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Paint removal and laser cleaning is becoming a trend in industry. With the availability of industrial grade ns pulsed lasers with an average power beyond 1 kW, the capability emerged to process many materials and structures at commercially feasible speeds. Usually, these lasers have pulse peak power beyond 100 kW and typically beyond 1 MW. Interaction with almost any material is guaranteed with the high peak power, leading to ablation, detachment or decomposition, depending on the beam size and the material properties. Such industrial grade lasers are based in either q-switching or power amplification of diode signals, providing high peak power pulses. Each pulse is a packet of energy that can evaporate a volume or set of coating, or melt and restructure its surface, or detach it from its substrate.
For defense and commercial aircrafts, a number of projects on laser paint and coating removal have already been performed. The LADS I & LADS II and the ARBSS systems have been developed and tested on aircraft and aircraft components. In 2017, Singapore airlines commissioned a commercial laser paint removal system, using a large 8 axes robot by NTS and advanced scanners by EWI. The Marine industry provides a wider range of applications, with a potential yearly market for commercial vessel paint removal merely standing at approximately $300 million, which exceeds that of aircraft estimated at $250 million worldwide.
If other more localized processes such as shaft and propeller resurfacing, selective rust and corrosion removal etc., are introduced to the market, the estimate increases to $2.3 billion per annum. On the other hand, market accessibility is low because the sector suffers low commercial utilization with just profitable areas remaining in the LPG/LNG transport, the cruise, private boat and ferry transport.
Coatings meant for marine applications are much thicker, usually ~1 mm. Variations in coating thickness are much less controlled, and coating consistency has frequently changed during service. Any degradation of the coating is usually accompanied by deep substrate corrosion. Access to complex structural geometries is almost impossible. In addition, the surface area of a commercial vessel is very important and dockyard delays had to be minimized.
Two major applications – paint removal and rust removal – are examined. Panamax, the average size commercial sea vessel, has about 19000 m2 of external surface area. CW and QCW CO2 are laser technologies that emit up to 30 kW. They indicate excellent interaction with organic paint, but have shown removal rates merely approaching 22,500 mm3/ kW.minute. Based on surface area and volume of paint of the Panamax sea vessel, the operation would take as much as 130 days with 1 kW of laser power.
Preferably, process speeds of about 10 times are needed so that a commercial case of using 4 to 6 kW of total laser power can be distributed around the vessel in 3 to 4 workshops and the task can be addressed within a week. Another option is to use low energy ns pulses of 0.1 to 12 mJ at high pulse repetition rates of 100 to 1000 kHz, attaining higher coverage range per raster and at the same time focusing into sufficiently small spots to preserve irradiance levels above the ablation threshold.
This results in much lower removal rate, close to 2,000 mm3/kW.minute because the increased coverage rate by decreased spot size and pulse energy, mathematically leads to slight increase of removal rate, in reverse proportion to pulse energy. The increase in removal rate is however restricted by the size of the smallest spot size theoretically and practically achievable. Finally, as heat continuously diffuses into the material at CW or above 200 kHz, both these technologies can thermally affect the substrate.
The methodologies mentioned above are based on an ablation removal technique, being exposed to thermal diffusion as the entire volume of the material removed needs to be vaporized. Since the late 80s, high energy pulses have been used in semiconductor processing to remove metal films without causing any significant damage to the substrate . The technology later evolved to offer a tool for transparent coating detachment in the optics sector  and semi-transparent paint removal . Coating removal efficiency is considerably increased by detachment. Most organic and polymer-based materials, like paints can sufficiently transmit 1 μm near infrared radiation. Even if white scattering additives or other pigments are introduced in the paint’s polymer matrix, these materials are semi-transparent.
Due to the larger wavelength and reduced molecular vibration interaction, polymers transmission in the NIR region is higher than the visible radiation. Therefore, using the simplified Beer-Lambert equation, the transmitted beam intensity reduces in negative exponential relationship to the coating depth, controlled by the absorption coefficient at the laser wavelength. At the coating to substrate interface, absorption is usually increased because of the 0 practical optical transmission of the metal and also due to surface roughness behaving as absorbing discontinuities.
Figure 1. a) Interfacial pressure P and b) hybrid detachment with P1, P2, P3 … Pulses To Detachment (PTD), induced by small (B1) and large (B2) pulse energy beams delivering irradiance It1 and It2 respectively.
Figure 2. Example paint removal efficiency thresholds for hybrid detachment of 300 μm and 600 μm thick semi-transparent paints.
