All types of marine organisms attach and grow on the surface of marine facilities and equipment during the course of their service in the marine environment. This results in fouling by marine organisms, accelerated corrosion of metal materials, failure of key parts of the marine equipment, and other problems that affect the normal operation of marine equipment. Therefore, the effective removal of marine biofilms from the surface of marine service materials has become a key breakthrough in the exploitation of marine resources. At present, effective cleaning methods for marine biofilms mainly include chemical removal with fungicides, mechanical removal with artificial eradication, cavitation water jet flushing, ionizing radiation, and ultrasonic adhesion prevention. However, to a certain extent, all these methods have drawbacks, such as a low cleaning efficiency, poor cleaning quality, environmental pollution, and uncontrollable damage to the substrate. Therefore, it is necessary to develop a green, efficient, and high-quality cleaning method to prevent biological fouling of the surfaces of marine service materials. As a green cleaning technology with significant potential for development in the 21st century, laser cleaning technology has the advantages of a high cleaning efficiency, high precision, high quality, and minimal damage. Owing to the uneven thickness, physical properties, and chemical composition of marine biofilm layers, as well as the special interface bonding properties between the organic membrane layer and the inorganic metal matrix, there are many new challenges in the laser cleaning of marine biofilm layers on metal surfaces in marine service environments. The mapping relationship among the laser-cleaning characteristics, laser-energy parameters, and cleaning quality requires further study.

In this study, a nanosecond pulsed laser was used to conduct laser-cleaning experiments on marine biofilm layers formed on the surface of 30Cr3 high-strength steel, which is commonly used in ocean engineering, after soaking in the Huanghai Sea. The surface morphologies of the substrate before and after laser cleaning were observed using an optical microscope and a laser confocal microscope. The surface roughness of the substrate before and after laser cleaning was measured, and the microscopic morphology of the substrate surface was observed using a scanning electron microscope. The composition and distribution of the elements on the substrate surface were analyzed using an energy spectrum analyzer before and after cleaning. The effects of different laser energy densities on the desorption behavior of marine biofilm coatings on high-strength steel surfaces during laser cleaning were observed and summarized using high-speed imaging equipments.

A marine biofilm on the surface of high-strength steel contains two components: an extracellular polymeric substance (EPS) layer composed of organic components and a hard attachment composed of limestone (Figs. 4-7). The laser with the energy density of 9.95 J/cm² has the best cleaning effect on the marine biofilm on the surface of high-strength steel, as it achieves a good removal of the EPS layer and hard attachments and causes little damage to the substrate. The laser with the high laser energy density of 11.05 J/cm² completely removes the marine biofilm, however, the thermal damage to the surface of the substrate is large. The cleaning effect of the laser with the low laser energy density of 7.74-5.53 J/cm² is relatively poor, and the cleaning effect decreases with a decrease in the laser energy density (Figs. 8-11). Following laser cleaning, the surface roughness of the substrate decreases with increasing laser energy density. For a laser energy density of 9.95 J/cm², the lowest surface roughness S_a=17.31 μm is reached, which is about 47.8% lower than that before cleaning, and corresponds to the best cleaning parameters described above. However, when the laser energy density is further increased, the substrate suffers thermal damage owing to excessive cleaning, resulting in a substantial increase in the surface roughness (Figs. 12-13). High-speed imaging observations reveal that only a thin EPS layer is removed from the surface by laser ablation at a low laser energy density during the process of laser cleaning of a marine biofilm layer. However, there is no obvious removal effect for hard surface attachments. At a higher laser energy density, the removal of the EPS layer and hard attachment is significant. The EPS layer is mainly removed by ablative decomposition and combustion, whereas the hard attachment mainly breaks off and flies off the surface through thermoelastic vibration (Figs. 14-17).

The components and surface states of the marine biofilm on the surface of high-strength steel soaked in the Huanghai Sea are complex and uneven, and the marine biofilm can be roughly divided into two components: an EPS layer with uneven thickness, mainly composed of organic components, and hard attachments, mainly composed of limestone. Under the premise of no damage to the substrate, the laser cleaning effect of the marine biofilm on the surface of high-strength steel improves with an increase in the laser energy density. The laser with the energy density of 9.95 J/cm² shows the best cleaning effect, with no residue of the marine biofilm left on the surface after cleaning. Following cleaning, the surface roughness is 17.31 μm, a 47.8% reduction from the initial roughness. The EPS layer is primarily cleaned using ablative decomposition and combustion, and the hard surface attachments are primarily cleaned using thermoelastic vibration.