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Meaningful Corrosion Sensors:

In the use of sensors for direct corrosion measurements, it is important that the sensors are actually representative of the structure or component that is being monitored. Seasoned professionals have often remarked that this is often a case of "the devil is in the details".

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In the words of one professional: "When you say the corrosion sensor is flush, make sure it is flush". For example, localized fluid turbulence created by a protruding sensor can potentially have a major impact on the damage mechanism(s) and the rate of damage.

In the use of corrosion coupons (representing simulated coating defects) for monitoring the performance of CP systems on buried pipelines, the following details may be important:
  • Surface condition (presence of scale and corrosion products, surface finish).

  • Temperature (will the coupon's temperature match that of the external pipe wall ?).

  • Degree of soil disturbance.

  • Metallurgical condition (chemical composition and microstructure).

  • Coupon size.

  • Position and orientation of coupon relative to pipeline and CP current flow.

  • Integrity of the electrical connections to the coupon and the pipewall, over many years of buried service.

  • Perhaps, even stress levels when monitoring stress corrosion cracking.


For successful corrosion monitoring programs, attention to sensor details is mostly essential - many failures can be retraced to shortcomings in this area. If the corrosion sensor is fundamentally flawed, investments in sophisticated monitoring hardware, computing systems, data transfer and software can be largely wasted. While sensor design, manufacture and installation can clearly be complex,  formal guidelines or standards are, unfortunately, rarely available. Therefore, at times, sensor designs and positioning strategies simulating worst case conditions may be deemed prudent.

Editor's note: also consider the saying "it only takes a mouse in the elephant show to ruin the whole circus".

 

References/Literature:

C.D. Stears, O.C. Moghissi and L. Bone III: "The Use of Coupons to Monitor Cathodic Protection of an Underground Pipeline", Materials Performance, February 1998, pp.23-31.

R.A. Gummow: "Using Coupons and Probes to Determine Cathodic Protection Levels", Materials Performance, August 1998, pp.24-29.

Links:

Link to another example from the dark ages, where you know who's  involvement was suspected ...

    

Copyright 2000-2005 M. Tullmin, All Rights Reserved
E-mail: tullmin@sympatico.ca

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