the areas of internal loss/corrosion. The increase in internal diameter was clearly visible in the graphical plots and 3D C-scans which accompany our report.

Conclusions

Following the inspection, the tubes that were identified as damaged were removed upon Cokebusters advice. It was recommended by Cokebusters that a metallurgic tube sample was obtained from the affected areas to carry out a full EDX/XRD analysis. The presence of metallic copper, iron hydroxide, magnetite and metal oxide scale would confirm that caustic attack is the mechanism responsible. All defect quantification and location were confirmed by the client using manual UT methods.

Caustic attack is a major concern for HRSG processes, but is avoidable if preventative measures are taken. It’s essential corrosion deposition is minimised by applying a suitable chemical treatment and monitoring arrangement within the feedwater supply system. The prevention of excessive sodium hydroxide concentrations within the feedwater system can be carried out using adequate boiler water treatment and accurate monitoring of the water supply. Regular descaling and inspection operations are recommended for total preventative maintenance.

Caustic gouging is a preventable yet potentially devastating issue in boiler systems. By implementing proactive maintenance strategies – including thorough tube inspection and mechanical cleaning – plant operators can reduce the risks associated with this form of corrosion. Partnering with experts in energy industry maintenance ensures that your furnace tubes remain in peak condition, helping to maintain efficiency, safety and reliability.

If your facility is at risk of caustic gouging or requires professional mechanical cleaning and inspection services, contact us today to learn how we can help protect your critical equipment.