Measuring Steel Thickness Through a Coating in Three Steps
On a coated pipe you want the remaining wall, not a stripped section. Ultrasound offers a route: send the sound through the coating and reflect only from the metal.
1. Not the same instrument as a coating gauge
A coating thickness gauge measures how thick the coating is. A through-coating ultrasonic gauge measures how thick the substrate is, ignoring the coating by passing sound through it. The first serves paint acceptance, the second serves corrosion assessment.
The two get confused often, and buying the wrong one is discovered on site, which is the expensive place to discover it.
2. The principle: one pulse, several echoes
The probe fires an ultrasonic pulse into the part. Sound reflects once at the coating-to-metal interface and again at the backwall. The instrument identifies the backwall echo and multiplies its round-trip time by the velocity in metal.
The difficulty is that the interface echo is often stronger than the backwall echo and is easily mistaken for it. These instruments rely on waveform recognition or a multiple-echo algorithm to pick the right one.
3. Three steps
Prepare. Remove oil, dust, loose rust and any coating that has come away. An uneven surface traps air between probe and coating, and the sound will not enter.
Calibrate. Zero on uncoated substrate of the same material, then set velocity for that material. Steel grades differ slightly, so critical work should be checked against a reference block from the same batch.
Measure. Hold the probe perpendicular, press lightly to expel air, and read once it settles. Average several points at one location to reduce local error.
4. Where it works, and where it does not
It works for in-service corrosion assessment on steel bridges, ship shells, tanks and pipe, and on painted or plated parts.
It does not work where the coating is very thick or is a multilayer composite, because attenuation is severe, or where the substrate contains internal layers that scramble the echoes. Those cases need another method or a local strip-back to verify.
Summary
The method exists so the coating never has to come off. The discipline exists so the number is worth putting into a corrosion assessment.
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