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Corrosion Detection and Measurement Using Eddy Current Methods Print E-mail


Corrosion is the deterioration of a metallic material by chemical (or electrochemical) attack. This is normally caused by the environment (most often water), and sometimes by another material.

The corrosion products generated are not electrically conductive, so we will be measuring the thinning of the material under test.
Image of Corrosion Detection and Measurement Using Eddy Current MethodsAn eddy current instrument and probe can be used for detection, and, using a specific procedure, it is often possible to perform a quantitative measurement.

There are several types of corrosion:
  • Uniform corrosion that extends evenly across the surface
  • Pitting corrosion that is uneven has smaller deep areas (pits)
  • Exfoliation corrosion that moves along layers of elongated grains
  • Intergranular corrosion that grows along grain boundaries
In most situations, particularly in the aerospace industry, the material under inspection will be a type of aluminum alloy, so we will concentrate on this application. Corrosion in steel is not normally detectable with eddy currents, although there are some exceptions, such as the use of the reflection remote field technique, mostly for tubing inspection.

Equipment Selection

Instruments. For corrosion detection it is best to choose instruments with high gain and low drift, preferably operating in the reflection (transmit-receive) mode. The availability of a low-pass filter (LPF) is also a useful feature that reduces the background noise that appears with some probes and often appears at high gain settings.

Probes. The best probes are usually reflection spot/surface types with small diameters below 0.5" (12mm), although larger sizes are sometimes used to cover larger areas. Special low noise, high gain reflection models are specially designed for aluminum corrosion detection.

Reference Standards. Calibration can be done using a step wedge type of reference standard of similar conductivity and thickness as the areas to be inspected. Areas of reduced thickness of 10%, 20%, and 30% are the most common.
 
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