Laser light can reveal condition of ship paint

With infrared laser light, researchers from DTU Fotonik can produce detailed 3D images of ship paint, so that the paints can become more durable and environmentally friendly.

Christian Rosenberg Petersen had a bright idea a couple of years ago. He decided to find out whether infrared light could be used to examine ship paint. Not just in the lab, but onsite at the ships.

The idea did not come completely out of the blue, but from an article published in Ingeniøren. The article was about a new type of ship paint developed and tested in a collaboration between the company Hempel and researchers from DTU. It described how the researchers continually observed a paint sample under the electron microscope to follow how its durability and corrosion protection evolved over time.

As a researcher at DTU Fotonik, Christian Rosenberg Petersen thought that it could be done smarter.

"We had created a system in the laboratory where we could show that we could see through different materials such as ceramics, paper, or plastic using mid-infrared light. We could, for example, see into the chip in a credit card. When I then came across the article in Ingeniøren, I thought it might be interesting to find out whether our technology could also be used to see into the paint on ships and observe variations of the thickness and distribution of the various paint components."

Instead of cutting off small samples and bringing them back into a laboratory, it might be possible to use the infrared light to obtain a detailed, three-dimensional image of the paint in a non-destructive manner, for example while the ship is docked.

Laser light penetrates in depth

At DTU Fotonik, research is being conducted into how best to control and use light, and Christian Rosenberg Petersen has specialized in a technology known as optical coherence tomography (OCT). It is very similar to ultrasound scanning, only with light instead of sound.

"For example, we will be able to see if the paint contains small bubbles, whether there are problems with adhesion, or whether there is initial rust formation underneath it."
Researcher Christian Rosenberg Petersen, DTU Fotonik

When emitting infrared laser light against a surface, some of the invisible light will penetrate the material, and here a small part of it will be reflected differently by the components of the material. The reflected light is collected and analysed in a computer, and the result is an image that reveals the internal structure of the material, similar to when reflected ultrasound produces the image of a foetus in an ultrasound scan of a pregnant woman.

The unique thing about OCT is that you can get a 3D image at a relatively good resolution—down to a few micrometres— completely without coming into contact with the subject.

Back in the 1990s, OCT was developed for eye examinations, and the technology still attracts most attention in medical contexts. The technology can also be used to diagnose skin cancer.

But OCT also has great potential in industry—and with a grant of just over DKK 1 million from the Danish Maritime Fund—Christian Rosenberg Petersen is now in the process of developing a compact, portable OCT system for inspection of ship paint.

"When it has all been assembled, we need to go and perform measurements on a ship. With the robotic arm, we can scan a relatively large area, limited only by the robot's range of 85 cm. For example, we will be able to see if the paint contains small bubbles, whether there are problems with adhesion, or whether there is initial rust formation underneath it," says Christian Rosenberg Petersen.

May lead to better coatings

The project has been named SHIP-COAT (Sub-Surface, High-Resolution, Inspection of Paints and Coatings Using Non-Destructive Laser Tomography). In February 2021, the portable measuring instrument should be ready for field testing.

Until then, the measurements are done in the laboratory with researchers from DTU Chemical Engineering observing. The research centre CoaST (The Hempel Foundation Coatings Science and Technology Centre) is located here, and the purpose of the research centre is to develop durable and sustainable paints—or coatings as they are called in the industry.

OCT can be used both in the development of coatings and later on when the coatings wear off.

"In connection with the development of a coating, it will, for example, be possible to check the properties of the coating during a test process. Once the coating has been introduced on the market and is used on ships, the quality can be checked on an ongoing basis through OCT-based inspections," says Christian Rosenberg Petersen.

His vision is that the SHIP-COAT project can lead to better, more environmentally friendly ship coatings, and he also stresses that the technology can be used for much more than coatings. Only the imagination sets any limits:

"I hope that more people outside medical circles will become aware of optical coherence tomography and come up with completely new applications that we haven't even considered yet."

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