Corrosion Behavior Of 304 Stainless Steel In Boron-containing Vapor

China's economy is growing and its demand for energy is huge.However,China's current energy consumption is still dominated by traditional fossil energy.On the one hand,the limited fossil energy is insufficient to support long-term sustainable development;on the other hand,the use of fossil energy will increase carbon emissions and accelerate the trend of global climate change.In order to optimize China's energy structure and promote the development of clean and low-carbon energy,it is urgent to accelerate the construction of nuclear power.Boric acid plays an irreplaceable role in nuclear power plants.However,it has been found that nuclear power materials have different degrees of corrosion in boron-containing media.Austenitic stainless steel is widely used in pressurized water reactor nuclear power plants due to its great heat resistance and corrosion resistance.However,corrosion of austenitic stainless steel will inevitably occur when it contacts with boron-containing medium at high temperature and pressure for a long time.Therefore,the study on the corrosion behavior of austenitic stainless steel in boron-containing medium is of great significance to the safe operation of nuclear power plant.In this paper,based on the actual situation of the equipment tip corrosion in the boron-containing solution circulating system of nuclear power plant,the corrosion behavior of304 austenitic stainless steel exposed to the vapor above boron-containing solution was studied.And the vapor corrosion was compared with the corresponding solution corrosion in order to clarify the difference between the vapor corrosion and the liquid corrosion.The oxide films formed on the surface of specimens were characterized by scanning electron microscopy(SEM),X-ray diffraction(XRD)and X-ray photoelectron spectroscopy(XPS).Finally,the corrosion behavior was elucidated by the growth mechanism of the oxide film.By exploring the corrosion behavior of 304 stainless steel exposed to the vapor above boron-containing solution,we found that: the oxidation rate of 304 stainless steel exposed to boron-containing vapor at 180?,first rapidly increased and then slowly increased before720 h,and sharply increased after 720h;After 1200 h corrosion,a double oxide layer was formed on the surface of the sample.The inner layer was rich in chromium and the outer layer was rich in iron and nickel.When the sample was exposed to the boron-containing vapor at180?,the presence of boric acid could promote the generation of local defects and thus accelerated corrosion.When the temperature was lower than 150?,the corrosion product on the sample surface was a monolayer oxide film with round spots distribution.With the increase of the temperature,the diameter of the round spots kept increasing.When the temperature was 180?,the edges of the round spots disappeared and a double oxide film was formed.By exploring the corrosion behavior of 304 stainless steel exposed to boron-containing solution,we found that: the oxidation rate of 304 stainless steel exposed to boron-containing solution at 180? first rapidly increased and then tended to gradually flat;After 1200 h corrosion,a double-layer oxide film was formed: the inner layer was fine and compact particles,and the outer layer was loose sheet oxide.When the samples were exposed to different concentrations of boron-containing solution at 180?,the obvious change of the sample surface morphology was not observed and while the corrosion rate increased slightly with the increase of boron concentration.When the sample was exposed to the boron-containing solution at different temperatures,the oxidation products were uniformly distributed on the surface of the samples,and the particle density gradually increased with the increase of corrosion temperature.The result indicated that the corrosion of 304 stainless steel exposed to boron-containing vapor was more serious than one exposed to boron-containing solution under the same conditions.The oxide films formed on the surface of the samples in both boron-containing vapor and solution were composed of Fe,Cr,Ni oxides and hydroxides.However,the oxide film formed in boron-containing vapor was thicker and the surface was rich in iron and nickel;but the oxide film formed in the corresponding boron-containing solution was very thinner and contains a large amount of chromium on the surface.The corrosion mechanism was preliminarily discussed as follows: the micro-cell formed on the surface of stainless steel to produce metal hydroxides;metal hydroxides were dehydrated to form metal oxides;oxidation products were attached to the surface of the sample to form an oxide film.