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Nguyen Bich Technical Director CorrOcean Canada, Inc. Calgary, Alberta, Canada

The Elephant According To the Five Fortunetellers - A Lesson in Corrosion Monitoring Extrapolation

Once upon a time, there were five blind fortunetellers who never knew what an elephant was like. As they were led to a real elephant, each was allowed to touch one and only one part of the animal. The first fortuneteller who came into contact with one of the legs described the elephant as a huge meat column. After touching an ear of the elephant, the second fortuneteller believed it was a gigantic fan. The third fortuneteller strongly affirmed his conviction that the elephant was really a big broom as he caressed its tail. The fourth fortuneteller contended that the animal was a large leather ottoman as he comfortably straddled on its back. Not to be outdone, the fifth fortuneteller testified that the elephant resembled a huge snake as he curled around its long nose.

A Common Corrosion Monitoring Pitfall: Extrapolation of Results

More than often, a corrosion monitoring device is used to predict corrosion of the entire system based on the response of the device at some location. Even with the best corrosion monitoring device money can buy, extrapolating the results from the device to the rest of the system can lead to errors of the size of an elephant. Typical reasons for such errors are:

• Wrong type of devices, for example using an LPR (linear polarization resistance probe) where there is no electrolyte.
  
• Wrong positioning, for example placing the ER (electrical resistance) probe's sensing element far away from the vessel wall when corrosion is due to water condensation.
  
• Wrong locations, for example placing the monitoring device near the well head where production fluids are turbulent and corrosion is due to phase settling further downstream.
  
• Misunderstanding the basic principle behind the device, for example using a hydrogen probe, which can only measure hydrogen flux, to determine corrosion rates, which are not always proportional to the measured hydrogen flux.
  
• Interference from solid deposits, hydrocarbons, etc.

How to Minimize Corrosion Monitoring Errors:

• Select the right device(s) for the right corrosion mechanisms.
  
• Place the device(s) at the worst corrosion locations. The locations should be determined from actual corrosion inspection or at least from a reliable corrosion model.
  
• Do not rely on a single device - always correlate to either another device or some other measurements such as pH, residuals, etc or verify against actual inspection.
  
• Understand the corrosion mechanisms before implementing any corrosion monitoring program.
  

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All rights reserved , Nguyen Bich

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