Research On The Influence Of Adulterating Sc Y La On Corrosion Resistance Of Zn-fe Alloy Coating

Zn-Fe alloy has become one of the major protective coatings for steel materials. The growing attention has been drawn to Zn-Fe alloy, since the alloy is economical and also possesses many properties, including an excellent resistance for salt spray corrosion and low hydrogen embrittlement. In this thesis, series of Zn-Fe alloy coatings have been obtained when different rare earth metal ions are added in sulfate bath system. The micro-structure, apparent morphology and corrosion resistance in sulfuric acid have been studied as well.Zn-Fe alloy coating on the surface of A3 steel plate was fabricated via direct current. The effects of temperature, time, pH and current density to the corrosion resistance were studied respectively on this basis. The optimal parameters for electroplating as follows: the temperature of electroplating solution is 25℃, the time for electroplating is 30 minutes, the pH of the bath is 4.4 and the current density is 25mA?cm-2.Scan electron microscope (SEM) and X-ray diffraction (XRD) were employed to monitor the microstructure and appearance of the Zn-Fe alloy coatings by adding scandium salt, lanthanum salt and yttrium salt into the bath. The Linear sweep voltammetry and AC impedance methods were used to study the corrosion resistance of the Zn-Fe alloy coatings. The results were shown below: (1). A uniform and dense coating could be obtained when adding certain amount of yttrium salt, which could change texture coefficient and the preferred orientation from (002) to (101). The initiating passivation current density (ipp) and maintaining passivity current density (ip) of coating corrosion reaction decreased significantly, and the value of ip changed from 28.87 mA·cm-2 to 2.056 mA·cm-2. The passivation potential shifted negatively at 30°C in 0.2 mol·L-1 H2SO4 solution. The Rp increased from 12 ?·cm2 to18?·cm2, which could cause the dramatically increasing corrosion resistance of Zn-Fe alloy coatings in acid solution. (2). A uniform and dense coating were prepared and the texture coefficient was changed by adding a certain amount of lanthanum salt, while the preferred orientation was still (002) and the diffraction peak intensity of main crystal face (002) decreased. The initiating passivation current density (ipp) and maintaining passivity current density (ip) of coating decreased and the passivation potential shifted negatively at 30°C in 0.2 mol·L-1 H2SO4 solution. The value of ip changed from 20.51 mA·cm-2 to 12.45 mA·cm-2 and the Rp from 10 ?·cm2 to 17 ?·cm2, which demonstrated that the corrosion resistance of Zn-Fe alloy coatings in acid solution were increased after adding lanthanum salt in bath. (3). Smaller crystal size and uniform texture of coating were obtained when adding certain amount of scandium salt, while the preferred orientation was still (002) and the diffraction peak intensity of main crystal face (002) decreased. The initiating passivation current density (ipp) and maintaining passivity current density (ip) of coatings corrosion reaction could decrease and the passivation potential shifted negatively at 30°C in 0.2 mol·L-1 H2SO4 solution. The value of ip changed from 20.51 mA·cm-2 to 1.75 mA·cm-2 and the Rp from 10 ?·cm2 to 16 ?·cm2, which indicated that the corrosion resistance of Zn-Fe alloy coatings in acid solution were increased after adding scandium salt in bath. By comparing the corrosion resistance of coatings obtained in the optimal conditions respectively, the conclusion indicated that the coating with the best corrosion resistance was obtained by adding scandium salt into the bath.