Study On The Deep-sea Corrosion Behaviors Of Fe-based Amorphous Coatings

With the exhaustion of resources on land,more and more attention is paid to the deep sea resources.However,complex and harsh corrosion environments in the deep sea bring new challenges to materials by decreasing the corrosion resistance.Study showed that metallic alloys and polymer coatings,which are commonly used in deep-sea environments,usually exhibit degraded corrosion resistance in deep-sea environments.In order to overcome this challenging problem in deep-sea environments,development of novel metallic materials with a ultrahigh corrosion resistance is highly desirable.Fe-based amorphous alloy coating,a new style of surface protective coating,is a potential material used in deep sea environment,owning to their unique mechanical and anti-corrosion performance.At atmospheric pressure,the amorphous coatings exhibited very stable passive films and low corrosion rates in a wide range of test environments.However,to our best knowledge,the study of corrosion behavior of amorphous coatings in high hydrostatic pressures environment has not been reported so far.We researched the effect of hydrostatic pressure on corrosion resistance of Fe-based amorphous coating with a composition of Fe48Mo14Cr15Y2C15B6(at.%),prepared by HVOF processes.X-ray diffraction(XRD),electrochemical testing system,scanning electron microscope(SEM);X-ray photoelectron spectroscopy(XPS)and high resolution transmission electron microscopy(HR-TEM)and nanoindentation tests were used to characterize the structure of amorphous alloy,electrochemical corrosion properties,structure and mechanical properties.We comparatively studied the effect of hydrostatic pressure on corrosion resistance of Fe-based amorphous coating and substrate.Results of potentiodynamic polarization tests and pitting incubation tests indicated that hydrostatic pressure deteriorated the pitting resistance of 316 L stainless steel by reducing the pitting potential and pitting incubation time.While hydrostatic pressure enhanced the pitting resistance of Fe-base amorphous coating by increasing the pitting potential and pitting incubation time.XPS and FIB/TEM results suggested that the deterioration of corrosion resistance of 316 L stainless steel at hydrostatic pressure resulted from the decrease in Fe(III)/Fe(II)ratio and Cr(III)-oxides as well as the structural inhomogeneity of the passive films.However,hydrostatic pressure accelerated the oxidation of metallic elements and the translation of Fe(?)to Fe(?),increasing the thickness(about 2.5 times)of passive film on the surface of Fe-based amorphous coating.Additionally,the decrease/increase in mechanical strength of the passive film also played a role in degradation/improvement of corrosion resistance of the 316 L stainless steel/Fe-based amorphous coating under hydrostatic pressure.Therefore,the enhanced pitting resistance of Fe-based amorphous coating under high hydrostatic pressure environment resulted from the enhancing and thickening of passive film.In order to illustrate the mechanism how amorphous structure affects the corrosion behaviors of amorphous alloys at high hydrostatic pressure,we researched the deep-sea corrosion behaviors of different kinds of bulk metallic glasses,and the effect of crystallization on deep-sea corrosion behaviors of bulk metallic glasses.Research showed that:(1)hydrostatic pressure enhanced the pitting resistance of Fe-base(Fe48Mo14Cr15Y2C15B6?Fe43.7Co7.3Cr14.7Mo12.6C15.5B4.3Y1.9)and Pb-based(Pd32Ni48P20)bulk metallic glasses,proving the important role of amorphous structure in improving pitting resistance of bulk metallic glasses;(2)Results of electrochemical tests showed that hydrostatic pressure enhanced the ionic conductivity of passive films on Fe-based BMG and crystallized Fe-based BMG via decreasing electrochemical impedance,accelerating the solution of metallic elements.But for Fe-based BMG,hydrostatic pressure increased its pitting resistance;for crystallized Fe-based BMG,hydrostatic pressure deteriorated its pitting resistance.So the results demonstrated that pitting resistance of metallic material under deep-sea environment is closely related to its structure.The homogeneity in aspect of microstructure and chemical component of amorphous alloy played a very important role in increasing its pitting resistance at high hydrostatic pressure.