| As a replacement for Dacromet, waterborne chrome-free zinc aluminum coating is a eco-friendly product due to completely abandoning the use of toxic Cr6+, which inherites the advantages of Dacromet, such as high corrosion resistance, no hydrogen embrittlement, high heat resistant and so on. However, the present water-based chrome-free Dacromet coatings exhibit worse corrosion resistance, much higher costs and short storage lifetime, thus limiting their widespread application in China. Aimed at several problems for the study of chromium-free Dacromet, from component design of the coating, the mechanism of inhibitors and binders, improvement of the ingredients and composition uniformity of the coating and hydrogen evolution inhibition of water-based coating, a series of low temperature curing water-based non-chromium Zn-Al-based multi-alloy coatings with different aluminum content were developed, which exhibited good resistance to corrosion in marine atmospheres. The influence of Al content on the coating protection mechanism and the anti-corrosion mechanism were analyzed deeply and systematically, which provided technical support for long-lasting corrosion protection for steel structures in the maritime atmosphere.Firstly, the components of the coating were designed and screened. The coating formulation and process of chrome-free Zn-Al alloy coating were designed and optimized by orthogonal test. The optimization formulation was as follows: the proportion of Zn-20 Al alloy flakes, AC66, phosphorus sodium molybdate, Tween-20 and ethylene glycol was 20%, 5%, 1~1.2%, 2.4% and 8%, respectively, the additive for silane hydrolysis methanol 、 the antifoaming Dow Corning and Thickener hydroxyethyl cellulose, the rest is deionized water. The best preparation conditions were: After dipping in the paints, samples were pre-dried at 100℃ for 10 minutes, heat to 280℃ at a rate of 10℃/min and then kept at a constant temperature for 25 minutes. After the second coating, the salt spray resistance of the coating in the thicknesses of 10~15μm was around 1200 h and the resistance to 10% Na Cl solution was more than 150 d. Based on the formulations of Zn-20 Al alloy coating, the Zn-30Al、Zn-40Al、Zn-55 Al and Zn-65 Al alloy coatings were prepared by adjusting the content of the dispersant. The 10% Na Cl solution accelerated immersion tests was used to evaluate the corrosion resistance of the coatings. The results showed that Zn-55 Al coating exhibited the best corrosion resistance to salt water, and no red rust occurred after 205 d of immersion.The degree of hydrolysis of the silane can determine the stability of the silane hydrolysate solution as well as the appearance、adhesion and corrosion resistance of the coatings. The variations of the conductivity of the solution with the hydrolyzed time prolonged were investigated which were caused by hydrolysis solvent, p H and additives, respectively. The optimal hydrolysis process was given: silane(AC66): methanol: H2O=1:1:3, p H=5, 0.15% ethylene glycol. Under this condition, a larger amount of Si-OH was able to obtain and could be stable, and the process of coating preparation was further optimized. The infrared spectroscopy results of the solution showed that the silane hydrolysate solution was compatible with lanthanum nitrate or phosphate sodium molybdate, which was beneficial in promoting the hydrolysis of silanes. The infrared spectroscopy technique was used to study the curing process of the coating, the action mechanism of epoxy silane AC66 in Zn-Al alloy coating was discussed combined with the chemical bonding theories proposed by B.Arkles, and the structure diagram for silane zinc-aluminum flake coating was given.The coating microstructure and composition of Zn-20 Al alloy were characterized by using SEM, EDS and XRD. The evolution of the corrosion products morphology and chemical composition of Zn-20 Al coating immersed in 10% Na Cl solution were studied, and the formation mechanism of corrosion products was explored. The electrochemical corrosion behavior of Zn-20 Al coating in 5% Na Cl solution was systematically investigated by corrosion potential(Ecorr), low frequency impedance value(|Z|0.01) and electrochemical impedance spectroscopy(EIS). The impedance data were fitted to appropriate equivalent circuits to explain the different electrochemical processes occurring at the electrode-electrolyte interface. The results showed that the coating corrosion process was divided into three stages, and the sacrificial anodic protection played a dominant role during the immersion process. The Ecorr and EIS data for the specimens with different scratch widths dipped in 10% Na Cl solution with time were investigated. The greater the scratch width was, the faster the potential shifted in a positive direction, the more significant the shape differences of EIS data between the artificial scratches coated specimens and the intact coating. Finally, the corrosion diagram for the different plots near the artificial scratches of Zn-Al coatings was given to explain the reason why there were differences in the corrosion behavior among the coatings with different widths scratch.The electrochemical corrosion behaviors of the Zn-Al alloy coatings with different Al content in salt water as well as the variation of morphology and composition of corrosion product with aluminum content were systematically studied. The Ecorr-t curves and the corrosion morphologies of the scratchs(the width of all scratches was 0.5mm) on the coatings with different Al content were also investigated. Then, the influence of the aluminum content of the coating on the corrosion mechanism changes was discussed deeply. The results indicated that the effect of sacrificial anode protection was weakened and the passivation(barrier protection) of corrosion product film was enhanced with the Al content in Zn-Al alloy coating increased. The protective mechanism of Zn-Al alloy coating was the interaction result of barrier protection and sacrificial anode protection, and the corrosion resistance of Zn-Al alloy coating was related to the contribution rate of two mechanisms.To further improve the overall performance of the coating, Al-Zn-Si-RE alloy was developed by adding micro alloying elements such as Si and RE into 55Al-Zn alloy, and then Cr-free Al-Zn-based alloy coatings were prepared. The electrochemical corrosion behavior of Al-Zn-Si-RE coating in 3.5% Na Cl solution was systematically investigated, the equivalent circuit models in different stages of corrosion were established, and the protection mechanism of the coating was discussed in depth. The corrosion process of Al-Zn-Si-RE coating was divided into four stages, and the main protection mechanisms of the coating were the sacrificial anode cathode protection and the shielding effect of corrosion products during the corrosion process. The evolution of the corrosion product morphology and chemical composition of Al-Zn-Si-RE coating immersed in 10% Na Cl solution were studied. It was found that a dense corrosion layer which consisted of amorphous and/or nanocrystalline formed on the surface of Al-Zn-Si-RE alloy coating in the corrosion process, and the formation mechanism of the corrosion products was discussed.Hydrogen-evolution reaction occurs severely in the aqueous Al-rich coatings. The best effective hydrogen evolution inhibitor and its optimum concentration for the Al-Zn-based alloy flake were determined by the gas volumetric measurement over a period of 72 h at 45℃: when m(TBAB): m(powder) was 0.5:1, the inhibition efficiency was 78.99%. The Al-Zn-Si flakes before and after hydrogen evolution were investigated with IR, SEM and XRD technics, and the pourbaix diagram was also used to analyze the corrosion mechanism of Al-Zn-Si alloy flake in aqueous media. Due to the microzone electrochemical inhomogeneity of Al-Zn-Si alloy powder, the micro-galvanic corrosion occurred on the bare area of metal flake surface which was not fully covered with the hydrogen inhibitor molecular, so that the hydrogen corrosion of Al-rich phase at relatively low potentials was accelerated. Finally the inhibition mechanism of the inhibitor TBAB was discussed. The quaternary ammonium ions(N+) were adsorbed continuously by the negatively charged surfaces of Al-Zn-Si alloy powder, and the monomolecular adsorption layer was formed on the metal powder interface. The hydrophobic membrane can hinder the aggressive particulate from transporting and contacting, inhibit the anodic reaction effectively and then slow down the rate of corrosion. |
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