Preparation Of Zn-Al Pseudo Alloy Coatings And Their Corrosion Resistance In Marine Environment

Zn-Al coatings are replacing the traditional pure Zn and Al coatings in the field of long life anti-corrosion for large steel constructions. Therefore, it is very important to study the preparation and corrosion resistance of Zn-Al coatings to create high performance coatings. In this paper, arc spraying technology was used to prepare Zn-Al pseudo alloy coatings with different contents of Al. The parameters were optimized through the BP artificial neural network technique. The microstructures of coatings were characterized by optical microscopy, scanning electron microscopy, energy spectrum analysis and X-ray diffraction. The coupon tests were conducted to evaluate the corrosion resistance of coatings in three kinds of simulated marine environments, including neutral salt-spray test, splash test and immersion test. At the same time, the corrosion mechanisms of coatings in different environments were discussed.The research results showed that model predictions obtained through the BP artificial neural network technique and experimental data agreed reasonably well, which indicated the model was reliable and available. Three kinds of Zn-Al pseudo alloy coatings with different contents of Al and Zn-15Al alloy coating were prepared under the optimum parameters. The appearances and microstructures of coatings were observed, which showed that the quality of coatings satisfied relevant national standard. The coating surfaces of Zn-14Al and Zn-46Al were rougher than those of Zn-15Al and Zn-27Al. The results of XRD showed that all the pseudo alloy coatings were composed of the Zn phase and the Al phase. The results of EDS showed that there were Zn enrichment zones and Al enrichment zones in the Zn-Al pseudo alloy coatings, however, the chemical composition was uniform in the Zn-15Al coating.The results of the electrochemical measurements showed that the open-circuit potential of these coatings shifted positively with increasing the content of Al in the solution of 3.5 wt% NaCl. However, the open-circuit potentials of these coatings did not differ much. In addition, passivation was observed from the polarization curves of coatings except that of the Zn-46Al pseudo alloy coating.The coupon tests of Zn-Al coatings in three kinds of simulated marine environments showed that the Zn-46Al pseudo alloy coating possessed better corrosion resistance in the neutral salt-spray test and immersion test, and Zn-15Al alloy coating had better corrosion resistance in the splash test.Through the observation of corrosion morphologies, the corrosion resistance of coatings was determined by the composition, corrosion product and exposure environment. For the Zn-Al pseudo alloy coating, corrosion product lost protection effect to the coating due to the splashing of solution, then the corrosion aggravated gradually. However, protection effect of corrosion product resulted in the occluded cell corrosion for the Zn-15Al alloy coating in the salt-fog environment and immersion environment, which made the corrosion resistance of Zn-15Al alloy coating dropped rapidly. The process of active dissolution was slowed down with increasing the content of Al in the Zn-Al pseudo alloy coatings, so the Zn-46Al pseudo alloy coating with higher content of Al had better corrosion resistance in the salt-fog environment and immersion environment.