Managing Localised Corrosion Using Online Wall Thickness Monitoring

by | Jul 26, 2022 | Digital Transformation, Industrial IoT, Measurement Instrumentation

The purpose of online integrity monitoring is to enhance the operational performance of an asset through improved decision making whilst the equipment is live or on-stream, avoiding loss of containment whilst maximising asset productivity. This is achieved using advanced corrosion and erosion monitoring technologies through a combination of:

  • Early detection of corrosion or erosion events, before the risk of loss of containment is too great for remedial action to avoid loss of containment.
  • Monitoring of the severity of corrosion or erosion and measuring the integrity of the equipment in real time, helping determine the root cause of cause of corrosion or erosion.
  • Designing and optimising any required corrosion or erosion mitigation strategy based on above insights, whilst optimising asset performance.
  • Validation that the corrosion or erosion mitigation strategy is adequately controlling corrosion or erosion, while maximising the operational performance of the asset.

The above deliverables from an integrity monitoring system require both high quality measurements and high frequency measurement delivery to desk. This blog post explains the best practice to deliver the above integrity monitoring capabilities in areas of fixed equipment that are at elevated risk of localised corrosion or erosion.

Solution for detecting localised corrosion in high-risk areas

The Rosemount™ Wireless Permasense system consists of many point measurement devices that automatically deliver wall thickness measurements to the desk of operators, corrosion engineers or inspectors. Point measurements allow high resolution and sensitivity to very small amounts of corrosion or erosion, in a similar way to manual ultrasonic testing (UT).

Corrosion Sensor ArrayOne benefit of permanently installed sensors like Rosemount Wireless Permasense systems over manual inspection is that the measurement location is always the same for consecutive measurements since the device is left mounted on the pipe. This, and the lack of variability in technician and equipment used, giving just 2.5 microns of repeatability between measurements. The high quality measurements are delivered automatically at a frequency that enables very small amounts of wall loss to be detected very quickly.

Rosemount Wireless Permasense sensors can be installed sparsely across the fixed equipment or installed in array formation in high-risk areas where the exact location of the attack is not yet known. The Rosemount Wireless Permasense system is regularly used to detect and measure localised (ie, non-uniform) corrosion or erosion attack.

Best practice to maximise the probability of detection of localised wall loss is to focus on monitoring area(s) of high risk, through knowledge of inspection history, fluid properties, metallurgy and geometry of the fixed equipment. A grid of sensors is installed in these high-risk areas. The number of sensors required to detect the localised attack depends on how early in the progression of the attack that detection is required, not the size of the individual corrosion pits.

An example of such a high-risk area with various stages of the localised attack within that area are shown below:

Stages of localised corrosion attack with increasing percentages of high-risk area affected.


Research performed by Imperial College London has shown that a sufficient number of point monitoring devices can be used to very confidently detect localised corrosion. The graph below defines the number of sensors required within each high-risk area, for a desired probability of detection at a given stage of attack. Each curve represents one of the percentages of area under attack shown in the diagram above.

Corrosion Chart_Detection and Number of Sensors RequiredFrom the green curve, we can see that 20 sensors installed within a monitored high-risk area experiencing corrosion attack over just 15% of the area would result in a confidence in detection of >95%. By the time this corrosion activity has spread to 25% of the area, 20 sensors will provide a near-certain detection of the corrosion activity (>99% confidence of detection).

Note that, at all times, each sensor is providing high quality wall thickness measurements at a frequency which enables rapid detection of corrosion activity and the calculation of reliable corrosion rates. This means that as soon as the corrosion occurs, it can be detected and then quantified through direct measurement of the pipe work integrity. This affords detection and measurement of corrosion activity long before the risk of corrosion induced failure rises to unacceptable levels in the monitored high-risk areas.

In addition to the detection and measurement of corrosion activity, the system can be employed to inform operators that corrosion activity is under control at the monitoring locations. Therefore a Rosemount Wireless Permasense system installed to detect localised corrosion is equally good at validating the effectiveness of any mitigation program that is put in place to control the corrosion.

Understanding the corrosion over time can be as useful as understanding the damage distribution across the system, especially to aid with root-cause analysis and online corrosion/erosion mitigation optimisation. The system deployed in grids at high-risk locations delivers the best available corrosion and erosion detection and measurement capabilities for localised attack.

Reliable point measurement technology

Whilst online thickness monitoring solutions such as the Rosemount Wireless Permasense system are best practise for more generalised corrosion mechanisms, some operators do not consider using such a solution for localised corrosion for various reasons.

This blog post explains that point measurement technology can reliably be used to detect localised metal loss, when installed appropriately as part of an effective inspection strategy. Emerson is the market leader for non-intrusive corrosion monitoring technology, with over 25,000 sensors deployed across 350 facilities worldwide. To learn more, visit

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The opinions expressed here are the personal opinions of the authors. Content published here is not read or approved by Emerson before it is posted and does not necessarily represent the views and opinions of Emerson.

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