Characterization Of Self-assembled Monolayers And Conversion Coatings On Magnesium Alloys For Corrosion Protection

In this study, the self-assembled monolayer and chemical conversion on magnesium alloys, AZ91D and AM60, were investigated, respectively. The experimental work consisted of two parts:â… . The formation of Self-assembled monolayer (SAM) of oleic imidazoline (OI) on Mg alloy (AZ91D) substrate.â…¡. The conversion coatings formed on AM60 substrate in molybdate/phosphate solution.In the first part, the SAMs of OI were successfully formed on Mg alloy (AZ91D) substrate treated in bi-distilled water solution. The assembled mode, surface performance and the assembled parameters, the protection efficiency (PE) of this kind of SAM for AZ91D substrate were studied by contact angle, FT-IR, EIS and linear polarization curves. These results showed that the functional group, -NH-, as head group reacted with Mg oxide and alkyl, -R(CH3(CH2)n-, n=17), afforded tail group up the surface for the OI-SAM. The optimal assembly parameters were summarized following as: assembled concentration, 5×10-4 mol.L-1; pH=8; room temperature (25℃). Well-ordered SAMs taken a long time, 72h, were necessary to achieve the ordered and dense adsorbed OI layer on the AM60 surface and the maximum PE value up to 98.1% was obtained by EIS data.In the second part, the mechanism, morphology, phase structure, optimal parameters and corrosion performance of the conversion coatings were analyzed using scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and electrochemical measurements in the molybdate and molybdate/phosphate baths containing some metal ions (Ca2+ and Mn2+) as well as additives on the AM60 substrate, The composite conversion coating formed in molybdate/phosphate bath had excellent performances beyond either simplex molybdate conversion or phosphating coating on the AM60 surface. The results showed that the Molybdate/Phosphate coating containing composite phases consisted of Mex(PO4)y, CaMoO4, MgAl2O4, MgO, Al2O3, MnO, as well as molybdate oxide with an"alveolate-crystallized"structure. However, molybdating coating presented an amorphous structure with"network-like"and the value of corrosion resistance was lower than that of molybdate/phosphate coating near 2 orders of magnitude. A optimal parameters of conversion bath were concluded following as: Na2MoO4, 0.15 mol.L-1;Mn (Ac)2, 0.05 mol.L-1;NaH2PO4, 0.30 mol.L-1;Ca(NO3)2, 0.01 mol.L-1;additives; temperature, 50℃;pH=5.0 and 5 min. The mechanism, corrosion/deposition process, of molybdate/phosphate conversion process was similar to the phosphating process on steel surface. Their difference was displayed by the fact that the composite coatings of molybdate/phosphating containing both metaphosphate and molybdate salts as well as oxides afford the corrosion protection for AM60 alloy substrate. The pH of conversion solution, molar ratio (MR) of MoO₄^2-/ H₂PO₄^- and the species of metal ions in conversion solution play important roles on the corrosion performance of conversion coatings. The optimal conversion coating performance appear for only MR is about 1/2 and pH=5, simultaneously, solution should contain Ca²⁺ and Mn²⁺. The better corrosion resistance of molybdate/phosphating coating was also confirmed in salt water immersion tests via comparing with the traditional chromate coatings.