| It is indispensable to take protective measures to protect steel substrate, becauseof steel directly exposed to seawater prone to corrosion. Zn-Al coating, which isobtained by chemical deposition of Zn-Al liquid on the surface of the metal, is ananti-corrosion coating, and it principally consists of zinc, aluminum flakes, andchromate. The protective mechanism of Zn-Al coaing is the barrier effect provided bythe metal flake and the accumulation of corrosion products, the sacrificial cathodicprotection and the passivation of chromate. Considering the benefits of excellentcorrosion resistance, heat resistance, no hydrogen embrittlement, well paint adhesionability, and the simple preparation technology, Zn-Al coating were widely used incorrosion protection of steel fasteners.Based on the existing preparation technology, futher optimization design wasmade to establish the best coating formulation and preparation process in this paper.The characrers of zinc powder on the corrosion resistant effects of Zn-Al coatingwere learned. The results showed that Zn-Al coating presented a higher corrosionresistant, and the coating thickness was10μm, possessed more than1500hours saltspray time, when the metal powder content in the liquid was25wt%, zinc aluminumratio was7:1. The coating which was gained by zinc powder B was relatively dense,had slower changes of open circuit potential, had a lower potential-0.8V afterimmersed for60days, and possessed higher charge transfer resistance Rct. Thecorrosion resistant effect of zinc powder B was more outstanding.In order to improve the corrosion resistance of substrate to meet more harshcorrosion environment, the composite coating was obtained by adding sherardizingcoating in the middle of substrate and Zn-Al coating. The corrosion-resistance of thecomposite coating system was studied. The research showed that the compositecoating had lower corrosion potential, presented the larger coating impedance(2.56×104·cm2) after immersed for30days, had good compatibility with substrate,and showed good sacrificial anode protection and shielding effect to45#steel. Thecorrosion resistance of the composite coating was superior to the single coating.In view of poor abrasion resistance, the Zn-Al coating prone to mechanicaldamage in the process of the production and use, resulting in lower corrosion resistant. The failure behavior of scratched coating was learned by artificial scratch simulationexperiment in this paper. The research showed that when the sample was scratched,open circuit potential was quickly changed. When the scratch width was3mm, thecorrosion current density was rapidly increased from0.5932μA·cm-2(which wastested by non-scratched sample) to3.1007μA·cm-2, and showed significant reductionof the corrosion resistance. SECM test showed that scratched coating existed obviousanode zone and cathode zone in the process of immersion, and the coating areaprovided protection to the scratched area, which would accelerate the corrosion rateof the coating. The coating was likely to corrode when it was closer to the scratch.According to the request of the compatibility of Zn-Al coating and actualworking condition characteristics of fasteners, two kinds of room temperature fastcuring repaired coatings were chosen to repair the scratched coating, one wasone-component zinc plating coating (A), the other was two-component epoxyzinc-rich coating (B). In this paper, the repair ability of the two coating wascomparatively learned by electrochemical tests and morphology analysis. Theresearch showed that:(1) Coating B had high adhesion with substrate. When it wasused to repair Zn-Al coating, it mainly offered the coating physical shielding effect tothe scratched coating, and coating A mainly offered the electrochemical cathodicprotection.(2) Comparing with1mm scratched coating, repaired coating A andrepaired coating B had lower open circuit potential, and had a larger charge transferresistance Rctwhich presented a gradually increasing trend, and played a goodprotection to steel substrate.(3) Comparing with repaired coating A, repaired coatingB showed better corrosion resistance, the open circuit potential was-0.75V afterimmersed for60days, the charge transfer resistance was3.55×105·cm2, thereexisted iron rust for40days by neutral salt test, resulting in the repair effect ofcoating B was superior to that of A.In summary, the properties, failure behavior and rapid damage repairingtechnology of Zn-Al composite coating were studied by means of electrochemicalmethods in this paper. It could provide theory basis for the practical application, andoffered important reference value to improve the service life of Zn-Al coating. |
PDF Full Text Request