Self-Assembly Method Of Alkyl Phosphate Inhibition Films In Aqueous Phase And Corrosion Protection Properties For Iron Substrate

Iron and its alloys are very important for our daily life and industrial production. The most important application of iron is to make steels. Iron is playing a growing role in the rapid development of society and economy. But, iron is easy to be corroded. It is reported that steel corrosion makes large loss in our country, and 10-20% of the annual output of steel are rotted due to the various types of corrosion. Therefore, it is very important and urgent to develop effective method for the corrosion protection of iron products. Up to now, there have been many research reports about how to protect iron and its alloys from corrosion.Alkyl phosphate is a kind of amphiphilic substance, which possesses hydrophilic groups and hydrophobic groups. Due to the hydrophilic groups absorb onto the iron surface with ease, alkyl phosphate may form protective thin films on the iron surface that protect the iron substrate from corrosion to some degree. So far, some studies about the formation of alkyl phosphates films on oxides surfaces have been reported, but predecessor’s research is scattered and not systematic. For these reasons, herein we studied the spontaneous assembly of mono-n-butyl phosphate (BP), mono-n-hexyl phosphate (HP) and bis (2-ethylhexyl) phosphate (BEP) films on iron surface and their inhibition effect on the corrosion of iron substrate. This dissertation can be divided into two parts as follows:(1) Spontaneous formation of mono-n-butyl phosphate and mono-n-hexyl phosphate thin films on the iron surface in aqueous solution and their corrosion protection property.At first, BP and HP were synthesized by the direct esterification reaction of phosphorus pentoxide with 1-butanol or 1-hexanol followed by purification. Then, The as-synthesized BP and HP products were dissolved by ultrapure water (～18 MΩ cm) to prepare three groups of aqueous solutions with different weight percentage concentrations (0.1%,0.01%, and 0.001%). The polished iron electrodes were immersed in the BP or HP aqueous solutions. After a period of time designated, the iron electrodes were took out of the aqueous solution and dried in air.Surface compositions and functional groups of as-formed the phosphate films on the iron surface were analyzed by means of Fourier transform infrared spectroscopy (FTIR), energy dispersive spectrometer (EDS) and X-ray photoelectron spectroscopy (XPS). The characterizations indicate that the phosphate molecules may adsorb on the iron surface via the electrostatic interaction and form self-assembled thin films with good corrosion protective property. The protection efficiency to the iron substrate was determined by Electrochemical Impedance Spectroscopy (EIS) and polarization curve methods. It is observed that, as compared to the bare iron electrode, the Nyquist impedance spectra of the BP or HP thin film modified iron electrodes showed a capacitive loop with the increased diameter, with the disappearance of previous Warburg impedance. At the same time, the corrosion current densities of the modified iron electrodes were decreased to different degrees. These results demonstrate that the BP and HP thin films are able to protect the iron substrate from corrosion to some extant. The calculated results still suggest that the protection efficiency of BP thin film is slightly lager than that of HP. Moreover, we used the contact angel to assess the hydrophobicity of these thin films. The hydrophobic performance of BP thin films was similar to that of HP thin films. On the basis of these results, the film formation mechanisms of BP and HP on the iron surface in aqueous phase were put forward.(2) The protection effect of bis (2-ethylhexyl) phosphate (BEP) thin films on the iron surface in different corrosive mediaAt first, BEP were dissolved by ethanol (95%) to prepare three groups of ethanol solutions with different weight percentage concentrations (1%,5%, and 10%). The polished iron electrodes were immersed in the BEP ethanol solutions. After a period of time designated, the BEP thin films were formed on the iron surface. Then, Electrochemical Impedance Spectroscopy (EIS) and polarization curves were used to study the protective effect of BEP thin films on the iron substrates in 0.5 M H2SO4 solutions. The results show that the BEP thin films can protect the iron substrate effectively in acidic conditions, however, the protective effect is very weak in neutral corrosive media (e.g.3.5% NaCl). To interpret the reason why BEP thin films on iron surface show different protective behaviors in different corrosion media, XPS and EDS were used to analyze the compositions and functional groups of the iron plate samples modified with BEP thin films. Through these characterizations and analyses of the surface, it is determined that, after the iron plate samples modified with BEP thin films were further treated by immersing in an acid solution, more BEP molecules adsorbed on the iron surface, which in turn led to the enhancement of corrosion resistance of BEP thin films.