| Magnesium and magnesium alloys not only have low density,high specific strength and good shock absorption performance,but also have good application prospects in the fields of aerospace and transportation.On the contrary,the standard electrode potential of magnesium alloys is extremely low and the electrochemical activity is high,resulting in poor corrosion resistance.Potassium ferricyanide conversion coating is a new type of chemical conversion coating with better corrosion resistance,and rare earth conversion coating has also been widely studied because of its good corrosion resistance and green concept.In this paper,potassium ferricyanide conversion film is treated with samarium salt to prepare a composite film with stronger adhesion and better corrosion resistance.In this paper,single experiments were used to explore the optimal p H and aging time of potassium ferricyanide conversion coatings;a two-step method was used to prepare a samarium salt/potassium ferricyanide composite film,and the optimal film-forming process was optimized.In this study,the corrosion resistance of the film in 3.5 wt.%Na Cl solution was investigated by dropping experiment,immersion experiment,electrochemical method and hydrogen evolution experiment.The coatings were characterized by scanning electron microscopy（SEM）,energy dispersive spectroscopy（EDS）,X-ray photoelectron spectroscopy（XPS）and X-ray diffraction（XRD）.The main results are as follows:（1）The main component of potassium ferricyanide conversion film is Fe4[Fe（CN）6]3,which can greatly improve the corrosion resistance of magnesium alloys.Among them,the coating with p H value of conversion solution of 4 and aging time of 12 h had the best corrosion resistance.At this time,the self-corrosion potential of the sample shifted significantly,and the self-corrosion current density decreased by about 3 orders of magnitude compared with the bare magnesium alloy.Moreover,the film layer has certain stability under acid-base solution and light conditions.（2）On the basis of reducing the amount of potassium ferricyanide,the samarium salt is screened out by a two-step method to form a composite film.Through single factor experiment and orthogonal experiment,the optimal film forming process of samarium salt composite film was determined as follows:the optimal samarium salt content is 5 g/L,the optimal film forming time is 20 min,and the optimal aging time is 12 h.At this time,the self-corrosion current density of the sample decreased by about 1 order of magnitude compared with the potassium ferricyanide conversion coating,reaching 2.370×10-10 A·cm-2.It shows that the composite film layer can further improve the corrosion resistance of magnesium alloys.（3）The bonding force between the composite film layer and the substrate is further improved,and the stability is good.The main components of the surface material are Sm Fe（CN）6 and Sm（OH）3.The film formation mechanism of this film belongs to the cathode film formation mechanism.The precipitation formed by the combination of Sm3+and OH-in the cathode area hinders the process of oxygen transport and electron transfer.In addition,it inhibits the diffusion process of anode ions(Mg2+)on the surface of the magnesium alloy substrate,reducing its electrochemical corrosion efficiency.The composite film layer inhibits the cathodic and anodic reactions of corrosion,reduces the corrosion power,and further improves the corrosion resistance of the substrate.（4）The analysis results of the corrosion resistance mechanism are as follows:in the early stage of corrosion,part of the electrolyte is adsorbed on the surface of the membrane layer,a small amount of the membrane layer is dissolved,and the corrosion products fill the pores or cracks on the surface of the membrane layer,blocking the penetration of chloride ions into the substrate.As a result,the film resistance increases in the early stage of corrosion.As the soaking time prolongs,the electrolyte penetrates into the substrate,the corrosion gradually progresses to a deeper and wider area,the film is damaged,and the film resistance decreases.Therefore,in the corrosive environment,the corrosion resistance of the film layer first increased and then weakened,and the maximum film resistance and charge transfer resistance both appeared after immersion for 24 h. |
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