Due to its high corrosion resistance, 304 stainless steel has several applications in various industries, including aerospace, military, electro-optics, refineries and petrochemicals. In recent years, studies on increasing the hydrophobic or oleophilic properties of this steel have been developed depending on the needs of different industries and working conditions. In the present study, the effect of laser engraving, chemical etching, electrochemical passivation and anodizing on the development of wettability behavior has been investigated. Laser engraving operations were performed on various samples of 304 stainless steel by changing parameters such as power, speed, pattern and distances between engraving lines. By examining the wettability angle and surface topology via light microscope, three samples were selected and eva‎luated in the continuation of the research. Chemical etching was performed to remove surface contaminants in an aqueous solution of 22.5 Vol.% of hydrofluoric acid at 50 °C for 5 and 60 seconds. Electrochemical passivation of selected samples in aqueous solution containing 10Vol.% hydrofluoric acid and 25Vol.% nitric acid at 40 ° C temperature and 1000 mV for 5 minutes was performed. Anodizing with ethylene glycol solution containing 0.1 M ammonium fluoride and 0.1 M distilled water at 60 ° C with a constant voltage of 60 V was performed in a single step for 60 seconds and 300 seconds and two steps for 300 seconds and then 120 seconds, respectively. Also, after each anodizing step, the samples were placed for 30 minutes in a furnace at 350 ° C to remove fluoride from an anodizing coating. In this study, the angle of contact with water, which indicates the degree of oleophilicity of the surface, and the angle of contact with diesel, which indicates the oleophilicity of the surface, were measured. Also, observing the surface topology of all samples to check the surface structures was done in different stages with the help of light microscope and the surface of 4 final samples were examined with the help of SEM, too. The laser roughness was eva‎luated from the surface of 4 final samples, which was a proof of the highest roughness in the best sample in terms of hydrophobicity and oleophilicity. The results show that laser engraving can cause significant changes in the hydrophobic properties of 304 stainless steel surface; The wetting angle with water was 18 degrees to 131 degrees. The chemical etching with a duration of 60 seconds removed the roughness formed from the laser engraving, therefore, the optimal duration of the process is 5 seconds. Examination of the samples of electrochemical passivation, samples of chemical etching and anodizing stage, it was concluded that oxidation of the sample surface is done in the anodizing stage and there is no need for passivation. Studies have shown that two-stage anodizing as following order, anodizing for 300 seconds - heat treatment at 350 ° C for 30 minutes - re-anodizing for 120 seconds - repetition of heat treatment for 30 minutes achieved the greatest difference in hydrophobicity and oleophilicity, so that in the best result, the wetting angle with water was 125 degrees and the wetting angle with gasoline was 4 degrees. In the electron microscope images, it was observed that the two final hydrophobic samples had nanometric roughness along with micrometer depressions, which seem to have a hierarchical structure. Also, the laser roughness results show that the largest difference in roughness height is for these two samples. From the results, it seems that the samples follow the Cassie Baxter model.

علي اشرفي، حميدرضا سليمي جزي

بهزاد نيرومند، علي شفيعي