Steel corrodes easily in an air environment. To improve its corrosion resistance, black mixed-crystal phases of Fe₃O₄, FeO, and Fe₂O₃ can be generated on its surfaces using blackening technology. Chemical oxidation, electrochemical oxidation, heat treatment, and other traditional blackening technologies cannot satisfy the requirements of green development owing to the use of toxic blackening agents, high energy consumption, environmental pollution, and low density of blackening film. Laser melting technology has been actively studied for improving the corrosion resistance of metal surfaces because of its high quality, high efficiency, and environment-friendliness. Research on blackening and rust prevention of steel surfaces using laser melting technology mainly focuses on the corrosion resistance of laser melting layers prepared with different laser powers or galvanometer scanning rates. However, the laser spot energy has a Gaussian distribution, and the single-pulse energy density and spot overlap rate cause rapid changes in the instantaneous heat accumulation and temperature field of the material surface during laser melting. In addition, repeated laser scanning leads to continuous heat accumulation and temperature field variations on the material surface during laser melting. These changes significantly influence laser melting, resulting in significant differences in the corrosion resistance of the prepared laser melting layers.

In this study, a laser melting layer was prepared on the surface of a Q235B steel plate sample using a 1064 nm pulsed laser. Based on the electrochemical analysis method, the effects of the single pulse energy density, spot overlap rate, and the number of laser scanning on the corrosion resistance of the laser melting layer of the Q235B steel plate were investigated. The optimal parameters of the laser melting were determined, and the laser melting layer with the best corrosion resistance was prepared. The corrosion resistance of the laser melting layer prepared based on the optimal laser parameters and that of the traditional alkaline blackening layer were compared and analyzed to verify the influence of the optimal laser parameters in improving the corrosion resistance of the laser melting layer.

First, laser melting experiments were performed on steel plate surfaces, each with a single laser scanning at a energy density interval of about 1.27 J/cm² ranging from 1.27 to 6.36 J/cm² (Fig. 2). At a single pulse energy density of 3.82 J/cm², the laser melting layer on steel plate surfaces with 70%, 80%, and 90% laser spot overlap rates had the maximum self-corrosion potential and minimum self-corrosion current density (Fig. 3). Therefore, the best single pulse energy density of the laser was determined to be 3.82 J/cm². Second, for a single laser scanning with a single pulse energy density of 3.82 J/cm², the laser melting layer with an 80% laser spot overlap rate had the largest self-corrosion potential and the lowest self-corrosion current density; in addition, the number of microcracks per unit area of the surface was the lowest, and the crack width was the narrowest (Figs. 3 and 4). Therefore, the optimal laser spot overlap rate was determined to be 80%. Third, laser melting experiments with different laser scanning times were conducted with the laser single-pulse energy density of 3.82 J/cm² and laser spot overlap rate of 80%. When the number of laser scanning was four, the laser melting layer showed the highest self-corrosion potential and lowest self-corrosion current density; furthermore, the number of microcracks per unit surface area was the lowest, and the crack width was the smallest (Figs. 5 and 6). Finally, energy spectrum and X-ray diffraction pattern tests revealed that the optimal laser melting layer prepared based on the optimal laser parameters mainly comprised Fe₃O₄ and FeO, thus complying with the national aviation industry standard (HB/Z 5079—1996) for steel blackening, with Fe₃O₄ as the main component of the corrosion-resistant layer (Fig. 7). The impedance arc radius and charge transfer resistance of the Q235B steel plate increased by approximately three times, and the impedance modulus was high (Figs. 8 and 9). A comparison of the surface roughness and scanning electron microscopy (SEM) data of the two corrosion-resistant layers further revealed that the optimal laser melting layer had a reduced surface roughness and good uniform density. This is more conducive to isolating the steel substrate from the corrosive environment and thus achieving improved corrosion resistance (Fig. 10).

A laser melting layer with high corrosion resistance was prepared on a Q235B steel plate surface using laser melting technology. The effects of the laser single-pulse energy density, spot overlap rate, and the number of laser scanning on the microstructure and electrochemical corrosion resistance of the laser melting layer were investigated. The following conclusions were drawn. First, the laser single-pulse energy density, spot overlap rate, and the number of laser scanning significantly influence the microcrack distribution, self-corrosion potential, and self-corrosion current density in the unit area of the laser melting layer. The optimal laser parameter can help achieve the strongest corrosion resistance of the laser melting layer. Second, based on the laser single-factor experiments of the single‐pulse energy density, spot overlap rate, and the number of laser scanning, the optimal laser parameters can be determined, and the laser melting layer with the strongest corrosion resistance can be prepared. Finally, the microstructure of the optimal laser melting layer prepared by the optimal laser parameter combination from the inside to the outside can be regarded as the transition from the gradual Fe oxidation layer to the stable Fe oxidation layer mainly composed of Fe₃O₄-FeO mixed crystals. The stable Fe oxidation layer exhibits decreased surface roughness and microcrack density, fewer oxidation leakage points, and prevention of excessive oxidation, thereby improving the corrosion resistance of the laser melting layer.