Fabrication And Characterization Of Hopeite Conversion Coating On Stainless Steel

Zinc phosphate chemical conversion?ZPCC? coating are widely used for anticorrosion of metal surface,improving paint adhesion and surface lubrication because of its high adherence to the substrate,good corrosion resistance and wear resistance.Furthermore,zinc phosphate?hopeite,named H?has good bioactivity and biocompatibility,which can be used as a potential biomedical material.Therefore,hopeite coating has attracted more attention in recent years for surface modification coating on implants.Usually,it is difficult to deposit a ZPCC coating on stainless steel?SS? by traditional chemical conversion method due to the passive oxide layer on the surface.In this research,it was focused on facile preparation of ZPCC coating on SS by traditional chemical conversion method.The effect of galvanic coupling and phosphating process on the microstructure of H coatings was studied,and the formation parameters of H coatingswere optimized.The optimized ZPCC solution formula was obtained by analyzing the effect of its composition on the H coating mass.And the uniform and fine ZPCC coating was fabricated on SS by traditional chemical conversion method.The phase composition,morphology,surface composition,microstructure,radicals,surface roughness and electrochemical properties of ZPCC coatings were studied by X-ray diffraction?XRD?,fourier transform infrared spectrum analyzer?FTIR?,field emission scanning electron microscopy?FE-SEM?,high resolution transmission electron microscopy?HRTEM?,X-ray photoelectron spectroscopy?XPS?,atomic force microscopy?AFM?and electrochemical workstation system,respectively.Based on the previous works,the effect of diffarameters on the morphology and thickness of the deposition of HA film on ZPCC surface was studied through changing the simulated body fluid?SBF?concentration,immersion time and solid/liquid ratio.Meanwhile,the biocompatibility of ZPCC coating also was peimarily discussed in this paper.The results revealed that the ZPCC coating was mainly composed of hopeite with minor phosphophyllite.However,the diffraction peaks of ZPCC showed that the planes of crystal was preferentially growing along?020?and?040?planes.Based on studying the influence of galvanic coupling and pretreatment on the formation of the ZPCC coatings,the optimum technical process were confirmed.The galvanic coupling of SS with pure iron was used.Pickling was performed on the specimens in a solution containing 7%V/V of phosphoric acid at room temperature for 10 min.Afterwards,activation was carried out in a solution of 3 g/L Ti colloids?Na₄TiO?PO₄?2,commercially obtained?at room temperature for 30 s.The fine and uniform coatings were formed at the concentration of 0.06-0.2 g/L of Fe²⁺.By orthogonal test,the compositions of ZPCC solution were zinc oxide:27.5 g/L,phosphoric acid?85%?:17.5 ml/L,nitric acid:30 ml/L,sodium chlorate:2.0 g/L,citric acid:5 g/L and calcium nitrate:5 g/L.Coating mass enhanced with increasing of pH value,temperature and time.The crystals became coarse when the pH value was below 2.0,while the coatings were fine and compact when pH value between 2.5 and 3.0.The substrate surface was completely covered with crystals as temperature from 45 to 85 ? as well as for 5 to 45 min treatments.The results of microstructure indicated that the distinct nanoscale structures were found with increasing of the pH value,the temperature and time,which showed homogenous distribution.The grain surface was characterized by the net-like microstructure with micro-nano structures,while the side surface of that was parallel sub-micro flake structures,the microstructure of which was beneficial for biological response of ZPCC coating.It was also found that the ZPCC coatings with a bilayered structure were adjacent outer coatings with porous,while the inner coatings were dense and uniform.Based on the potential-time curves,the mechanism of coating formation was discussed.The formation of ZPCC coating was defined into four stages:the dissolution of galvanic coupling substrates,amorphous precipitation,crystallization and growth of the crystals as well as crystal reorganization.The pull-off strength of ZPCC coatings increased to the maximum values under pH of 2.75 and at 75 ? for 30 min treatment,the values of that were 27.67±0.63 MPa,25.42±0.52 MPa and 28.96±0.32 MPa,respectively.Furthermore,the coatings formed at those conditions also showed the best corrosion resistance behavior and hydrophily.As such,it could be deduced that the fine,uniform coating was formed under pH value of 2.75,at 75 ? for 30 min,which possessed good corrosion resistance and highly pull-off strength.The HA film deposited on the ZPCC coating was uniform as the simulated body fluid?SBF?concentration increased to 1.5 times.The thickness of HA films was about 1 ?m after immersing for 15 d.While it increased to 2 ?m after immersing for 30 d.In addition,an uniform HA film was also formed on the surface of ZPCC coating when the solid/liquid ratio was 8:1,and its thichness increased 10 times comparing with that rario was 1:1.The deposited HA film was composed of lamellar structure with flower-like porpus crystals.