In industrial settings where safety and maintenance are non-negotiable, accurately assessing material thickness—especially when coatings or corrosion are present—is a vital task. From pipelines in oil refineries to ship hulls, storage tanks, and structural supports, understanding how much material remains beneath the surface is essential for operational efficiency and safety compliance.
An Ultrasonic Thickness Gauge offers a powerful solution for non-destructive testing (NDT), allowing technicians to measure material thickness through coatings and over corrosion. But measuring accurately in these challenging conditions requires the right approach, equipment, and understanding of ultrasonic technology.
Understanding the Basics of Ultrasonic Thickness Measurement
Before tackling difficult surfaces, it’s helpful to know how an Ultrasonic Thickness Gauge works.
The device sends a high-frequency sound pulse into a material using a transducer. That pulse travels through the material, reflects off the back surface, and returns to the transducer. The time it takes for the echo to return is used to calculate the thickness based on the known velocity of sound through the material.
This method is quick, non-destructive, and widely used across industries like marine, oil and gas, energy, and manufacturing.
However, the presence of coatings (like paint or epoxy) or corrosion (such as pitting, scaling, or rust) can interfere with the signal, making accurate measurement more complex.
The Challenge with Coated and Corroded Surfaces
1. Coatings Add Extra Layers
Paints, protective films, and coatings are often applied to prevent rust and wear. While helpful, these layers introduce an extra barrier that affects how the ultrasonic wave travels. In standard single-echo measurements, the thickness reading may include the coating, leading to inaccurate results.
2. Corrosion Causes Irregular Surfaces
Corroded materials have pits, voids, and uneven back walls. These inconsistencies scatter or weaken the returning sound waves, which can lead to unstable readings or even missed detections.
Overcoming these challenges requires advanced ultrasonic methods and proper technique.
Key Ultrasonic Modes for Coated or Corroded Surfaces
Modern ultrasonic thickness gauges offer multiple measurement modes, some of which are designed specifically for challenging surfaces.
Single-Echo Mode
Measures from the surface to the first back-wall echo.
Includes coatings in the measurement.
Not ideal for painted or corroded materials where only base material thickness is needed.
Echo-to-Echo Mode
Measures the time between multiple back-wall echoes.
Ignores surface coatings by starting measurement from the metal layer beneath.
Ideal for coated but relatively smooth substrates.
Multiple-Echo Mode
Sends a pulse and waits for several back-wall echoes.
Uses advanced algorithms to filter out coating thickness and average out irregularities.
Provides high reliability on painted, corroded, or scaled surfaces.
Preferred for marine, offshore, and industrial inspection environments.
Choosing the Right Ultrasonic Thickness Gauge
To measure coated or corroded surfaces effectively, not just any gauge will do. You need one with:
Multiple-echo capabilities to ignore coatings
Low-frequency transducers to penetrate heavy corrosion
High-gain amplifiers for capturing weak return signals
A rugged, waterproof casing for harsh environments
One well-known and trusted provider is Cygnus Instruments, a manufacturer and supplier of high-performance ultrasonic thickness gauges. Their multiple-echo technology is industry-leading, allowing accurate thickness readings through coatings without removing protective layers. Cygnus gauges are widely used in marine inspection, offshore rigs, storage tank testing, and structural integrity monitoring—proving their value in the field.
Step-by-Step Guide to Measuring Coated or Corroded Surfaces
Step 1: Select the Appropriate Gauge and Mode
Choose a gauge with echo-to-echo or multiple-echo capability. For heavy corrosion, use a lower-frequency probe (around 2.25 MHz) which penetrates better through rough, thick, or degraded material.
Set the gauge to the correct mode (e.g., multiple-echo) depending on whether you need to exclude coatings or navigate surface damage.
Step 2: Apply Couplant
Use a suitable couplant (usually a gel or liquid) between the transducer and the surface. It eliminates air gaps and ensures good sound transmission. For rough surfaces, a gel with higher viscosity may be more effective.
Step 3: Clean the Measurement Area (if needed)
While ultrasonic gauges work on painted and rusted surfaces, removing loose debris or flaky corrosion can help improve measurement accuracy. Don’t strip the paint if you're using a multiple-echo mode—this is designed to measure through coatings.
Step 4: Place the Transducer Correctly
Hold the probe firmly against the surface at a perpendicular angle. Avoid sliding or tilting, as this can distort readings. In corroded areas, take multiple measurements and use the lowest consistent reading to identify the minimum remaining wall thickness.
Step 5: Take Multiple Readings
Corroded materials often have uneven wear. Take measurements at several points across the surface. In critical applications (like pressure vessels), grid mapping or scanning tools can provide a thickness profile.
Step 6: Record and Analyze Data
Many modern gauges offer onboard storage and Bluetooth or USB export. Record your data for analysis, compliance documentation, or long-term monitoring.
Best Practices for Reliable Measurements
Always calibrate your gauge with a reference block or known thickness standard, ideally made of the same material you're testing.
Use the correct velocity for the material. Steel, for example, typically uses 5,900 m/s, but this varies for other materials.
Avoid taking readings near welds or curved edges, which can scatter or distort sound waves.
If in doubt, re-test the area using a different probe frequency or technique.
Common Industries and Applications
Ultrasonic gauges that can handle coatings and corrosion are indispensable in several industries:
Oil and Gas
Inspection of pipelines, storage tanks, and pressure vessels
Detecting wall thinning from internal corrosion
Marine and Shipping
Monitoring hull integrity through anti-corrosive paint
Checking ballast tank corrosion without dry docking
Construction and Infrastructure
Assessing steel beams and reinforcements inside painted structures
Ensuring safety in bridges and towers
Manufacturing and Industrial Maintenance
Equipment integrity in harsh environments
Regular thickness checks on coated machinery
When to Use Ultrasonic Testing Over Other Methods
Compared to visual inspection, radiography, or destructive testing, an ultrasonic thickness gauge is:
Non-destructive – No need to cut, drill, or strip paint.
Cost-effective – Reduces the need for dismantling or downtime.
Portable – Handheld devices can be used in the field.
Accurate – Can detect thinning of just a fraction of a millimetre.
That makes ultrasonic testing especially useful for in-service inspections and preventive maintenance strategies.
Final Thoughts
Measuring coated or corroded surfaces can be challenging—but with the right equipment and technique, you can achieve accurate, reliable results without damaging the material or removing protective coatings.
An Ultrasonic Thickness Gauge with advanced features like multiple-echo technology, low-frequency transducers, and smart calibration options makes this possible, even in the harshest industrial environments.
When safety, compliance, and material longevity are on the line, investing in quality matters. That’s why professionals around the world trust manufacturers like Cygnus Instruments for their ultrasonic thickness testing needs. With decades of experience and a commitment to innovation, Cygnus provides tools that deliver precision where it counts most.
So the next time you need to inspect a painted tank, a corroded pipe, or a structural beam hidden beneath layers of coating, remember—ultrasonic gauges are built for just that. And with the right knowledge, every measurement counts.
