The Nonequilbrium Statistical Theory Of Pitting Corrosion Of Metal Materials

Pitting corrosion is a stochastic process.The theoretical calculation model for pitting growth of metal materials is established based on non-equilibrium statistical theory.Based on the stochastic theory and the mechanism of the pit growth,the time evolution equations of corrosion growth rate and pit depth are obtained.According to the statistical theory of pitting corrosion,the partial differential equation of pitting corrosion of metal materials is established.Then the probability density function,failure probability,reliability,pitting life distribution function and average pitting life of metal materials are deduced.Finally,taking the aircraft aluminum alloy LD2 structural material and Q235 steel as examples,the theoretical model is applied and the calculation results are analyzed.The main contents are as follows:Background: Owing to the high mechanical strength,excellent heat-resistance property and good dimensional stability of metal materials,they are widely used in people's life and production.However,metal materials are vulnerable to corrosion in both the natural environment and the industrial environment.Pitting corrosion is one of the common and highly destructive localized corrosion,which is a corrosion form caused by localized destruction of the passive film on the surface of metal materials.In the corrosive environment,pits continue to grow in depth and to be more concealed,which is likely to cause catastrophic damage in metal materials.And then the service life and reliability of materials are greatly reduced.Accordingly,the investigation of pitting corrosion of metal materials has a strong practical significance.Contents and methods: The growth process of pitting corrosion is regarded as a non-equilibrium irreversible process,and the inhomogeneous fluctuations of materials and corrosive environment have been considered.Probability density function of space and time are derived by establishing and solving Fock-Plank equation,the failure probability and reliability of materials are calculated by using the weakest link model of pitting corrosion.Further,the pitting life distribution function and the average pitting life of metal materials are obtained.Finally,the theoretical model is applied to analyze the growth process of pitting corrosion of aluminum alloy LD2 and Q235 steel.Thereby we get the applicability of the non-equilibrium statistical theory to analysis of pitting corrosion of metal materials.Research results: The theoretical curves of pit depth and corrosion growth rate versus corrosion time of aluminum alloy LD2 and Q235 steel are obtained,and the theoretical results are in good agreement with the experimental data.Meanwhile,the curves of probability density function of space and time of two materials are plotted,so the most probable pit depth in different corrosion time ranges and the most probable corrosion time range of different pit depths can be deduced.Moreover,according to the theoretical calculation method of the failure probability and reliability of materials,we draw the curves of the failure probability and reliability versus corrosion time of aluminum alloy LD2.Thus the safety period of metal materials is deduced.We hope it can be applied to predict the pitting life of metal materials and analyze the reliability of materials.