| The high temperature corrosion rate of heat resistant steel and titanium alloy would be significantly accelerated in the marine atmospheric environment which is characterized by high humidity and salinity,which could lead to rapid corrosion and early failure of high temperature components and cause significant economic losses.High temperature resistant organic silicone coating is a kind of high temperature resistant coatings commonly used for protection of high temperature components such as chimneys,exhaust pipes,smoke exhaust pipes,and steam pipes.They have excellent high temperature and corrosion resistance and are commonly used below 250℃.Dealing with the serious corrosion of heat resistant steel and titanium alloy in high temperature marine atmospheric environment,a variety of nano-and micro-sized particles were added to the organic silicone coating to improve its high temperature corrosion resistance.The influences of the organic silicone based coatings on high temperature corrosion resistance of 304 stainless steel and Ti-6Al-4V alloy were investigated in different corrosion environment and the effects of nano-and micro-sized particles on the high temperature corrosion properties of the organic silicone coating was studied.The self-densification phenomenon of the coating due to the volume expansion caused by the oxidation of the active nano-sized particles and the flow of micro-sized glass powders at high temperature was observed,revealing the different roles of the active nano-sized particles and inert micro-sized particles in improving the high temperature corrosion resistance of organic silicon coating.The main research results are as follows:1.The curing mechanism of organic silicone resin at room temperature and the thermal degradation behavior of cured resin at 600℃ were studied.The results show that curing of organic silicone resin at room temperature can be achieved through the cross-linking reaction between-NCO in polyurethane and-OH in organic silicone resin.At 600℃,the organic components in the organic silicone coating undergo thermal decomposition and volatilization,and the Si-O-Si backbone in the organic silicone resin tended to be highly cross-linked.The thermal degradation product of the organic silicone resin is porous amorphous SiO2.2.The influences of the organic silicone composite coating modified by nano-sizedα-Al2O3,TiO2,ZrO2,Al,Ti powders,and micro-sized glass powders on the oxidation and corrosion behavior of 304 stainless steel at 600℃ in air and under cyclically alternate salt spray followed by thermal exposure at 400℃ were studied respectively.The results showed that at the initial stage of high temperature exposure,the organic components in the organic silicon coating modified by nano-and micro-sized particles were thermally oxidized and decomposed and then volatilized,and the nano-and microsized particles filled most of the voids left by the thermal decomposition of the organic components in the coating.The dispersed active nano-sized Al and Ti were oxidized and led to volume expansion,which further improving the density of the coating.The organic silicon coating was transformed into an inert inorganic coating which was composed of a continuous amorphous SiO2 matrix,dispersive Al,Al2O3,TiO2,ZrO2 and Al2SiO5 particles and discrete voids.The latter effectively inhibited the inner migration of corrosion media,including O2,NaCl,and H2O,from the environment during high temperature oxidation and cyclically alternate salt spray followed by thermal exposure at 400℃ and significantly improved the high temperature oxidation and corrosion resistance of the stainless steel.3.The influences of the organic silicone composite coating modified by nano-sized ZrO2 and Al powders,micro-sized quartz and glass powders on the oxidation and water vapor corrosion behavior of Ti-6Al-4V alloy at 600℃ were investigated.The results show that the presence of water vapor in the environment significantly accelerated the oxidation of Ti-6A1-4V alloy at 600℃.The organic silicone coating modified by nanoand micro-sized particles apparently improved the oxidation resistance and water vapor corrosion resistance of Ti-6Al-4V alloy at 600℃.At the initial stage of oxidation and water vapor corrosion at 600℃,the organic silicone coating was transformed into an inert inorganic coating consisting of the amorphous SiO2 skeleton,the ductile Al agglomerates covered by its oxides,the inert ZrO2,quartz and Al2SiO5 formed by the solid state reaction of glass powder with active nano-Al at high temperature and the glass powders which have a certain fluidity in high temperature and filled the voids in the organic silicone coating and enhancing the bonding between the SiO2 skeleton and high temperature resistant fillers.The newly formed inorganic composite coating effectively retarded the inward migration of the corrosive media O2 and H2O in dry and moist air at 600℃,thereby significantly improved the oxidation and corrosion resistance of the substrate alloy.4.Furthermore,the effects of the modified organic silicone coating on the corrosion behavior of Ti-6Al-4V alloy induced by solid NaCl deposit in dry and moist air at 600℃ were studied.The effect of pre-oxidation treatment on the high temperature corrosion resistance of the organic silicone coating was also investigated.In the above high temperature corrosion environment,Ti-6Al-4V alloy suffered severe corrosion.The organic silicone coating modified by nano-and micro-sized particles provided high temperature corrosion protection for Ti-6Al-4V alloy to a certain extent.At the initial stage of high temperature exposure in the above corrosive environments,the inorganic coating formed due to the oxidation and decomposition of the organic components in the organic silicone coating has poor thermal stability,low density,and high porosity,so a large amount of corrosive media(NaCl,water vapor,and O2)entered into the coating and arrived at the coating/alloy interface.TiO2 and other oxides generated by active oxidation of the alloy rapidly grew along the voids in the coating and resulted in the formation of a thicker interfacial reaction zone between the coating and the alloy.The inorganic coating formed after the pre-oxidation treatment of organic silicone coating at 600℃ has good thermal stability,high density,and low porosity,effectively reducing the amount of corrosive media entered and reducing the active oxidation rate of the titanium alloy.Therefore,the organic silicone coating after the pre-oxidation treatment significantly improved the high temperature corrosion resistance of the substrate alloy in the above two environments. |
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