Role Of Coating Defects In Corrosion Behavior Of Fe-based Amorphous Metallic Coatings

Fe-based metallic glasses(MGs)are thought to be promising materials due to their superior mechanical properties and anti-corrosion performance.However,the poor ductility and limited sample size of bulk Fe-based MGs hinder their applications as structural materials.When applied as surface coatings with thicknesses of several hundreds of micrometers,the coating fabrication can be easily achieved by thermal spray technologies and the brittleness of Fe-based MGs can be effectively avoided.It is worthy of noting,the thermal spray process can introduce defects,such as pores,oxides and crystalline phases,to the Fe-based amorphous metallic coatings(AMCs).The coating defects have critical effects on the coating performance.In this paper,the influence of coating defects on the corrosion behaviors of Fe-based AMCs as well as the intrinsic mechanisms have been systematically investigated,the synergistic effects of pores,oxides and composition heterogeneity in the coatings were discussed.Subsequently,the ambient conditions in different service environments of the coatings were taken into consideration,and the corrosion behaviors of Fe-based AMCs under wet-dry cyclic conditions and boric acid solution environments were investigated.The study aims to provide a deep and systematic understanding of the intrinsic mechanisms of coating corrosion,solve the corrosion problems introduced by coating defects and make a promotion of engineering applications for the Fe-based AMCs.The main research results are summarized as follows:(1)The Fe-based AMCs,with coating thickness of about 500 ?m and coating porosity of(1.46±0.07)%,were successfully fabricated by high velocity air fuel(HVAF)technique.The influence of coating pores on corrosion behavior was investigated.The open-circuit potential(OCP)curves and the passive region of potentiodynamic polarization curves of the as-sprayed coating exhibit a lot of transient peaks,the passive films of the as-sprayed coating are unstable.The potentiodynamic polarization curves measured through micro-electrode samples show the corrosion resistance of the coatings decreases with increasing porosity.The pores are prone to trigger localized corrosion and thus decrease the corrosion resistance of Fe-based AMCs.The epoxy resin with low viscosity was employed to seal the coating pores.After the sealing treatment,the corrosion resistance of the coatings was improved,with reduced corrosion current densities,improved passive film stability and enhanced re-passivation ability.(2)The interparticle oxide layers formed during thermal spray process is also an important problem deteriorating corrosion resistance of the coatings due to the formation of composition heterogeneous zones and structural defects around the spray formed oxides.To lower the influence of composition heterogeneity around the interparticale oxide layers on corrosion resistance,a dense Fe-based metallic coating with high Cr-content was designed.The correlation between the interparticle oxide layers and the localized corrosion was investigated through long term immersion tests combined with electrochemical impedance spectroscopy measurements in 3.5 wt.%NaCl.The results show that electrolyte solution would diffuse to the inner coating through the interparticle oxide layers and induce localized corrosion.Subsequently,the localized corrosion develops along the interparticle oxide layers,which eventually causes coating failure.The diluted aluminum phosphate solution could penetrate into the interparticle oxide layers.The diffusion pathways in the as-sprayed coatings could be sealed permanently after the aluminum phosphate is cured in the interparticle oxide layers.After the coating was sealed with aluminum phosphate,the corrosion current density was decreased and the long term corrosion resistance was improved.(3)The localized corrosion problem of the Fe-based AMCs cannot be completely resolved by a pure sealing treatment as the sealing treatment cannot alleviate corrosion problem induced by composition heterogeneity.The composition distribution of the coating was analyzed by SIMS,which demonstrated the existence of localized Cr depletion at the peripheries of defects in Fe-based AMCs,e.g.pores,oxides and crystalline phases.The localized Cr-depleted zones in the coatings are the main reason for decreasing passive film stability.The quasi in-situ confocal laser scanning microscopy(CLSM)observation found the localized corrosion preferentially initiates at the Cr-depleted zones.An electrochemical polarization treatment was proposed to selectively dissolve Cr-depleted zones in the coatings.With the elimination of Cr-depleted zones the corrosion resistance of the coatings was enhanced,with an order of magnitude reduction in the passive current density together with the improvement of the passive films stability.(4)Under wet-dry cyclic conditions,it is extremely difficult to unravel corrosion features under wet-dry cyclic conditions by conventional electrochemical measurements as the solution resistance is extremely high.To address this problem,the corrosion evolution of the coatings under wet-dry cyclic conditions was investigated by electrochemical impedance spectroscopy(EIS)on a two-electrode cell.Accurate charge transfer resistance was obtained through fitting EIS results using a modified transmission line equivalent circuit model and a landscape map of charge transfer resistance was constructed for a panoramic investigation on coating corrosion.The results show that the corrosion rate increases rapidly when the solution film over the coating becomes very thin due to fast oxygen transport under ultrathin solution film.When the solution film over the coating is very thin,an oxygen concentration gradient will form between the inside and the outside of coating pores as well as the interparticle oxide layers,which promotes the localized corrosion initiation in the pores and the interparticle oxide layers.In general,the substrate is well protected by the coatings against corrosion under wet-dry cyclic conditions.(5)Fe-based AMCs are promising surface protective materials for spent nuclear fuel storage application due to the excellent thermal neutron absorption performance and anti-corrosion performance.The corrosion behaviors of the coatings under the spent fuel wet storage environment were examined in the 1.5 wt.%H3BO3 solution.The Fe-based AMCs exhibit very low corrosion current densities(about 4.24 × 10-7 A cm-2)and can self-passivate in the H3BO3 solution.When the polarization potential is lower than 0.4 VSCE,the passive films are stable;When the polarization potential is higher than 0.5 VSCE,the passive films are unstable.The X-ray photoelectron spectroscopy(XPS)results show high polarization potential promotes the dissolution of Mo in the passive films,which is responsible for the decreased passive film stability.In H3BO3 solution,the passive films on the Cr-depleted zones are more prone to breakdown,thus the corrosion is always localized at the pores and the interparticle regions.The Fe-based AMCs perform a favourable long term endurance in the 1.5 wt.%H3BO3 solution.The localized corrosion penetrates the coating after 130 d immersion,while only slight corrosion can be observed on the mild steel substrate after 180 d immersion.The coating still exhibits a well protective performance for the substrate in H3BO3 solution even the coating is penetrated by localized corrosion.