| With the increased environmental awareness of people, an intense research effort is being undertaken to replace chromates aestivation by new non-chromate treatments adhesive agents. Molybdate conversion coating is considered as one of the most possible alternatives to chromate conversion coating, which presents very little toxicity and no carcinogenic nature has been studied ,and molybdate conversion coating have been widely used to improve the corrosion resistance and the painting capacities of the metals. Silane coupling agents is a new and environment-protecting surface-treatment technology applied in metal pretreatments, which has great advantages such as low cost, wide application and excellent adhesive strength to organic coatings, therefore attracts many experts' attention.Preparation of the molybdate conversion film and silane film for hot-dip galvanized (HDG) steel has been systematically developed in this paper, so as the growth, structure and anticorrosive properties of all kinds of the films. The optimal passivation technics was obtained. However, the anticorrosive properties both of molybdate conversion film and silane film still have a gap compare with chromate conversion coating. Based on the treatment technology of every single film, the continuous thicker double-layer composite films are gained by two-step treatments, i.e. top-coating silane film on the molybdate conversion film. Surface morphologies, composition and structure of above films were observed and characterized by scanning electronic microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES) and infrared spectroscopy (RA-IR), meanwhile whose formation and corrosion resistance mechanism were investigated. The results show that:(1) the molybdate conversion film is amorphous, the chemical composition content of Mo, P, O, Zn of film have continuous gradient change along the depth direction of the whole film from the surface. But if the thickness of the conversion film is more than a certain value (about 100 nm), it is apt to cracking but not throughout the whole film. (2) The silane molecules were not combined with HDG coating by the simple physical absorption, but chemical action was generated between Si-OH from silane solution and Zn-OH on HDG coating, then form metallosiloxane bonds, Si-O-Zn. This action made the interface of the silane films be combined with HDG coating tightly. The remaining silanol groups in the films condensed and formed Si-O-Si bonds, and further formed dense three-dimensional network structure on HDG coating. (3) Compared with every single film, the continuous thicker double-layer composite films are gained by two-step treatments. The HDG coating surface was mostly covered by molybdate conversion film, and they were covered by silane films which reacted with molybdate film surface and absorded to molybdate conversion coatings. Through the interaction between these two kinds of films, continuous, compact and complete composite coatings were formed on the galvanized steel substrate.The results of neutral salt spray (NSS) test show that: there is 5 to 8% area of white rust appeared on the untreated hot-dip galvanized (HDG) steel within 2 hours in the salt spray environment. If the HDG sample covered by the single molybdate coatings or silane coatings, the appearance of the 5 to 8% area white rust would delay to 1 day; if the HDG sample covered by the composite coatings, the time of Salt spray could extend to 2 day.The results of tafel polarization curve measurement show that: the anodic and cathodic processes of zinc corrosion are suppressed more markedly after hot dip galvanized steel with treatments by a variety of ways. The polarization resistance of the molybdate conversion coating (1.32~7.952 k?·cm2) obtained from the optimal technics treatment is far more than untreated hot dip galvanized steel (500~800?·cm2). The cathodic processes of zinc corrosion are suppressed more markedly than anodic processes by the molybdate conversion coating; The silane coating act as a barrier on the surface of zinc layer, which can control both anode and cathode reaction. The polarization resistance of the silane coating is 4.4~5.2 k?·cm2. Both the anodic and the cathodic processes of zinc corrosion are suppressed by the composite coatings. The corrosion current density value of the composite coatings is much smaller than that of the single coatings; the polarization resistance of the coating reach 59.02 k?·cm2, which is more than chromate conversion coating (31.4 k?·cm2). For the composite coatings, the synergistic corrosion protection effect of the molybdate films and silane films, the protection properties of the composite coatings are markedly enhanced and are highly superior to those of the chromate conversion coatings. Therefore, the composite coatings are promising to be as the replacers for the chromate conversion coatings. The results of electrochemical impedance spectroscopy (EIS) measurements show that: for the three coatings, the Low-frequency impedance (ZLF) value of the coatings is more than that of the hot dip galvanized steel; the anti-corrosion property order of three coatings were tested by electrochemical impedance spectroscopy (EIS) measurements is as same as that of neutral salt spray test (NSS) and tafel polarization curve measurement; in the Bode and Nyquist diagram, there are 2 to 3 time constant of each coating appeared, among these time constants, the low frequency time constant corresponds to the Charge transfer resistance (Rc t )and electric double layer capacitance (Cdl), high-frequency time constant corresponds to the film resistance (Rf ) and film capacitor (Cf). Because of the existence of oxygen concentration and the slowly oxygen diffusion, there are diffusion resistance occurring in the zinc-based interface of the Molybdate coatings and composite coatings. In the EIS diagram of molybdate film, silane film and composite film, the first three time constants appeared, which reflects another thin layer on the coating surface, film resistance (Rf1) and film capacitor (Cf1), and the film resistance (Rf1) value is very small, may correspond to the existence of a loose or cracks (electrolyte can penetrate) thin layer. |
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