When the far wall is eaten away by pitting, the wall no longer has a single thickness. The measurement serves to find the worst point, and to find it you have to look for it.
- ✓ Dual element probes, tolerant of irregular surfaces
- ✓ Echo-echo mode to read through the coating
- ✓ Fast scanning so as not to miss the thinnest point
- ✓ A-Scan and B-Scan to validate doubtful readings
Corrosion alters the measurement on two fronts. The first is the outer surface, the one the probe rests on: flaking rust, scale and pitting make contact discontinuous, and without uniform coupling the signal becomes unstable or absent. The second is the far wall, that is, the internal surface of the component, which corrosive attack has made irregular: an eaten-away wall no longer reflects the pulse back to the probe in an orderly way, but scatters it in several directions.
An important conceptual difference from measurement on a sound part follows. On a corroded wall there is no single thickness value: there is a distribution of remaining thicknesses, and what matters for integrity is the minimum. A single reading, however stable, says nothing about the worst condition of the component; a scan of the area is required, with closely spaced readings and an explicit search for the most thinned point.
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The probe suited to this condition is the dual crystal one. Having one element dedicated to transmission and one to reception, it copes better with non-flat surfaces and irregular far walls, and keeps a usable reading where a single crystal probe would lose the signal. This is why dual element transducers are the established standard for corrosion inspection on piping, tanks and steelwork.
How the inspection is set up
| Aspect | Measure to take |
|---|---|
| Surface preparation | Removal of loose rust and deposits, without cutting into sound metal |
| Probe type | Dual element transducer, suited to irregular surfaces and far walls |
| Coating present | Echo-echo mode to measure the metal only, range 2.54-152.4 mm |
| Measurement strategy | Scan of the area in search of the minimum, not a single reading |
| Validation | A-Scan or B-Scan to recognise unreliable readings |
Coatings, scanning and recording
A corroded component is almost always painted as well, and the coating added to the metal would falsify the reading. Echo-echo mode solves the problem by measuring the interval between two successive backwall echoes, that is, the metal only: on the CMX series the declared range is 2.54-152.4 mm.
For the search for the minimum, what counts instead is the update rate: the scan function of the CMX series acquires up to 250 readings per second, so that as the probe is moved over the area the lowest value does not escape. The data collected is then archived in the instrument’s data logger for comparison over time.
Recommended instruments
| Product | Why it is relevant |
|---|---|
| Dakota CMX10-DL | Complete corrosion gauge, with echo-echo mode and fast scanning |
| Dakota MX2-DL | A-Scan and B-Scan to validate readings on irregular far walls |
| Transducers for corrosion thickness gauges | Dual element probes, suited to corroded and non-flat surfaces |
| DakMaster | Data download and reporting for comparing campaigns over time |
Frequently asked questions
Not necessarily. In echo-echo mode the instrument measures the interval between two consecutive backwall echoes and returns the metal thickness only: on the CMX series the declared range for this mode is 2.54-152.4 mm.
Because on a corroded wall the remaining thickness varies from point to point and what counts is the minimum. A scan of the area with closely spaced readings is needed, not an isolated value.
By looking at the signal rather than the figure alone. With A-Scan or B-Scan a regular backwall echo can be told apart from an anomalous reflection produced by an eaten-away far wall, which might otherwise appear as a plausible value.
Back to Materials and difficult cases or write to us from the Contact page.
Further information and contacts
Systems division and sensor integration partner : www.roder.it
More information about RODER SRL : about us
Contact for general information : info@roder.it
Contact for information about these products : info@roder.it
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