| The corrosion problems of oceanic equipments serving in marine water and marine atmosphere environment are serious. Conventional organic coatings always need maintenance and don't meet the demand of un-maintenance or less-maintenance at a long time. Therefore, technology and theory studies on long-term anticorrosion coatings against the equipments are of great importance.In the paper, high velocity arc spraying technology (HVAS) which is one of the key remanufacturing technologies was used to produce Zn-Al-Mg-RE coating, and then organic coatings were applied on the coating. The synergy effect between the coatings was investigated by immersion testing, chloride copper acetic acid spraying testing (CASS), electrode potential testing and EIS testing. The anticorrosion mechanism of the composite coatings was studied with SEM, EDAX, XRD, immersing samples with artificial defects in 5%NaCl solution, EIS testing and testing combining device which was designed by the author and ion chromatograph. The influence of physicochemical property of corrosion products on failure behavior of the organic coatings was studied on the basis of theory compute and testing. After surveying typical components failure process of some ship and some amphibious equipment, the composite coatings were applied, and anticorrosion property of the composite coatings in real oceanic environment was examined at the same time. Main conclusions were drawn as following:Comparing with Al coating, the Zn-Al-Mg-RE coating had better synergy effect with organic coatings both in solution corrosive medium and salt spraying corrosive medium. In the organic coatings/Al coating interface, the adhesion was destroyed by the alkalinous corrosion products and the delamination of organic coatings was accelerated by osmotic pressure, which led to the appearance of blister. While, the life of stable corrosion products that acted as a passivating film on the surface of Zn-Al-Mg-RE coating was longer. EIS results indicated that organic coatings had better synergy effect with Zn-Al-Mg-RE coating when there were micro-pores or macro-pores in the organic coatings. The pore resistance of the composite anticorrosion coatings system kept higher than 10~7Ωcm~2 during all the testing, which indicated that the organic coatings protected the metallic coating effectively all the time. The coating capacitance kept constant at a long time at the later stage of testing, which indicated that micro-pores in the organic coatings were remedied by the Zn-Al-Mg-RE coating corrosion products. When there were macro-pores in the organic coatings, micro-pores around macro-pores were preferentially blocked by the corrosion products, then the new micro-pores, the micro-pores between the corrosion products and the organic coatings and in the corrosion products were blocked by the corrosion products. By this way, the microstructure of corrosion products in macro-pores became compact and the destruction of corrosion medium to interface was inhibited.The double self-sealing anticorrosion mechanism of the composite anticorrosion coatings was brought forward. After samples with artificial defects immersed in 5%NaCl solution for 430 hours, the artificial defects in the organic coatings were blocked by the Zn-Al-Mg-RE coating corrosion products. The compact microstructure of the corrosion products was observed by SEM. The results showed that the composite coatings had the function of blocking micro-pores in the organic coatings. Free-film permeation testing indicated that the diffusion of chloride ions into the Zn-Al-Mg-RE coating free-film slowed down with time and the chloride ions concentration in the solution kept constant at the later stage of testing, which indicated that the corrosion products of Zn-Al-Mg-RE coating had the function of blocking the micro-pores in the coating and the permeation of corrosion medium was held.The nanocrystals in the corrosion products were discovered and the formation mechanism of the corrosion products was brought forward. The samples with Zn-Al-Mg-RE coating were immersed in 5%NaCl solution for 30 days, 120 days and 210 days, and the grain sizes of the corrosion products was calculated from the XRD testing. Results indicated that there were nanometer corrosion products in the corrosion products and microstructure of the corrosion products became much compact, which made it clear that the reason of the composite anticorrosion coatings had double a self-sealing characteristic was ascribed to the formation of the nanometer corrosion products.Brim spreading velocity of osmotic pressure blister and critical bending moment of volume expansion blister were computed. The main factor influencing blister of organic coatings on Al coating was osmotic pressure, while, the main factor influencing blister of organic coatings on Zn-Al-Mg-RE coating was volume expansion force during corrosion products generating, which were identified by testing.The composite coatings were applied to ships and amphibious equipments. Results showed that the composite coatings protected the equipments effectively. The organic coatings system originally applied on the equipments was not changed and problems such as short life, badly protecting and high cost were settled.
|
PDF Full Text Request