The Behavior Of Alloy Oxides In High-temperature Molten Salts Application Environment

Molten salts,as an excellent heat-transfer fluids（HTFs）or thermal energy storage（TES）candidate,have been widely used in various fields,such as molten salt reactors（MSR）and concentrated solar power（CSP）plants,due to their high decomposition temperatures and good thermal properties.However,the high-temperature molten salts application environments posed a huge challenge to the structural materials,which is supposed to touch high-temperature molten salts on one side and high-temperature air on the other side.Therefore,the structural materials are expected to be good in both oxidation resistance and molten salts corrosion resistance at high-temperature.And in most high-temperature environments,corrosion resistance,mainly oxidation resistance in materials is achieved by forming a dense and robust oxide layer（Cr₂O₃、Al₂O₃）on the alloy surface.But the alloying constituents such as chromium and aluminum tend to be easily attacked in molten salts at high temperature,and most oxides are not stable in molten salts,except for dense alumina that exhibits corrosion resistance in molten chlorides.Thus,how to balance the content of each element in the alloy to form a dense oxide layer against high-temperature air oxidation,and shows good corrosion resistance to high-temperature molten salts has become an important issue that needs to be solved in high-temperature molten salts applications.In this paper,the behavior of two promising alloy systems for use in molten salts environments above 800℃ in air and molten chlorides was studied at high temperature:（1）The effect of tungsten content on the oxidation resistance of Ni-x（5-30 wt.%）W-6Cr alloy in air at 850℃.（2）The oxides that Ni-xAl-Cr（Ni-0,1,4,6 wt.%Al-6Cr and Ni-0,1,2,3 wt.%Al-20Cr）automatically formed on the surface in molten chlorides at 800℃ and the related effect on the corrosion resistance of the alloy.（1）Ni-26W-6Cr is considered to be used in molten salts at the temperature above 800^oC,which shows better mechanical properties and molten fluorides corrosion resistance at high temperature than those of Hastelloy N（the maximum allowable temperature is704℃）.However,it is believed that the combination of high tungsten content and low chromium content can reduce the high-temperature oxidation resistance of alloy,in addition,there are few previous studies about the oxidation behavior of the alloys with high tungsten content（more than 20 wt.%）and low chromium content.In this paper,μ-XRD,μ-XRF andμ-XANES based on synchrotron radiation facilities,as well as SEM-EDS,XRD,EPMA are combined to clarify the distribution of surface oxides on the four Ni-x（5-30 wt.%）W-6Cr alloys after oxidation in air at 850℃ for 100 h,among which Ni-25W-6Cr alloy shows the best oxidation resistance.Tungsten acts as secondary getter in the Ni-xW-6Cr alloys and promotes the formation of an external continuous oxide scale（NiCr₂O₄）,which is responsible for the increased oxidation resistance as the tungsten content increasing when the tungsten content is below 25wt.%,while the excessive tungsten can facilitate the growth of CrWO₄ through consuming Cr₂O₃,which prevents the formation of NiCr₂O₄,and CrWO₄ breaks the continuity of the chrome oxide scale,creating the breakway in oxides scale for the diffusion of oxygen and metal ions.（2）The corrosion of alloys in molten chloride salts is primarily driven by the impurities（H₂O,O₂,OH^-,H⁺）present in the salt which is mainly due to the inherently hygroscopic nature of chloride salts,similar to the corrosion of alloys in molten fluoride salts.The difference is that the fluxing action of molten chloride salts is not so serious as that of molten fluoride salts,and oxides such as Al₂O₃ could exist in molten chlorides.Therefore,it is possible for alloys to form passivation oxide film on surface by reacting with the moisture or oxygen in the molten chloride salts to prevent corrosion.In this work,two kinds of Ni-xAl-Cr alloy systems were immersed in molten chloride salts at800℃ for 300 h or 400 h,and then the corrosion products on the alloy surface and the elemental distribution of the cross section were characterized by XRD,SEM-EDS and EPMA.The results proved that dense and continous Al₂O₃ scale locally formed on Ni-6Al-6Cr surface during the immersion corrosion,and which can actually avoid the outward diffusion of chromium,while some dispersed Al₂O₃ particles on the alloy surface didn’t work in this way.In addition,Al₂O₃ is not formed in the initial stage of corrosion,instead,chromium and aluminum on the alloy outermost surface diffuse into the molten salts at first,leaving a nickel-rich outer scale on the alloy surface that can not stop the penetration of oxygen into the alloy and then internal Al₂O₃ is formed.Yet,no dense Al₂O₃ film was found on the surfaces of Ni-（1,2,3 wt%）Al-20Cr alloys,but an outer oxide layer composed of Al,Mg,O locally formed on Ni-2Al-20Cr alloy,which can inhibit the penetration of Fe（impurity in salts）into the alloy but can not prevent the loss of Cr.Since the demanding requirement for the in-situ formation of a protective dense Al₂O₃ layer on the alloy surface in high-temperature chlorides,it is difficult for Ni-xAl-Cr alloy（without any pretreatment,such as pre-oxidation）to spontaneously form a passivated surface film through reacting with the impurities in the high-temperature chloride salts.