Thermal Spraying Zinc Coating And Zinc Rich Coating In The Atmospheric Environment And The Study Of Corrosion In Simulated Accelerated Test

Atmospheric corrosion is the most widespread form of steel deterioration, while metallic coatings such as zinc, aluminium and zinc/aluminium alloy coatings have been one of the main ways of protecting steel against atmospheric corrosion. In this work, thermal sprayed zinc coating, thermal sprayed zinc composite coating (sealed zinc coating) and zinc-rich coating were prepared on Q235steel substrates respectively, and the microstructures of these three coatings were characterized by scanning electron microscope (SEM) and energy dispersive X-ray spectroscopy (EDS). Then, exposures of these coatings in the atmosphere of Changsha city, as well as cyclic spray tests were conducted. And the correlation between the atmospheric exposure and the cyclic spray tests was analyzed by means of SEM, EDS and X-ray diffraction (XRD). Moreover, the corrosion behaviors of coatings in the simulated atmospheric corrosive medium were studied by SEM, electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization, and then the corrosion mechanisms of the coatings were discussed. Results are as follows:The surface of the as-sprayed zinc coating was considerably rough, companioned with many open pores. In the cross section, the as-sprayed zinc coating could be sorted into an outer coarse layer and an inner compact layer based on the amount of the micro-defects. In the initial immersion stage, the micro-defects on the outer layer of the as-sprayed coating were blocked due to the formation of the compact corrosion products. With increasing the immersion time, the zinc coating continued to dissolve deeper in the coating, and at the same time the corrosion products became thicker and more compact which would improve the corrosion resistance of the as-sprayed coating in the later immersion stage.As for sealed zinc coating, the organic sealants not only covered the most surface of the as-sprayed zinc coating, but penetrated into the original pores and micro-cracks of the outer coarse layer. Therefore, the sealing layer reduced the effective reaction area for corroding and porosity, blocking the transfer of corrosive electrolyte or corrosion products between the sealed coating and solution, and thus delayed the corrosion process of underlying zinc coating. The sealed zinc coating showed corrosion potential much positive than the unsealed zinc coating and the corrosion current density (icorr) decreased approximately three times compared to the value of icorr for the unsealed zinc coating, indicating that the corrosion resistance of the as-sprayed coating in the simulated corrosive medium was enhanced a great extent after sealing treatment.The zinc-rich coating was prepared with well-size-distributed zinc powders and little porosity, and thus had high density. The zinc-rich coating could prevent the attack of the electrolyte in the initial immersion. With longer immersion time, the zinc powders corroded fast due to the penetration of corrosive medium, providing cathodic protection for steel substrate. In the later immersion stage, the transfer of corrosive electrolyte was blocked by the corrosion products, showing a blocking and shielding effect against corrosion.The corrosion weight losses of these coatings were all in linear range along with the atmospheric exposure time, and the equations of linear regression that predict the corrosion rate of the coatings were proposed respectively. The cyclic spray tests could accelerate corrosion of the coatings, where the same corrosion products as atmospheric corrosion were generated. Also, the corrosion kinetics of these three coatings followed a linear law approximately in the cyclic spray tests. Therefore, the cyclic spray tests can be applied to simulate the corrosion behaviors of these coatings in the atmosphere of Changsha city.