Study On The Assembly Of Organic-inorganic Complex Conversion Layers On The Cold-rolled Steel Substrate And Their Corrosion Resistance Performance

In the field of metal corrosion protection,chemical conversion layer behaves like the "double-sided adhesive"between the metal substrate and the primer,which can not only protect the metal substrate from corrosion to some extent,but also improve the adhesion between the parimer and the metal substrate.Therefore,it is widely used as the pre-treatment process before coating.The traditional chemical conversion layers mainly include chromate and phosphate coatings.However,the waste water discharged by chromate passivation process contains the carcinogenic hexavalent chromium,which leads to harmful influence on human being and the ecosystem environment.Similarly,the waste water and waste residue generated by the phosphating process contain phosphate,heavy metal ions and other pollutants.Although phosphate is less toxic than hexavalent chromium,the discharge of a great amount of phosphate can cause the phosphorous pollution that also needs to be dealt with.In view of human health and environmental problems,the above-mentioned two technologies have been banned or greatly restricted in many areas,so it is urgent to develop a new generation of environmentally-friendly chemical conversion layer products.Organic phosphonic acid have the advantages of stable structure,low toxicity and low irritation,moreover,P,O and other heteroatoms in their molecules can be anchored onto the metal surface as the adsorption center and the as-formed adsorbed layer can work as an insulation layer to prevent the metal substrate from corrosion to some degree.In recent years,a lot of work has been focused on the application of organic phosphonic acid as the corrosion inhibitors.However,the organic phosphonic acid self-assembled films are very thin(nanometer scale)and have some defects.In order to solve this problem,the complexation effect between organic phosphonic acid and inorganic metal ions is used to construct a metal complex film with three-dimensional structure on the surface of metal substrate,which provides a new method for the preparation of environmentally friendly conversion layer.In this paper,the coordination-driven self-assembly technology was used to construct different types of organic-inorganic complex conversion layers on cold-rolled steel sheets.Firstly,three kinds of 2-phosphonobutane-1,2,4-tricarboxylic acid-rare earth complex layers(PBTCA-RE layers for short;RE = La,Ce and Nd)were fabricated on the cold-rolled steel sheets by using the above method,and the protective efficiency,microstructure and the film forming mechanism of three PBTCA-RE layers were studied in detail.Then,based on the coordination-driven self-assembly technology,by applying sodium molybdate to further modify the 2-hydroxyphosphonoacetic acid-zinc complex conversion layer(HPAA-Zn layer for short),the optimum conditions for the fabrication of high quality Mo-incorporated conversion layer were determined,and we investigated the effect of sodium molybdate doping on corrosion resistance,surface morphology and chemical compositions of the conversion layer.The main contents of this thesis include:(1)Environmentally friendly rare earth complex-based chemical conversion layers with good corrosion resistance performance on the cold-rolled steelsubstratePBTCA-La layer,PBTCA-Ce layer,PBTCA-Nd layer were fabricated on the cold-rolled steel substrate by using water heat treatment and self-assembly technology.The obtained results showed that,within a certain concentration range,there existed the strong synergistic effect between rare earth ions and PBTCA.Because rare earth elements had more empty orbitals,oxygen atoms in the phosphonic acid group andcarboxyl group of PBTCA had lone pair electrons,rare earth elements would react with PBTCA,generating coordination complex which will deposite on the surface of cold-rolled steel sheet.In addition,the rare earth oxides produced in the process of film formation were doped in the conversion film,which further enhanced the corrosion resistance of the conversion layer.The results of XPS and EDS showed that three rare earth complex conversion layers were composed of PBTCA-RE complex and corresponding rare earth oxides.However,it can be seen from the results of electrochemical impedance spectroscopy and steady-state polarization curve that,three kinds of rare earth complex layers prepared under the same conditions displayed different anticorrosive efficiency,and the corresponding sequence from large to small was:PBTCA-La layer>PBTCA-Ce layer>PBTCA-Nd layer.On the basis of SEM observations,the PBTCA-La layer on the surface of cold-rolled steel sheets was compact and uniform,while the surface of PBTCA-Ce layer and PBTCA-Nd layer had some cracks,which provided pathways for corrosive electrolyte to diffuse into the substrate surface,thus producing the adverse impact on the corrosion resistance of the conversion layer.The width of cracks in PBTCA-Nd layer was larger than that in PBTCA-Ce layer.(2)Corrosion protection performance of Mo-incorporated 2-hydroxyphosphonoacetic acid-Zn complex conversion layers on the cold-rolled steel substrateOn the basis of the fabrication of HPAA-Zn conversion layer that was fabricated by utilizing the complexation effect between organic phosphonic acid and metal ions,we further prepared Mo-incorporated HPAA-Zn layer on cold-rolled steel substrate.Electrochemical impedance spectroscopy(EIS)was used to evaluate the effects of the molar ratio of HPAA/Zn2+,film-forming time and sodium molybdate addition amount on the corrosion resistance of chemical conversion layers.The optimum conditions for the fabrication of high quality Mo-incorporated conversion layer were determined:HPAA/Zn2+ molar ratio was 1/1,the film-forming time was 10 min,the concentration of sodium molybdate was 0.1 wt%,and pH of the film-forming solution was 4.Electrochemical tests and morphological characterizations showed that Mo-incorporated HPAA-Zn layer had the better corrosion protection ability than Mo-free HPAA-Zn layer,which was attributed to the better uniformity and improved barrier properties of the Mo-incorporated layer.Mo-incorporated HPAA-Zn layer was mainly composed of ZnO and HPAA-Zn(?)complex,and the occurrence of Mo-O-P bonds facilitated the formation of HPAA-Zn layer on the cold-rolled steel substrate.