Application: Precise measurement, through a layer of paint or similar coating, of the residual thickness of the metal wall of pipes, tanks, beams, ship hulls and other structures.
Background : In many industrial and petrochemical sectors, maintenance requirements call for measuring the residual thickness of metal walls affected by corrosion. Often, this procedure has to be carried out despite one or more layers of paint, or similar non-metallic coatings, covering the part to be inspected. When this inspection is carried out using conventional ultrasonic thickness gauges, the presence of paint causes measurement errors; more often than not, the apparent metal thickness is increased by more than twice the thickness of the paint layer, due to the very slow propagation speed in the paint. There are two possible solutions to this problem: measurement between echoes and THRU-COAT measurement.
Equipment: Between-echo and THRU-COAT measurement functions are available on two models of Olympus corrosion thickness gauges: the 38DL PLUS and the 45MG. Typically, measurement between echoes is performed with one of the following separate transmit-receive probes : D790, D791, D797 or D798. THRU-OAT measurement, on the other hand, requires one of two specialized separate transmit-receive probes, the D7906-SM or D7908.
Separate transmit-receive probes have long been the industry standard for corrosion control applications, due to their superior reactivity to rough surfaces or pitting corrosion. As a result, they are generally recommended for all common corrosion control applications. Single-element delay-line probes can be recommended for inspection of smooth, painted metal parts requiring high-precision measurements.
Operating mode Typical transverse wave propagation velocity in steel is around 5900 m/s, whereas it is less than 2500 m/s in paint and similar coatings. Conventional ultrasonic equipment will measure the total thickness of the painted metal wall, erroneously applying the propagation velocity in steel to the paint layer. As a result, the paint layer will be considered 2.35 times thicker than it actually is (ratio between the different propagation speeds in the two materials). In applications involving thick coatings and tight tolerances, the error induced by the paint layer can make up a significant proportion of the total measurement. The solution is therefore to measure the thickness in such a way as to exclude the coating from the calculation.
Thickness measurement between echoes takes advantage of a long-established technique which involves calculating the interval between two successive background echoes, representing the successive sound paths of the ultrasonic wave through the inspected material. In the case of painted metal, these background echoes occur inside the metal only, and not in the coating. Consequently, the interval between any pair of echoes (background echo 1 to 2, background echo 2 to 3, etc.) represents the thickness of the metal only; the thickness of the coating is cancelled out.
THRU-COAT measurement involves the use of a patented software approach to determine the time interval represented by a round trip inside the coating. This interval is used to calculate and display the coating thickness; by subtracting it from the total measurement, the device is also able to calculate and display the thickness of the metal substrate.
Each of these techniques offers advantages and disadvantages that must be taken into account when choosing the one best suited to a given application:
Advantages of measurement between echoes :
- A wide range of common probes can be used
- Often effective, even through rough surface coatings
- Operation possible at high temperatures (up to approx. 500°C), using suitable probes
Disadvantages of measurement between echoes :
- Many background echoes required: may be non-existent in highly corroded metals
Thickness range sometimes narrower than with THRU-COAT function
Benefits of THRU-COAT measurement :
- Applicable to a wide range of thicknesses, generally from 1 mm to over 50 mm in steel
- Only one background echo required
- Precise residual thickness measurement for pitting corrosion
Disadvantages of THRU-COAT measurement :
- The coating must be non-metallic and at least 0.125 mm thick.
- The surface must be relatively smooth
- Use of one of two specialized probes is required
- The approximate maximum surface temperature is 50°C.
Procedure: The procedures for activating the between-echo and THRU-COAT functions and taking measurements with the 38DL and 45MG instruments and the appropriate probes are described in the user manuals for these instruments. You'll find the information you need there, plus further details on how to calibrate propagation velocity and optimize gain and echo suppression, if required.
Scope THRU-COAT measurement : The THRU-COAT function used with D7906-SM or D7908 probes can measure non-metallic coatings only (paint or epoxy) with a minimum thickness of 0.12 mm. If no coating measurement is displayed when you take a measurement on coated steel, either the coating thickness is below the minimum measurable by the THRU-COAT function, or the measurement simply cannot be taken. In many cases, however, the coating thickness will still be subtracted from the measurement and the steel thickness will be displayed. You can also try taking the measurement using the between echoes function, or you may have to remove the coating.
Generally speaking, the thickness measurable in THRU-COAT mode ranges from around 1 mm to over 50 mm, depending on the acoustic properties of the metal and the surface conditions inside.
Scope measurement between echoes : In between-echo mode, the thickness range depends on the choice of probe, the type of instrument, the acoustic properties of the metal and the surface conditions. By definition, between-echo measurement requires at least two background echoes. As a result, it may not work when materials are extremely corroded, very rough, have very high scattering or very high attenuation. Indeed, under these conditions, the material does not support the second background echo. In such cases, the THRU-COAT measurement should be used whenever possible.
For the 38DL and 45MG thickness gauges, typical thickness ranges in inter-echo mode are as follows:
Sensor type Thickness range D798 1.5 mm to 7.50 mm D790/D791 2.0 mm to 50 mm D797 12.5 mm to 125 mm M201 1.25 mm to 12.5 mm The thickness range depends on the material, surface conditions and temperature.