| The chromium coating has been widely used in automotive,electronics,daily hardware and aerospace fields because of its high hardness,good wear resistance,good corrosion resistance and decorative appearance.Traditional hexavalent chromium electroplating is harmful to the environment and human health because of its high toxicity,which can not meet the needs of green development.Compared with hexavalent chromium,the trivalent chromium electroplating has some advantages such as low toxicity,no current interrupt affection,no equipment corrosion and so on,which possess good market application prospect.At present,the sulfate trivalent chromium plating is usually applied to decorative coatings,the electrodeposition of hard chromium is still not yet mature,pending further study.In this paper,based on our previous research of decorative sulphate trivalent chromium plating,hard trivalent chromium plating has been successfully developed through a large number of experiments.The characteristics of hard chromium coating were studied by X-ray fluorescence measurement(XRF),scanning electron microscopy(SEM),X-ray energy dispersive spectroscopy(EDS),X-ray diffraction(XRD),microhardness tester and Tafel curve.The effects of heat treatment and Fe2+concentration on the coating were investigated.In addition,the mechanism of Cr3+electrodeposition was studied by linear sweep voltammetry(LSV)and cyclic voltammetry(CV).The effect of Fe2+ and additive in the bath was also investigated.The main contents and results of this thesis are as follows:1.Hard chromium electrodeposition from sulfate trivalent chromium bath and coating characterizationWe have successfully developed hard chromium process from sulfate trivalent chromium bath through a large number of tests,which can be used in practical production.The optimum bath composition and process conditions are as follows:Cr2(S04)3·6H2O 25 g·L-1,H2C2O4·2H2O 10 g·L-1,H3B03 70 g·L-1,Na2S04 100 9·L-1,sulfur-containing organic compound 200 mg·L-1,pH 3.0?4.0,temperature 35?55 ?,the anode is Ti-IrO2(DSA),the current density of 3?6 A·dm-2,magnetic stirring.The dispersion power of the bath is 95%and the coverage power is 100%.The trivalent chromium electrodeposition current efficiency is more than 20%with current density of 3?6 A·dm-2.And the chromium deposition rate was 0.12 ?m·min-1 at current density of 3 A·dm-2 and within 60 min.The thickness of the chromium coating was up to 10.6 ?m at the current density of 3 A·dm-2.and plating time of 120 min.The coating has nodular morphology and nanocrystalline structure,which process good adhesion to bright nickel layer.The microhardness of the coating reaches to 789.2 Hv.The corrosion nothetial(Ecorr)of the coating in the 3.5wt%NaCl solution is-0.29 V and the corrosion current density(jcorr)is 9.26×10-5 A·dm-2.2.Effect of heat treatment and Fe2+ on properties of hard chromium coatingThe morphology and structure of the coating changed after heat treatment.After heat treatment at 100 ?,the morphology of the coating transformed from nodular to flat,dense and micro-crack.With heat treatment temperature increased,the coating surface was more compact and the crack structure gradually deepened,and the micro-hole appeared.Heat treatment temperature is further increased to 600 ?,the coating surface is still relatively flat and dense,but began to gradually have other crystal precipitation;The coating surface has been oxidized and covered with crystal,when heat treatment temperature was up to 700 ?.Heating the coating at 100?300 ?,the nanocrystalline began to agglomerate.Heating at 400?500?,the coating transformed into crystalline structure,and the chrome grains began to coarsen.When the heat treatment temperature is raised to 600?700 ?,Cr3C2,Fe3C and Cr2O3 were precipitated with chrome grains coarsening.After heat treatment,the characteristics of the coating became better.The microhardness of the coating reached a maximum of 1158.4 Hv after heat treatment at 400 ?.The corrosion potential of the coating was-0.16 V and the corrosion current density was 1.78 × 10-5 A·dm-2 after heat treatment at 500 ?,which had the best corrosion resistance.Overall,the optimum heat treatment temperature of the coating is 400 ?.Fe2+ was in co-deposition with Cr3+,which can affect the structure and properties of the coating.When the content of Fe2+ in the bath was 100 mg·L-1,the content of Fe in the coating was 3.62 wt.%,and the coating had a micro-crack morphology,which was nanocrystalline.With the increase of Fe2+ content in the bath,the Fe content in the coating also increased.When the content of Fe2+ in the bath was 300 mg·L-1,the content of Fe in the coating was up to 21.86 wt.%,the morphology of the coating changed from micro-crack to nodular,and the coating was amorphous.With the further increase of Fe2+ content in the bath,the Fe content in the coating was also further increased,but the morphology and structure of the coating were no longer changed.The microhardness of the coating increased with the increase of iron content,and the microhardness reached 928.9 Hv when the iron content was 23.78 wt%.With 21.86 wt%iron content in the coating,the corrosion resistance of the coating reached to the best,the corrosion potential in the 3.5wt%NaCl solution was-0.25 v,and the corrosion current density was 7.45 × 10-5 A·dm-2.3.Study on mechanism of trivalent chromium electrodeposition from sulfate bathThe trivalent chromium electrodeposition involved two consecutive reduction steps.The first step,Cr3+ + e ? Cr2+,is controlled by the electrochemical' reduction and diffusion process,and the second step,Cr2+ + 2e ? Cr,is an irreversible process under the diffusion control.The increase of temperature and the concentration of trivalent chromium both weakened the reduction polarization of Cr3+,so that the reduction potential became more positive and the current density at the same reduction potential increased.Fe2+ can promote the electrodeposition of Cr3+,mainly in the promotion of Cr2+ to Cr,but not with the process of Cr3+ to Cr2+.The,additives played the role of depolarization of cathodic reduction of Cr3+,which increased the deposition rate of Cr3+ and then promoted the deposition of Cr3+. |
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