Studying The Influence Mechanism Of Lead-hydrogen Coupling On Passive Film Of Alloy 690TT

In the steam generator of PWR,heat transfer tubes is suffering from severe service environment for a long time,such as high temperature,high pressure,irradiation and corrosive water medium,which is easy damage,affecting the safe operation of nuclear power plant.The phenomenon of lead-induced stress corrosion cracking(PbSCC)in the secondary circuit is one of the important ways of heat transfer tube damage.In the secondary circuit,there are many sources of hydrogen for heat transfer tubes,and hydrogen can promote corrosion and stress corrosion cracking(SCC)of metals and alloys.Therefore,it is a worthy question to research whether hydrogen plays an important role in the process of PbSCC.The occurrence of SCC is closely related to the local rupture of passive film.Therefore,studying the effect of hydrogen on the degradation behavious of passive film on heat transfer tube induced by lead is very important to research the mechanism of PbSCC.This paper takes Alloy 690TT with 59 Ni-30 Cr-10 Fe as the research object.Immersion and electrochemical experiments,current sensior atomic force microscopy(CS-AFM),scanning kelvin probe force microscope(SKPFM),time of flight secondary ionization mass spectrometry(ToF-SIMS),X-ray photoelectron spectroscopy(XPS),auger electron spectroscopy(AES),combined with first principle calculation based on density functional theory are used to systematically and deeply explore the coupling effect of lead and hydrogen on passive film of Alloy 690TT and study the coupling effect of lead and different surface states on the passive film of Alloy 690TT subsequently.The specific summary is as follows:(1)The degradation mechanism of passive film on Alloy 690TT induced by trace Pb was studied.On the one hand,lead will enter into the oxide barrier layer to inhibit the outward diffusion of oxygen vacancies,resulting in the decrease of oxygen content in the passive film,thus inhibiting the inward growth of passive film.On the other hand,Pb(OH)2 adsorbed and deposited on the surface can promote the dehydration process of Cr(OH)3 but inhibit the dehydration process of Ni(OH)2.While,lead increases the content of less protective hydroxides in the passive film finally.These two mechanisms work together to change the composition and structure of the passive film of Alloy 690TT,resulting in a decrease of two orders of magnitude in the resistivity of the passive film in the grain interior and a significant decrease in the electrical resistance of the passive film on both sides of the grain boundary carbide.Finally,lead causes serious degradation of the protective properties of the passive film of Alloy 690TT,and accelerates the outward diffusion and dissolution processes of nickel and iron in the alloy.The mechanism of lead-induced passive film degradation of Alloy 690TT is revealed at atomic scale,which provides the basis for the study of PbSCC mechanism and theoretical guidance for corrosion protection and material design of heat transfer tube.(2)The coupling effect of lead and hydrogen on the passive film of Alloy 690TT was studied.It’s found that there is a coupling effect between lead and hydrogen,which can inhibit the entry of oxygen into the passive film and increase the content of hydroxides in the passive film of Alloy 690TT,promoting the diffusion and dissolution of iron and nickel in the alloy.In alkaline solution,a large amount of dissolved iron and nickel are easy to form hydroxides and adsorb on the surface of passive film,which results in the formation of double-layer p-type semiconductor passive film on Pb,H and Pb-H samples.Space charge layer at the double-layer p-type semiconductor interface increases the barrier of electron transfer in the passive film,which increases the electrical resistivity of passive films.However,due to the increase of hydroxide content in the passive film,the protective properties of passive films on the Pb,H and Pb-H samples still become worse.Besides,lead,hydrogen and the coupling of lead and hydrogen reduce the electrical resistance of passive film at the interfaces between grain boundary carbides/TiN and matrix,resulting in Alloy 690TT being more sensitive to corrosion at these interfaces.Combined with the localized analysis methods,it is found that the coupling lead and hydrogen reduces the corrosion trend of matrix passive film surface compared with lead-containing condition,which causes the change of the most easily corroded position of passive film from the grain boundary carbides/matrix interface of to the interface between TiN and its nucleus.This provides a new insight for the interpretation of the PbSCC phenomenon of Alloy 690TT,which is sometimes IG and sometimes TG.(3)Different levels of cold-work deformation layers were obtained by different surface treatment methods,and the degradation mechanism of passive film on Alloy 690TT with different surface states induced by lead in high temperature and high pressure water environment was studied by surface characterization technologies.The results show that due to the existence of cold-working deformation layer on the surface,the electrical resistivities of passive film on the ground and mechanical polishing(MP)samples decreases by one and two orders of magnitude,respectively.However,there is almost no cold-working deformation on the surface of electropolished(EP)samples,and the electrical resistivity of passive film decreases only three times due to lead.Finally,the joint action of lead and cold-working deformation layer results in more serious degradation of the protective properties of passive film on Alloy 690TT,and accelerates the diffusion and dissolution of nickel and iron in the alloy.From the perspective of electron transport,it is revealed that there is a coupling effect between lead and cold-working deformation,which can resulting in the degradation of passive film on Alloy 690TT.These will provide a theoretical basis for the formulation of PbSCC control measures and operation specifications.