Study On Preparation Of Titanizing Coatings On 316L Stainless Steel And Their Pitting Corrosion Behaviors

Austenitic stainless steel has been widely used in chemical,shipbuilding,foods and biomedical fields.Nevertheless,when the stainless steel is exposed to an aggressive environment containing Cl^-ions,the passive film is susceptible to localized breakdown and then corrosion must occur.Due to the dense oxide film is easily formed on the surface of metallic titanium,thereby preparing a titanium coating on the surface of stainless steel may enhance its corrosion resistance.Pack cementation has attracted people's attention due to its advantages of low cost,simple operation and the tight adhesion between the coating and the substrate.There is the limited understanding on the microstructure features of the titanium coating on the surface of stainless steel and its corrosion behavior in environments such as highly corrosive hot salt water.In this dissertation,the titanium coatings are prepared on the surface of 316L stainless steel by pack cementation.By using electrochemical testing techniques,scanning electron microscopy(SEM),X-ray energy spectroscopy(EDS)and X-ray diffraction(XRD)and other characterization methods,the effects of the pack cementation parameters on the microstructure and corrosion behavior of the coatings are studied,and the coating growth behavior is discussed.The main results obtained are as follows:The effects of Ti content,packing time and temperature on the microstructure and phase composition of the titanium coating were studied.The coatings has a compact structure,which can be divided into an inter-diffusion layer and an outer layer,and forms a metallurgical bonding with the matrix.The coating contains Ni Ti₂,Fe Ti,Ni Ti,Al₂O₃,?-ferrite,metallic Ti and Cr₉Fe₄₂Mo₂Ti₅phases.When the pack contains 57%Ti,it has the largest activity.The increase in temperature and extension of the packing time are beneficial to the growth of coating.Kinetic calculations indicate that the coatings form through an outward-grown process and follow the parabolic law.The activation energy of the coating is 140 k J/mol.,and the growth is controlled by the diffusion of Ti in the 316L matrix.The Ti content,time and temperature will not change the microstructure and growth mechanism of the coatings.The effects of the Ti content,time and the temperature on the corrosion behaviors of the titanium coatings were studied in a 3.5%Na Cl solution at 80?.The titanium coatings can passivate spontaneously in the solution and show a lower passive current density.The coatings can greatly enhance the corrosion resistance of 316L stainless steel and significantly improve the pitting resistance of 316L substrate.When Ti content increases from 47%to 77%,the corrosion resistance and pitting potential of coating shows a peak at 57%Ti in the pack mixture.Both increasing the temperature and extending the time can improve the pitting resistance of the coatings,and the pitting potential can reach 950 m V_SCE.The constituent phases of titanizing coatings display different corrosion resistances.The high corrosion properties primarily results from the Ti/Ni Ti₂ phases in the outer layer.Increasing the outer layer thickness can effectively improve the coating corrosion resistance.