Hybrid Treatment Of Chromium Electroplating And Nitriding On Mild Steel

It has been a hot topic to prepare nanostructured electroplated layers by traditional electroplating method in the field of nano-surface engineering. Electroplated chromium layer was always studied for its corrosion or wear resistance, but seldom for its structure. However, there are a number of defects in the electroplated layer for the sake of electrical crystallization. Because of thermodynamic instability, these defects will change with the temperature rising. Moreover, nanostructure is in the state of high energy, it will lead to gain growth when temperature rising. All the above matters can influence the performance of electroplated layer. Therefore, study of structure and thermal stability of nanostructured electroplated chromium layer, as well as changing of interdiffusion and microhardness, are highly needed.The nanostructure of electroplated chromium layer with the thickness of 10μm that prepared by industrial chromium electroplating method was studied on Q235 mild steel by means of X-ray diffraction (XRD), scanning electron microscope (SEM) with energy dispersion spectrometer (EDS), transmission electron microscope (TEM), as well as microhardness test. Its corresponding thermal stability ranging from 200℃to 600℃was analyzed. The results showed that the original grains, about 10 nm in size, grew to more than 100 nm after annealing at 500℃. The interdiffusion layer between chromium layer and mild steel was thicker and thicker with the annealing temperature rising. Meanwhile, the microhardness of chromium layer declined continuously, when over 400℃, the falling rate of hardness started to speed up.Though chromium electroplating can improve the hardness, wear resistance, as well as corrosion resistance on the surface of material, these performances may lower at high temperature. In order to solve this problem, it was treated by hybrid treatment in this paper, that is, industrial gas nitriding, ionitriding as well as high temperature nitriding were carried out on the as-electroplated chromium layer in order to form CrN layer. Because the high corrosion resistance of CrN and the microcracks on the surface in the plating state were closed and covered by the nitride, the surface corrosion resistance of the whole material was greatly improved after hybrid treatment. By using XRD, SEM and EDS, the microstructure and morphology of the hybrid treated samples were analyzed. The corrosion resistance was also studied after hybrid treatment by salt spray test.The results showed that a hybrid treatment layer could not formed by industrial gas nitriding and ionitriding. But high temperature nitriding could form the chromium nitride layer and the depth of this layer was deeper and deeper with the nitriding temperature rising and the nitriding time prolonging. Also the chromium nitride gradually changed form Cr₂N to CrN. Besides, the content of NH₄Cl, the flow ratio of N₂, the dryness of furnace, as well as the process of in and out furnace, had some affects to the nitriding results. High-temperature-nitriding at 900℃for 6h (a little NH₄Cl) was superior to other nitriding technologies. It is the much better hybrid treatment technology on this experimental condition. The corrosion resistance of hybrid treated samples excelled to as-electroplated samples.