| The purpose of this work is to improve the corrosion performance of bisphenol-A epoxy resin(EP44)coatings,and to explore its anticorrosion mechanism.Based on the above requirements,EP44 were graft modified by a organosilane small molecule??3,3,3-trifluoropropylmethyldimethoxysilane(TMDMS),and then filled different contents of 1,1,1,3,3,3-hexamethyldisilazane(HMDS)surface treated nanosilica powder to improve the anticorrosive performance of these composite coatings not only by the hydrophobic barrier of the TMDMS,effectively inhibit the absorption of water to the coating,but also take advantage of the mechanical barrier of inorganic ceramic nanoparticles to achieve the purpose of retarding corrosion by filling the pores,cracks,and free valume present in the coating,thus delaying the infiltration process of water and the corrosive ions to along the thinkness of the coating.TMDMS has successfully been grafted onto EP44 molecules to substitute trifluoropropyls for hydroxyls via significant alcoholysis of the TMDMS methoxyls with the EP44 hydroxyls,as corroborated by FTIR and ~1H NMR spectroscopies,under anhydrous DBTDL-catalyst conditions at 95 ~oC for 8 h.Meanwhile,a nanosilica powder has successfully been modified with HMDS to convert its surface hydroxyls into methyls through electrophilic substitution of the nanosilica hydroxyls by the HMDS trimethylsilyls at 50 ~oC for 20 h,as confirmed by FTIR spectroscopy and TGA.It is revealed from contact angle(θ)measurements of the coatings that theθof water against,and thus the hydrophobicity of,the EP44 coating considerably is increased from 83.8°to 105.5°upon its grafting with TMDMS,likely,this is due predominantly to the introduction of superhydrophobic trifluoropropyl groups,as well as secondarily to a reduction in the hydrophilic hydroxyl concentration present,both upon the graft modification,which then changes little with further addition of the increasing concentrations of the modified nanosilica.The impedance magnitudes(|Z|)at a frequency of 0.01 Hz and the coating resistance(R_c)origin from electrochemical equivalent-circuit models simulation of TMDMS-grafted EP44 coating were got by electrochemical impedance spectroscopy(EIS)all are larger than EP44 coating upon each of the immersion times(1,24,240,480,720 and 960 h)in electrolyte(i.e.NaCl)aqueous solution,revealing that the anticorrosive performance of the former evidently is improved compared with the latter,as a consequence probably of the significantly increased hydrophobic barrier to the corrosives mediated by water,as well as possibly of the water digestion from hydrolysis of the residual,unreacted TMDMS methoxyls,both present in the coating upon the graft modification.Further,when the TMDMS-grafted EP44 coating is filled with the increasing contents(0~5 wt%)of the modified nanosilica,the|Z|at a frequency of 0.01Hz and the R_c(i.e.anticorrosive performance)first intensified,and subsequently weakened among 0~0.5 wt%,and finally improved again at about 3 wt%.The little changes in its hydrophobicity dictate that the anticorrosive performance of the composite coatings has to be dominated by a corrosion inhibition mechanism by the nanosilica particles.It has interestingly been found that,as the nanosilica particles concentration steadily is raised from 0 to 5 wt%,the corrosion inhibition of the composite coatings first intensifies probably thanks to an enhancement of the filling rate of the pores,subsequently weakens presumably due to an increase in the porosity from increased size-exclusion of aggregated nanoparticles from the pores,and finally improves again possibly owing to a densification of the outside-of-pore barriers of greatly aggregated.This constitutes dual critical concentrations of the nanosilica at0~0.5 and about 3 wt%,respectively,that give rise to two extrema(a maximum followed by a minimum)in the corrosion inhibition of the composite coatings. |
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