The Studies Of Corrosion Feature Of Fe₃Al-based Materials In Seawater And Its Surface Protection Methods

FeAl-based alloys as promising materials have numerous attractive properties for high temperature applications. Being low cost compared to conventional stainless steels , these materials are also considered for corrosion-resistant applications in replacement of stainless steels used in seawater. But the corrosion behavior and the kinetic of Fe₃Al-based alloys and composites have not been studied, neither does its surface protection methods. So to study the corrosive mechanism of Fe₃Al-based materials and to find out the methods of controlling its corrosion are very important. In this thesis, we attempted on one hand to study the corrosion electrochemical behaviors of Fe-Al based materials in seawater and give explanations to its corrosion mechanism, on the other hand to investigate the surface protection methods about Fe₃Al-Cr alloy, and try to find better methods for protection. For improving the corrosion resistant of these alloy, we studied rare-earth metal conversion coating and super-hydrophobic coating.Firstly, the corrosion behaviors of Fe₃Al(3%Cr), Fe₃Al(5%Cr) by casting and pure Fe₃Al by powder alloying were studied, and also Q235 mild steel and 304 stainless steel were studied as compared. And the Fe₃Al/ZrO2 composites with Fe₃Al 75%,50%,30% were also studied. The corrosion behaviors of Fe₃Al based materials were investigated by electrochemical measurements such as electrochemical impedance spectroscopy (EIS), polarization curve; and weight loss measurements. And surface techniques such as SEM and Video-microscope were used to discuss the corrosion mechanism. The results showed that in seawater, the corrosion resistant of Fe₃Al(5%Cr) was the best, and on the order of 304ss;there were not significant difference between Fe₃Al(3%Cr) and powder alloying Fe₃Al, and their corrosion resistant were much better than Q235. The corrosion form of these alloys was pitting corrosion caused by the blowholes of these materials. The open corrosion potential of Fe₃Al/ZrO2 composites was elevated by the content of ZrO2, and the passivation period was disappeared, and the corrosion rate was accelerated. It was because the content of Fe₃Al was so low that there was no integrity passive film on it. The corrosion form of these composites were selective corrosion, and Fe₃Al was dissolved by preference as anode.Secondly, cerium conversion coating was formed by immersing the Fe₃Al in CeCl3 solution, the electrochemical measurements and weight loss measurements results showed the conversion coating had good corrosion resistant. Then the optimum technology(CeCl3: 10g/L; H2O2: 3g/L; T: 50℃;t:1h) was obtained by orthogonal experiment. The cerium conversion coating formed on Fe₃Al3%Cr, Fe₃Al5%Cr, and activated Fe₃Al5%Cr had good corrosion resistant proved by electrochemical measurements. The surface morphology was studied by metallurgical microscope. The films formed in H2O2 containing solution deposited faster, and the firms were loose. The depositing mechanism was supported by calculating. The inhibition of conversion coating deposited on passive surface was prevent anodic process, and the coating on the activated surface provided corrosion protection by covering the whole surface.Self-assembled monolayers of n-tetradecanoic acid (CH3(CH2)12COOH) were formed on the native oxide surfaces of Fe₃Al(3%Cr). The structure of the monolayers was probed with contact angle measurement and SEM. The results suggested that the structure of the adsorbed film is similar to lotus and the contact angle was larger than 150°for seawater. Moreover, the corrosion behaviour of bare Fe₃Al and Fe₃Al with super-hydrophobic surface, with two different surface microstructure, were investigated using EIS and weight loss measurements in seawater. Both methods revealed that corrosion rate of modified Fe₃Al decreased dramatically because of its special microstructure. These results indicated that the super-hydrophobic surface inhibited the corrosion process of Fe₃Al in the seawater in a certain degree.