Lanthanum Conversion Coating Pretreated With Silicate On Hot-Dip Galvanized Steel

The traditional chromate-based passivation treatments （Chromate conversion coatings）due to its benefits of high toxicity, carcinogenic and environment hazards with the presenceof Cr（VI）, which have made its use be more and more restricted. Based on this, the study ofnew environment-friendly chromium-free passivation technology for replacing chromatepassivation be imperative, among them, silicate passivation and rare earth metal saltschromium-free passivation technology has become a research hotpot, be getting more andmore attention by researchers from different countries, but both can not completely replacethe chromate, while the combined study of silicate and rare metal salt reported less at present.This paper adopts3.5molar ratio （SiO₂/Na₂O）of sodium silicate solution as thepretreatment solution, processing of galvanized steel specimen immersed in the solution withrare earth lanthanum salt （La（OH）₃） as main salt and H₂O₂as an additive. By orthogonal test,The optimal experiments of the conversion coatings as follow: SiO₂3.5Na₂O0.5g/L, ambienttemperature, time at1min; La（NO₃）₃6H₂O9g/L, H₂O₂10ml/L, T70°C, t10min.The morphologies and composition structure of the coatings was investigated byscanning electron microscope （SEM）, energy dispersive spectroscopy （EDS）, Atomic forcemicroscopy （AFM）, X-ray photoelectron spectroscopy （XPS）, and the growth mechanisms ofthe conversion coating, which combined with Weight test, were discussed. The corrosionresistance of the coatings was further studied by Neutral salt spray test （NNS）, Tafelpolarization and Electrochemical impedance spectroscopy （EIS） curve. EIS and equivalentcircuit were used to investigate the process of growth and corrosion of the coatings.Research showed that, compared with the single lanthanum conversion coating, the rateof growth of the conversion coatings which obtained by silicate pretreatment and thenimmersed in the rare earth lanthanum solution was faster, and the coatings were uniformduring the growth. At the beginning of lanthanum salt processing, the film was smooth, anddid not appear obvious cracks, the thickness of the coating obtained from1min immersion was about440nm. The coatings appeared cracking phenomenon and the crackshomogeneously distributed throughout the film surface with the immersion time of10min. Itcould be found that the locality coatings flaked off clearly with the immersion time of30minand some white granular attachments presented on the surface of the film with the time of60min. Weight gain experiment results indicated that, the film showed a linear growth lawwith the immersion time range in10min, and then kept basic balance between layer growthand flaking off after20min.The surface state of hot-dip galvanized specimen after pretreatment with sodium silicatehad changed apparently, and then the specimen was deactivated in rare earth lanthanum salt.During the formation of the lanthanum conversion coating was formed with the depositionof La（OH）₃and a little Zn（OH）₂deposition at the same time. EDS and XPS analysisindicates that the film mainly composed of La₂O₃/La（OH）₃、ZnO/Zn（OH）₂.NSS experiments showed that, compared with the single lanthanum conversion coating,the white rust-resisting property of the conversion coatings was improved obviously, andnearly quite with chromate. Tafel result indicated that, the coatings can inhibit both thecathodic and anodic process of zinc corrosion, the sample was immersed by lanthanum saltwith10min had the largest polarization resistance and the smallest current density.Compared with the untreated HDG, the polarization resistance of the sample increasednearly50times, compared with the single lanthanum conversion coating, the polarizationresistance of the sample increased nearly2times. EIS result and the equivalent circuitanalysis revealed that the corrosion resistance of the coatings increased with the immersiontime, having the maximum corrosion resistance with the immersion time for10min, and thendecreasing gradually with continuing to increase the time.The corrosion process of the optimum technological coating is divided into three stages:at the early stage, the film was dissolved and destroyed, electrolyte solution reach to thefilm/substrate interface and the formation of the corrosion reaction; at the middle stage,electrolyte solution diffuse into the film/substrate interface; at the last stage, the corrosion ofzinc become he dominate reaction.The scratching corrosion test indicated that, The conversion coatings, which obtained bypretreatment with sodium silicate and then immersed in rare earth lanthanum salt, have good self-heal ability, which is better than the single lanthanum conversion coating...