Laser process can cut industrial chemical usage in vehicle manufacture

27 Sep 2021

LIS (laser interference structuring) treats aluminum alloy for corrosion protection.

Scientists at the U.S. Department of Energy’s Oak Ridge National Laboratory have applied a laser-interference structuring, or LIS, which they say “makes significant strides” toward eliminating the need for these hazardous chemicals used for corrosion protection in military vehicles and aircraft systems.

Chromate conversion coating, or CCC, uses hexavalent chromium, a known carcinogen, to inhibit corrosion. Sulfuric acid anodizing, SAA, uses sulfuric acid, which can severely irritate skin and eyes, and when inhaled, can lead to permanent lung damage.

The U.S. Department of Defense (DoD) operates hundreds of industrial facilities that make and repair its vehicles and equipment, spending more than $20 billion in corrosion protection annually.

The agency’s Strategic Environmental Research and Development Program, or SERDP, planned and executed with the Department of Energy and the Environmental Protection Agency, is “focused on developing alternative technologies to eliminate materials and processes that are of environmental concern,” said Robin Nissan, program manager of SERDP and its sister program, the Environmental Security Technology Certification Program.

“Our defense systems require repair and refurbishment,” he said. “Our programs are investing in the development of alternative processes that can ensure robust performance, sustainable practices and eliminate environmental risk.”

In three successive publications, ORNL materials scientist Adrian Sabau and a team of chemists and manufacturing scientists described, demonstrated and analyzed an LIS technique and compared its performance to the traditional solvent-intensive methods.

Sabau, who specializes in materials processing such as metal casting and solidification, and his team had recently completed a project using LIS for bonding in automotive applications. When he read DoD’s call for research on nonsolvent surface preparation, Sabau recognized that a similar technique could be effective for coating adhesion as well.

They treated aluminum alloy sheets by splitting the primary beam of a pulsed nanosecond laser into two beams and focusing them on the same spot on the specimen surface. This process roughened the surface with periodic structures, changed the surface chemistry and sub-surface microstructure.

Meyer, a physical chemist, and Leonard, a microscopist, contributed to the characterization work outlined in Optics and Laser Technology. He conducted surface chemical analysis using X-ray photoelectron spectroscopy, or XPS. “XPS is a material characterization technique that can determine what elements are on the surface — the top 5 to 8 nanometers — of solid materials,” said Meyer.

“Before laser processing, XPS was used to determine the chemical composition of aluminum alloy sheets, which showed high amounts of carbon. XPS was used again to determine if the laser processing cleaned the surface. The results showed a significant reduction of the carbon and was one of our key findings,” he added

Sabau added, “In looking at subsurface characterization, we found a beneficial aspect that we bumped into by accident. In the top layer, we saw the dissolution of copper-rich precipitates, where corrosion can initiate.”

After an aluminum alloy sheet is cleaned, often the surface energy prohibits the coating from sticking properly, a known issue in industrial surface coatings. The team’s next publication, in International Journal of Adhesion and Adhesives, considered coating adhesion and found that the LIS method provided adhesion as well as the industry-standard and solvent-intensive CCC and SAA techniques. A patent for coating adhesion was awarded in 2021 based on this LIS technique.

The third paper, published in Corrosion: The Journal of Science and Engineering, outlined the final tests that Sabau’s team conducted with an epoxy primer used by the U.S. military for airplane wings and bodies.

Sample testing

“The laser interference-treated substrate exhibited higher corrosion resistance,” said Jun, who attributed the result to copper-rich precipitates dissolving. However, on the samples coated with primer or primer and topcoat, LIS did not perform as well as the chemical solvent techniques.

The team tested a second set of samples that were simply wiped down with acetone prior to the application of primer, resulting in very little corrosion, and the formation of blisters was delayed by hundreds of hours.

Jun said further investigation to optimize LIS would be worthwhile. “Our research approach, combining lab-scale electrochemical measurements and industrially adopted ASTM [American Society for Testing Materials] salt spray testing, was very successful and aided in-depth understanding of the effects of laser interference treatment,” he said.