In the oil, gas and chemical engineering industries, 3PE anti-corrosion coatings (three-layer polyethylene coatings) are widely used to protect metal pipelines because of their excellent corrosion resistance. In the daily pipeline inspection process, how to accurately measure the wall thickness of the metal substrate without damaging the anti-corrosion layer is an important technical challenge to ensure pipeline safety. Ultrasonic thickness measurement technology, which is non-destructive, efficient and accurate, has become the preferred method to directly measure the wall thickness of metal through the 3PE coating. This paper systematically describes the measurement method and operating instructions based on the structural properties of 3PE coatings and the principles of ultrasonic propagation.

Measurement Principles and Technical Advantages

1.How to penetrate multi-layer medium?

The 3PE coating is divided into three layers from the inside out: an epoxy powder layer (approx. 0.3 mm), an adhesive layer (approx. 0.1 mm) and a polyethylene layer (approx. 2-3 mm), with a total thickness of typically 2.5 mm to 3.7 mm. An ultrasonic thickness gauge emits ultrasound waves (2-5 MHz) through the probe. The signal penetrates the three layers of coating to the inner wall of the metal pipe and is then reflected back to the probe. The instrument utilizes the time difference between the propagation of the sound waves through the coating and the metal, combined with a preset speed of sound through the material, to calculate the actual wall thickness of the metal.

2. What are the technical advantages?

Non-destructive inspection: Avoids damage to the coating and protects the pipeline.

Highly efficient: No need to pre-treat large areas of coating; measurement time for a single point is less than 10 seconds.

Applicable: Suitable for acceptance of new pipelines, routine inspection of in-service pipelines and complex areas such as bends and welded seams.

Preparation for Measurements

1.About Instrument

To realize the accurate measurement through 3PE anti-corrosion coating, we have to select an ultrasonic thickness gauge with "through-coating measurement mode(E-E)". In this case, we used UM-5D Ultrasonic Thickness Gauge. This high-precision thickness gauge has real-time A-scan display mode and supports users to adjust relative parameters to identify the waveform.

2.About the Probe 

For pipes with a radius of curvature greater than 100 mm, use a 5 MHz flat probe (10-15 mm crystal diameter) or a small curvature probe (50-100 mm radius). 

On-site Measurement Procedures

Set up the probe and calibrated the probe

Select the "Through-coating measurement mode"

Calibrate the sound velocity with the standard test block made in the same material as the pipe.

Measurement Results

Measuring the wall thickness of metallic pipes through 3PE coatings using YUSHI UM-5 Series Ultrasonic Thickness Gauges requires precise signal identification and sound velocity calibration in multi-layer coating. Highly accurate measurements can be achieved without damaging the coating by standardizing surface treatments, optimizing probe parameters and following signal interpretation protocols. Practical applications should combine the age of the pipe, the corrosivity of the medium and complementary techniques (e.g. eddy current or radio-graphic testing) for a comprehensive corrosion assessment.