Preparation And Electrochemical Research Of Boron-containing Coating On Low Carbon Steel

Low carbon steel, as an important structural material, is widely used in many fields suchas chemical industry, metallurgy, electronic information,etc. However, due to its soft surfaceand high chemical activiey, it can not meet some specific requirements. Therefore, to improvethe surface properties of low carbon steel is an urgent and also an important research directionin material science. In recent years, the rapid development of surface treatment technology isgaining widespread attention. This paper mainly studied the alloy layer and the compositesediments on the low carbon steel surface as well as the associated electrochemical researchin order to improve the performance of low-carbon steel surface, especially the corrosionresistance, hardness and abrasion resistance.The type of coating and the preparation method are mainly divided into two categories:The first category is a new thermal diffusion method based on the ultrafine powder coatingtechnology which is proposed from the previous studies on the solid-phase thermal diffusionmethod. The ultrafine powders are loaded on the surface of the low carbon steel. Afterinsulated in hydrogen atmosphere, Fe-B alloy layer will generate on the surface of the lowcarbon steel. The second category uses composite electrodeposition method to prepare theFe-FeB, Fe-Ni/FeB and Ni-B4C composite sediment on the low-carbon steels which is basedon the unique physical and mechanical properties of composite sedimentary. The crystalnucleation/growth process in the single system and composite system were analyzed by thecyclic voltammetry (C-V), constant potential step (i-t) and electrochemical impedancespectroscopy (EIS).The corrosion resistance of the boron-containing coating was also studiedby EIS and Tafel. Meanwhile, this paper also examed the passivation behavior of low-carbonsteel surface covered with boron-containing layer.For the boron-containing layer samples, the study mainly focused on the surfacemorphology, roughness, corrosion resistance, hardness and abrasion resistance of the Fe-Balloy layer samples, which were prepared at different composition ratio and different thermalinsulation temperature. The results showed that the corrosion current density of the samplewas lower than the low carbon steel substrate. The influence trend of maintaining the temperature on the corrosion current density was the same as on the corrosion potential: thecorrosion current density of the sample decreases with the increase of the temperature ofincubation. The sample reached the minimum corrosion current density at900℃and themolar ratio of Fe/B in the powder was2/1. Under this condition, corrosion resistance wasthe best and it had the best ability to protect the matrix, the highest micro-hardness and thebest abrasive resistance.For the study of the Fe-Feb Fe-Ni/FeB and Ni-nano-B4C composite deposition layer, thispaper started from the composite electrodeposition influencing factors, including theconcentration of particles in solution, the cathode current density, pH, plating temperature andstirring rate of the solution, and further studied the related properties of three types of theboron-containing compound. Ni-nano-B4C composite sediments samples showed the bestcorrosion resistance and Fe-Ni/FeB composite sediments samples took the second place, andFe-Feb complex deposition layer samples showed relatively poor corrosion resistance whichwas similar to the matrix. One well-known drawback of most composite deposited layer is theporosity and internal stress. After heat-treatment at appropriate temperature, porosity andinternal stress of the composite deposited layer will be improved to some extent. Therefore,after heat treatment, the corrosion resistance of the three kinds of composite sedimentssamples was improved. At the meanwhile, these three kinds of composite sediments samplesall showed sigficanlty higher surface hardness and better wear resistance than the low carbonsteel substrate.In borate buffer solution, both the Fe-B alloy layer made by thermal expansionpenetrates and the compound deposited layer with boron-containing made byelectrodeposition performed better on the passivation than the low carbon steel substrate. Thepassivation film formed on the surface of the low-carbon steel was n-type semiconductor,which was the same as the Fe-B alloy layer sample and Fe-Ni/FeB compound deposition layer.And the majority carrier density in the film was electrons. However, passive film formed onthe surface of Ni-nano-B4C composite deposition layer performed p-type semiconductingcharacteristics, and the majority carrier was hole. The carrier density of the surfacepassivation film was also significantly lower than that on the surface of low carbon steel.Classic PDM mode was used to calculate the diffusion coefficient of the point defects for the deposited layer of the Fe-B alloy layer samples and the Ni-nano-B4C composite layer surfacepassivation film. The diffusion coefficient of the Fe-B alloy layer was10-17~10-14cm2/s, andthe Ni-nano-B4C composite sedimentary layer was10-14cm2/s. By using the quantitativeanalysis through the carrier density of the diffusion coefficient, it was determined that theelectro-conductibility of surface passivation film was mainly affected by the film carrierdensity.