Laser surface structuring could reduce use of hazardous chemicals in aerospace and automotive

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Adrian Sabau holds a primer-coated specimen, which was pretreated with a laser-interference structuring technique and shows coating adhesion. (Image: Carlos Jones/ORNL, U.S. Dept. of Energy)

Oak Ridge National Laboratory (ORNL) researchers aim to use laser-interference structuring (LIS) to eliminate the use of hazardous chemicals in anti-corrosion pre-treatment processes for vehicle/aircraft aluminium alloy surfaces.

The work looks to answer a call from the US Department of Defense (DoD) for research projects that explore non-chemical alternatives for corrosion protection in military vehicles and aircraft systems.

Long-lasting protection from corrosion is essential for materials used for vehicles and aircraft to ensure structural integrity amid extreme operating conditions. Two chemical pre-treatment processes, chromate conversion coating and sulfuric acid anodising, are widely used in industrial settings to prepare for coating adhesion and protect aluminium alloy surfaces against corrosion. 

While highly regulated, both processes use large quantities of hazardous compounds with known environmental and health risks. Chromate conversion coating uses hexavalent chromium to inhibit corrosion, which is a known carcinogen. Sulfuric acid anodising on the other hand uses sulfuric acid, which can severely irritate skin and eyes, and when inhaled can lead to permanent lung damage. Millions of gallons of such chemicals need to be disposed of annually as hazardous waste.

Answering the call

The US military operates more than 12,000 aircraft, 10,000 tanks, hundreds of ships and a multitude of other vehicles and weapons systems. The DoD owns and operates hundreds of industrial facilities that manufacture and repair these vehicles and equipment, spending more than $20 billion in corrosion protection annually. 

The DoD’s Strategic Environmental Research and Development Program (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 defence systems require repair and refurbishment. Our programs are investing in the development of alternative processes that can ensure robust performance, sustainable practices and eliminate environmental risk.’

In three successive publications1,2,3, materials scientist Adrian Sabau and a team of chemists and manufacturing scientists at the US Department of Energy’s ORNL described, demonstrated and analysed a LIS technique and compared its performance to the two traditional solvent-intensive methods. Sabau and his team had recently completed a project using LIS for bonding in automotive applications when he read the DoD’s call for research on nonsolvent surface preparation, recognising that a similar technique could be effective for coating adhesion as well.

Experimentation and results

In their experiments the researchers treated aluminium alloy sheets by splitting the primary beam of a Q-switched Nd:YAG 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.

The corrosion behavior of laser-interference specimens was then assessed against that of specimens prepared by chromate conversion coating and sulfuric acid anodizing treatments. This was done by first coating each specimen with primers and topcoats to exacting DoD specifications, and then exposing them to salt spray.

It was found that most specimens prepared at a laser fluence of 1.78J/cm2 were found to develop one very small blister after only 96 hours of exposure. However, the growth of these blisters was not significant even after 1,000 hours of salt spray exposure. 

‘For a process that was conducted at ambient temperature without solvents, most of the samples performed extremely well,’ said Sabau. ‘This technique is a huge step in the right direction towards non-chemical intensive surface preparation for coatings.’

The team then experimented by wiping specimens structured with a laser fluence of 1.24J/cm2 with the organic chemical compound acetone, which occurs naturally in plants, trees, and is produced as a breakdown product of animal fat metabolism. 

They found that only a fraction of the specimens were found to develop several tiny blisters after 790 hours of exposure.

While the researchers were impressed at these results, they believe further investigation to optimise LIS would still be worthwhile.

'These results indicate that the LIS technique with the additional acetone wiping has the potential to be further developed as a minor chemical surface preparation technique for chromate-containing epoxy primers coatings,' Sabau concluded.

[1] Laser-interference pulse number dependence of surface chemistry and sub-surface microstructure of AA2024-T3 alloy 

[2] Coating adhesion of a chromate-containing epoxy primer on Al2024-T3 surface processed by laser-interference

[3] Corrosion Behavior of Laser-Interference Structured AA2024 Coated with a Chromate-Containing Epoxy Primer







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