| Corrosion is one of the main factors affecting the service life and safety of steel structures and it has become an urgent practical issue to be addressed.In some cases,the structures have reached the point where they must be reinforced in a timely manner.However,as the atmospheric exposure test requires a long period of time and the test is restricted to a certain region,such direct test approach is unfavourable.Hence,it prompts an experimental study on accelerated corrosion simulations,aiming to study the correlation between accelerated corrosion and corrosion in the natural atmosphere which is a core practical issue.With the support of National Natural Science Foundation of China(Grants No.51578357),this study investigates Q235B commonly used by the energy and chemical industries in Shanxi Province,China.Based on the typical atmospheric exposure condition of urban industries,accelerated corrosion tests in a cyclic wet/dry acidic Cl~--containing laboratory environment,and machining simulation corrosion for Q235B steel,similarities concerning the corrosion behavior,topography of etch pits,static performance,and fatigue performance were studied.The main research results are as follows:(1)Through the atmospheric exposure,corrosion behavior study,the dry/wet cycle accelerated corrosion test conducted in this study demonstrates a significant corrosion rate which is about 8 times of that in the industrial atmospheric environment.It follows the power function,W=At~n;and the corrosion tendency is basically the same.Although the steel has the same corrosion product in the two different corrosion environments,the formation mechanism and process of corrosion products are different,so the morphology of rust layers is different.(2)The three-dimensional non-contact surface topography instrument was used to study the rust pit morphology of urban industrial atmospheric exposure and dry/wet cycle accelerated corrosion test and to study the evaluation indices of the corrosion model including pit shape,maximum pit depth,average pit depth,pit diameter-depth ratio,etch pit distribution,and thickness indices.The time-varying mathematical models between these parameters and the corrosion degree are given.As a result,the pit depth distribution follows the normal distribution and a probability distribution model related to the rust rate is established.A corrosion model suitable for the common Q235 steel used in civil engineering under the acid rain corrosion environment is proposed.(3)Under three different corrosion methods,static tensile tests were performed on 80 test specimens and the influences of corrosion methods and corrosion degrees on the constitutive law of rusted steel plates were investigated based on the topographies of etch pits.In addition to the net strength calculated based on the minimum cross section,other mechanical properties decrease with the increasing corrosion degree and the dispersion increases.The deformation ability is more obvious than the decrease in strength.When the weight loss rate is?16%,the yield platform of the stress-strain curve of corroded steel plate is significantly shortened.The weight loss rate of 30%is defined as the critical point at which the yielding platform disappears under the dry/wet cycle accelerated corrosion test.The critical corrosion rates of the main mechanical indices that do not meet the requirements of the specification are put forward.The relationship models between the etch pit depth and mechanical property degradation are established.A deterministic constitutive model related to the corrosion rate is also established.(4)Constant-amplitude axial tensile fatigue tests were performed for 55specimens un-corroded under the urban industrial atmosphere and the dry/wet cycle accelerated corrosion test conditions.The results show that the fatigue life decreases sharply with the increasing rust degree under the same stress range and the decline trend is similar.When the corrosion degree is close,the fatigue limit is close.The fatigue design curves suitable for the common Q235 steel under the acid rain corrosion environment are developed.In the dry/wet cycle accelerated corrosion environment,when the weight loss rate is about 30%,guarantee rate=97.72%and stress amplitude>187.98MPa,it is unsafe to use the standard design S-N curve and improved structural detail to the corroded steel plate is required.(5)It is applicable to apply the infrared thermal imaging method based on the energy theory to study the fatigue performance of corroded steel.The temperature change of rusted steel during the fatigue loading still conforms to the typical three-stage theory of temperature rise.When the corrosion degree is close,the greater the stress,the higher the temperature rise of the specimens;when the fatigue load is the same,the more severe the degree of corrosion,the more serious the defects caused by corrosion,the greater the energy dissipation,the higher the temperature rise of the specimens,and the more significant the fatigue performance decreases.The quantitative model of the infrared thermal imaging method can quickly predict the fatigue limit and S-N curve of corroded steel.The error between the fatigue life measured by the conventional fatigue test is small and therefore the consistency is generally good.(6)Based on the gray correlation theory,the correlations between the corrosion model assessment indices and the mechanical performance indices in different corrosive environments were analyzed and the results show that all correlations are good,validating that the corrosion model is suitable for the common Q235 steel under the acid rain corrosion environment.The degradation of steel mechanical property is not sensitive to the differences in rust layer topography and corrosion process.As long as the microscopic mechanisms of corrosion are identical or the topographies of etch pits are similar,the accelerated corrosion test can accurately simulate the atmospheric corrosion and predict the degradation pattern of steel mechanical properties in an atmospheric environment. |
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