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Laser rust removal is an entirely different process than paint removal. Typical steel rust is a combination of iron oxides such as ferric oxide, Fe2O3, ferrous oxide, FeO, and magnetite Fe3O4. These are partially transparent to NIR radiation, resembling semiconductors. However, rust is far from being a homogenous or continuous layer.

Figure 3 : Sketches of 3 modelled levels of rust development

A model describing the different levels of rust and relevant surface structuring is depicted in Figure 3.

For Level 1 type rust, the rust only exists as surface layers and occasionally creates shallow pits of maximum 50 μm depth with the edges of the pits in line of sight to a single reference point above the surface.

Level 2 with eroded cavities as well as multi-grain rust filled pits with parts of the pit walls not in line of sight to an elevated reference point.

Level 3, with deep cavitation and multi-grain rust filled pits that have progressed underneath the substrate surface, thus generating concealed features from the line of sight of an elevated reference point.

Erosion typically progresses in multiple stages, with new erosion seed locations being activated on successive stages. Rust-filled pits may also contain suspensions of non-oxidised steel.

Figure 4: a) Level 3 steel rust. b) Rust removed with 5% spatial pulse overlap. c) Rust removed with 60% spatial pulse overlap

Tests using the Powerlase Rigel i1600e laser were conducted on steel, featuring rust penetration depths between 40 and 230 μm. Figure 4a shows a typical example of a Level 3 rusted plate.

Pulses were released with 5% spatial overlap to investigate process speed based on a detachment model. A maximum speed of 0.75 m2/min was demonstrated using 12kHz pulse repetition rate, 70 ns pulse duration, 1.62 MW/mm2 irradiance and 1.5kW average power transmitted through a fiber. Figure 4b depicts the result of processing, showing signs of remaining pockets of rust.

When overlapping the pulses by 65%, process speed increased to 1.35 m2/min at 12 kHz and 0.85 MW/mm2 irradiance. Figure 4c depicts an area processed by overlapped pulses showing fewer pockets of rust and more surface melting. This indicates an ablation based process where the overlapping pulses melt or ablate a significant volume of un-oxidized metal until reaching concealed corrosion cavities.

To conclude, the hybrid ablation-detachment approach can return commercially interesting paint removal rates around 40,000 mm3/ kW-min. Rust removal at rates of 1 m2/min is demonstrated for moderately oxidized plates using 1.5kW Q-Switched Nd:YAG laser. The applications prove a competitive edge for laser for the marine industry.