Implementation of Laser Ablation Coating Removal Technique for Steel Components on VDOT Bridges

One of the most common methods to mitigate corrosion of metallic structures is the application of paint coatings. However, with time and weathering, coatings deteriorate and require ongoing maintenance to ensure that they provide the necessary protection for the underlying metallic substrate. At the present, the Virginia Department of Transportation (VDOT) utilizes grit blasting to remove old coatings and provide a fresh surface for the reapplication of new coatings. This procedure is expensive due to containment structures such as tarps and scaffolding, traffic management, and physical load on the workers. In addition, the current process generates hazardous waste that is difficult to contain and remove. A relatively new technique termed Laser Ablation Coating Removal (LACR) has been developed that combines a hand-held laser ablation system with a high efficiency multi-filtration vacuum system. The LACR process incorporates a high-power pulsed laser operating in the near IR (λ=1064 nm, pulse width 83 ns), and a multi-stage HEPA filtration system that provides the ability to remove coatings while reducing containment needs, meeting industrial hygiene safety requirements, along with a compact, confined modular filter disposable system.
The goal of this project was to evaluate if the use of LACR would provide VDOT with an acceptable new alternative for removing existing coatings. In this study, the LACR coating removal process was documented both in the laboratory and in the field for the feasibility of implementing this process on specific areas of VDOT bridges. There were two outstanding questions to be answered: first, whether VDOT could effectively implement LACR technology in coating removal operations on bridges, and second, to identify any additional coating removal techniques that VDOT should evaluate. To meet the goals of the research conducted here, the study was divided into four phases in collaboration with the Virginia Transportation Research Center (VTRC) and VDOT. The environmental and industrial hygiene requirements in Phases 1 and 2 were used to establish the viability of using LACR by VDOT. Each of the subsequent phases was then used to address questions and further develop LACR as a method for removing coatings from VDOT structures.
The Phase 1 study consisted of LACR processing that was performed in a controlled lab environment to remove lead-based paint and rust from heavily weathered I-beam sections made of A36 steel (ASTM A36 Standard Specification for Carbon Structural Steel). The sections were taken from a decommissioned bridge structure in the VDOT Lynchburg District. The bridge was put in service in the 1920s, prior to the implementation of grit blasting, and a layer of iron oxide (mill scale) was present below three layers of paint. Two comparison samples were also processed using the incumbent method of grit blasting. The Phase 2 study included an on-site (field) demonstration where LACR was evaluated on an in-service bridge structure in the VDOT Lynchburg District (off of Route 460 in Farmville, VA). In the Phase 3 study, sections of a bridge bearing were processed by LACR to determine whether the current system was capable of cleaning the surfaces of recessed areas of the bridge beam.
Results from electron and optical microscopy showed that LACR effectively removed multiple layers of paint and elemental hazards, including lead. I-beam cross sections showed that regions of the underlying mill scale layer (iron oxide) ranging from 20 to 100 μm in thickness remained on the surface with morphological evidence of melting and a solidification region on the order of 1 μm in depth. Hardness measurements and tensile testing of the base metal, grit blasted, and laser cleaned samples showed that LACR has no detrimental effects on the surface or on the mechanical properties of laser cleaned bridge steel. Base metal and laser cleaned samples showed virtually no change in microhardness, both averaging about 139 HV from the surface through to the bulk of the steel. Tensile testing showed that both the yield and ultimate tensile strengths of base, grit blasted, and laser cleaned metal were 281 MPa and 440 MPa respectively, all on parity with that of ASTM A36 structural steel. Furthermore, fatigue testing of LACR samples shows no statistical difference in fatigue behavior from that of the base metal, with an endurance limit measured within the range of 300 MPa for laser cleaned steel, versus 269 MPa based on repeated loading conditions of base metal found in literature. Surface roughness was performed on LACR and grit blasted samples. Roughness values recorded parallel and perpendicular to the laser beam orientation averaged 5.26 and 5.41 μm arithmetic average roughness, Ra. In comparison the grit blasted samples exhibited a Ra average surface roughness of 9.86 μm. Pull-off adhesion testing on re-painted LACR surfaces showed that there was little variation between the repainted samples following LACR or grit blasting, with average pull-off strengths of 1800 psi. After the cyclic testing, the majority of the pull-off failure occurred due to cohesion failure of the coating film. It is worth noting that the surface cleaning methods do not seem to affect the adhesion strength differently from each other. Based on this work, it was concluded that there was little variation in the adhesion strength between the two surface cleaning methods.
Industrial hygiene surveys conducted both remotely and in-person showed that no heavy metals or volatile organic compounds (VOCs) were detected during LACR. Lead levels were recorded to be at 0.0043 mg/m3 for one laser operator and 0.0014 mg/m3 for the other operator, both of which were well below the occupational safety and health administration (OSHA) action level (AL) of 0.03 mg/m3.
Metallurgical characterization was performed using optical microscopy, scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), x-ray fluorescence (XRF), and x-ray diffraction (XRD). Adhesion testing was performed with a pneumatic adhesion tensile testing instrument (PATTI) tester. Surface roughness was measured by mechanical profilometry using a Mitutoyo mechanical stylus. Tensile and fatigue testing was performed on a 22 kip Instron loadframe under pull-pull fatigue conditions of R = 0.1 and at 10 Hz.