Adosrption,Film Formation And Corrosion Inhibition Mechanism Of Octadecylamine On Carbon Steel Sruface In The Secondary Circuit Of Pressurized Water Reactor Nuclear Power Plant

The carbon steel corrosion is a prominent problem in the normal operation and lay up stage of the secondary circuit of pressurised water reactor nuclear power plants.Among a range of corrosion protection methods,the injection of Octadecylamine（ODA）into the circuit has proved to be an extremely convenient and efficient method.However,the further application of ODA in nuclear power plants is seriously hampered by the fact that the corrosion inhibition mechanism of ODA is not well understood.In this paper,the adsorption and film formation mechanisms of ODA on carbon steel surfaces were analysed through molecular dynamics（MD）simulations,and the effects of temperature and pressure on the adsorption configuration and stability of ODA molecules were investigated.The diffusion characteristics of the three corrosion particles（O₂,H₂O and Cl^-）in the ODA corrosion inhibition film were explored,and the ODA corrosion inhibition mechanism was elucidated from a microscopic perspective.The research results are as follows.A microscopic adsorption model was developed to explain the process and mechanism of ODA adsorption and film formation on carbon steel surfaces.The ODA molecule is adsorbed on the carbon steel surface through the formation of coordination bonds between the nitrogen atom and the iron atoms in the carbon steel;the adsorption process can be divided into two stages,which are dominated by the interaction between water molecules and ODA molecules and the interaction between iron atoms and ODA molecules.At room temperature and pressure（80°C,0.1 MPa）,the ODA molecule adsorption configuration is nitrogen atoms adsorbed on the iron surface,and the remaining alkyl chains are suspended in solution;while at high temperature and pressure（140°C,8 MPa）,the entire ODA molecule is adsorbed on the carbon steel surface,and has a higher adsorption energy.As the concentration of ODA increases,the corrosion inhibition film conformation evolves from a monolayer to a bilayer film,and the corrosion inhibition film denseness and corrosion inhibition properties are continuously improved;there is a threshold concentration,when the ODA concentration exceeds this value,the excess ODA molecules will no longer have an impact on the corrosion inhibition film conformation,but will be aggregated into microclusters and suspended in the aqueous solution.The particle diffusion coefficient was calculated to evaluate the corrosion inhibition of ODA films from a diffusion perspective.Corrosion inhibition film exists in two ways to function.One is as a physical barrier that isolates the contact between corrosion particles and the surface of carbon steel;the second is by inhibiting the diffusion ability of corrosion particles inside the corrosion inhibition film,reducing the possibility of corrosion particle movement to the surface of carbon steel.The diffusion trajectories and coefficients of water molecules at three different locations within the corrosion inhibition film（at the junction between the corrosion inhibition film and the Fe（001）surface,inside the corrosion inhibition film,and in the solution）were analyzed and calculated;the effect of the corrosion inhibition film on inhibiting the corrosion of carbon steel caused by oxygen was investigated through MD simulations of applied oxygen molecules.According to the results of the diffusion coefficient,the corrosion inhibition performance of ODA on water and oxygen molecules is as high as 83.8%and76.88%respectively.Based on the diffusion behaviour of corrosion particles within the corrosion inhibitor film,the corrosion inhibition mechanism of ODA corrosion inhibitor film was clarified.The corrosion inhibition film are self-motioned,and there are cavites inside the corrosion inhibitor film,so that a certain size of connecting channel can be formed between the cavities and the corrosion particles can diffuse through the corrosion inhibitor film by moving from one cavity to another.Temperature,free volume fraction,cohesive energy density and the self-motion of the corrosion inhibitor film are important factors influencing the movement of corrosion particles within the film.Higher temperatures lead to increased particle movement,increasing the likelihood of corrosion particles moving towards the carbon steel substrate.The smaller the free volume fraction,the greater the cohesive energy density,which facilitates the corrosion inhibition film to further inhibit the diffusion ability of the corrosion particles,making the particles trapped in a certain cavity to continuously vibrate and achieve the purpose of protecting the carbon steel material.Due to electrostatic interactions,the ODA corrosion inhibition film has a higher inhibition capacity for charged particles than for electrically neutral particles.The research in this paper can provide theoretical guidance for the scientific application of ODA in the secondary circuit of pressurised water reactor nuclear power plants,mitigating the corrosion of carbon steel in the secondary circuit and protecting the safe operation of the units.