|Meaningful Corrosion Sensors:
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".
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".
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.
Link to another example from the dark ages,
where you know who's involvement was suspected ...