Preparation And Properties Of Ceramic Coating On Q235Low-carbon Steel By Micro-arc Oxidation

The ceramic coating was obtained on Q235low-carbon steel by combination ofhot-dip aluminizing and micro-arc oxidation (MAO). In the step of hot-dip aluminizing,the effects of hot-dipping temperature, immersion time and pulling rate on the coatingthickness has been studied, and the hot-dipping technologic parameters fitting formicro-arc oxidation was determined. In the step of micro-arc oxidation, the influences ofthe composition and concentration of the electrolyte, current density and treating time onthe morphology, composition and properties of MAO coatings were studied in detail. Themorphologies of surface and cross-section and the composition of ceramic coating wereanalyzed respectively by SEM and XRD and the hardness, corrosion resistance, hightemperature oxidation resistance and thermal shock resistance of the ceramic coating werealso tested.Experimental results show that:(1) The hot-dipped aluminum coating is made up of internal alloy layer and externalrich-aluminum layer. Thickness of alloy layer is affected mostly by immersion timeamong the hot-dipping temperature, immersion time and pulling rate, while the thicknessof rich-aluminum layer is influenced greatly by pulling rate. The hot-dipping temperaturehas obvious effect on both of two layers. Within the experiment range, the maximalthickness of aluminized coating is about170μm, and the corresponding technologicparameter is: hot-dipping temperature760°С, immersion time120s and pulling rate20mm/s. XRD patterns shows that the main components of surface layer is Al, whichcoexists with a small quantity of Fe-Al intermetallic compound. (2) A large number of crater-like discharging channel and spherical-similar particlesexist on the surface of MAO coating. Compared with Na2SiO3electrolyte, the MAOcoating prepared in NaAlO2electrolyte have flat surface with less holes and particles. Inthe same electrolyte, the higher the concentration of main salt, the larger the particlesproduced on the coating surface are, and the higher the current density, the wider the sizedistribution of the particles produced on the coating surface are.(3) The XRD patterns of MAO coating shows that, the main components of MAOcoating is κ-Al2O3, θ-Al2O3and a small number of amorphous Al2O3. Besides, thecoating prepared in Na2SiO3electrolyte also contains Al2O3-SiO2compound and SiO2.(4) The hardness of MAO coating reaches about HV900, which is4times of Q235steel. The hardness of MAO coating prepared in Na2SiO3electrolyte is slightly higherthan that in NaAlO2electrolyte, and increase with extension of treatment time.(5) The corrosion resistance of MAO coating is much better than steel substrate. Andthe corrosion resistance of MAO coating prepared in Na2SiO3electrolyte is little betterthan that in NaAlO2electrolyte. Within the range of experimental parameter, the corrosionresistance of MAO coating improves first and then drops with increase of current density,and improving as treatment time goes by.(6) The high temperature oxidation resistance of MAO coating is better than304stainless steel and Q235steel. The coatings prepared in Na2SiO3and NaAlO2electrolyteshave nearly the same high temperature oxidation resistance.(7) The thermal shock resistance of MAO coating prepared in Na2SiO3and NaAlO2electrolytes are nearly the same, and which is increased with increasing of current density,but decreased with extension of treatment time.