Further, the two layers that contribute to huge interfacial pressure trap the vaporized material. This pressure powers coating removal through ejection. As a result, only a small volume of material needs to be vaporized to attain detachment of material with thickness orders of magnitude greater than the vaporized layer. In addition, laser beam intensity equivalent to the detachment threshold needs to reach the interface through the Beer-Lambert equation, but this ablation threshold is usually lower than a clean and smooth semi-transparent coating surface. High pulse energy q-switched lasers essentially provide adequately high intensity I0, accomplishing single pulse detachment of most paints up to 100 μm thick.
Therefore once the detachment threshold is reached, process efficiency is enhanced in a non-linear fashion. Increasing pulse energy further and distributing over larger surfaces to preserve irradiation just above the detachment threshold will further reduce the detachment threshold. This phenomenon takes place because of an increased number of evaporated species at the interface and the increasing area over which the detachment pressure is applied. Therefore, less pressure is required to overcome the shear forces on the perimeter of the irradiated area that holds back the coating (Figure 1a). The coating removal rate and efficiency thus increase at a much steeper gradient when compared to the volume ablation process.
Single pulse detachment is difficult in the case of thick coatings used in marine vessels. However, an ablation-detachment hybrid process is used to achieve high removal efficiency. In this hybrid process, the first few overlapping pulses focus at reducing the thickness of the coating on a pulse to pulse basis (patent pending GB1710188.2). Once sufficient radiation is transmitted via the remaining thickness to initiate detachment, the remaining material can be removed at high efficiency (Figure 1b). Again, higher energy pulses usually above 100 mJ at 50 to 100 ns, unlock the non-linear efficiency growth threshold indicating removal rates greater than 40 mm3/kW.minute.
Shown in Figure 2 is the increased process speed for white paint on primer for a 600 μm thick coating and 300 μm thick coating. The onset for high efficiency removal is seen at 180 and 260 mJ, respectively. Average power is sustained at 1.5 kW, and the process is optimized for overlap and spot size per datum.
As the pulse energy increases, pulse duration changes from 114 ns to 42 ns. Pulse shortening merely contributes to a linear increase in efficiency and does not attain a non-linear efficiency boost without the accompanying increase in pulse energy. Results were obtained with the help of a Powerlase Rigel i1600 Q-switched DPSS Nd:YAG laser.
Rust removal is an entirely different process than paint removal. Typical steel rust includes a combination of iron oxides such as magnetite Fe3O4, ferrous oxide FeO, and ferric oxide Fe2O3. These look like semiconductors and are partially transparent to NIR radiation, but rust is far from being a continuous or homogenous layer. Figure 3 shows a model describing the varying levels of rust and relevant surface structuring. Level 1, the rust exists only as surface layers and sometimes produces shallow pits of maximum 50 μm depth with the pits’ edges in line of sight to a single reference point over the surface.
Level 2 with eroded cavities and multi-grain rust filled pits, with some 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 below the surface of the substrate and therefore generates concealed features from the line of sight to an elevated reference point. Usually, erosion progresses in several stages, with new erosion seed locations being triggered on consecutive stages. Suspensions of non-oxidized steel may be present in rust filled pits.
Figure 3. Sketches of 3 modeled levels of rust development.
Figure 4. a) Level 3 steel rust. b) Rust removed with 5% pulse overlap. c) Rust removed with 60% pulse overlap.
Using the Rigel i1600 laser, tests were performed on steel that features rust penetration depths ranging between 40 and 230 μm. A typical example of a level 3 rusty plate is shown in Figure 4a. Pulses were released with 5% overlap to study the process speed depending on a detachment model. A maximum speed of 0.75 m2/minute was shown using 1.62 MW/mm2 irradiance, 12 kHz pulse repetition rate, 70 ns pulse duration and 1.5 kW average power transmitted via a fiber. Shown in Figure 4b is the result of processing, revealing signs of remaining pockets of rust.
When the pulses were overlapped by 65%, process speed increased to 0.85 MW/mm2 irradiance and 1.35 m2/minute at 12 kHz. Shown in Figure 4c is an area processed by overlapped pulses indicating more surface melting and less pockets of rust. This suggests an ablation-based process where the overlapping pulses ablate or melt a considerable volume of un-oxidized metal until concealed corrosion cavities are reached.
In conclusion, hybrid ablation-detachment method can return commercially viable paint removal rates around 40,000 mm3/ kW.minute. Rust removal at rates of 1 m2/minute has been shown for moderately oxidized plates using 1.5 kW Q-Switched Nd:YAG laser. These applications demonstrate a competitive edge of laser for the marine sector.
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This information has been sourced, reviewed and adapted from materials provided by Andritz Powerlase Limited.
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