Initial Corrosion Behavior Of Weathering Steel In Simulated Marine Atmosphere Environment

Digital image correlation technology(DIC)was used in this paper to observe the failure process of weathering steels subjected to different corrosion times under uniaxial tensile stress.The strain change process was recorded and the mechanical performance degradation of corroded specimen was obtained.Through the scanning electron microscope(SEM)analysis and electrochemical test of the rusted samples and the samples after chemical derusting under different rusting times,the corrosion mechanism of weathering steel is analyzed.Finally,using the three-dimensional scanning system,the real corrosion models corresponding to different corrosion areas are obtained and analyzed to figure out the degradation reasons of mechanical properties.The main conclusions are as follows:(1)As the corrosion time increases,the rust layer on the weathering steel surface becomes dense and stable.In the standard salt spray test,the corrosion rate fluctuates in the range of 4-5.8um/d.After 12 days of corrosion,the Q355 NH rust layer becomes denser and denser.The corrosion speed gradually decreases,as a result the thickness loss trend decreases gradually.(2)At 2d,a large number of relatively uniform corrosion pits appeared on the surface of the steel substrate after chemical rust removal;at 21 d,the surface layer gradually became flat,and some of the corrosion pits showed a tendency to expand;at30d,the corrosion pits on the surface of the weathering steel substrate almost disappeared.As the degree of corrosion increases,the corrosion state of weathering steel tends to stabilize during the process of corrosion,and the density of the rust layer gradually increases,which improves the protection for the steel matrix and enhances the corrosion resistance of weathering steel.(3)When weathering steel is tested in NaCl electrolyte,the self-corrosion potential drops from-546 m V to-720 m V,and then rises to-642 m V.The trend is rebounded after moving downwards.The change in corrosion current is the same as the corrosion potential,and the corrosion trend increases firstly,and then decreases,finally fluctuates stably.When the electrolyte lies in an alkaline environment,the potential of the weathering steel increases and the corrosion tendency decreases.When the measurement was performed after 24 hours in an alkaline electrolyte environment,the weathering steel was corroded in this case.The reason is that the weathering steel would form a passivation film on the surface of the steel substrate in an alkaline environment to prevent the occurrence of rust.(4)After 30 days exposure in a salt enviroment,no obvious corrosion products appeared on the surface of 2205 stainless steel.Electrochemical tests were carried out on the 2205 stainless steel.The test results showed that the corrosion potential of stainless steel was always greater than-200 m V,and the corrosion current density was always less than 1?A/cm2,indicating that 2205 stainless steel did not corrode in the salt spray environment,and the concentration of chloride ions in the salt spray fails to penetrate the passive film on the surface of the stainless steel.(5)Using digital image correlation technology(DIC),the change cloud map of the strain field during the teat process can be clearly exhibited,which can reveal the degradation mechanism of mechanical performance caused by regional corrosion.The corrosion area and the weakest section have obvious effects on the yield and tensile failure of weathering steel.The final damage position of the steel starts from the weakest section.Therefore,the acquisition of corrosion parameters at the weakest section is extremely important for the performance evaluation of steel.(6)As the corrosion rate increases,the nominal yield strength and the nominal ultimate strength show a downward trend.For steels in a fully corroded state,the decrease in mechanical properties is linearly related to the corrosion rate.For locally corroded steels,the decline of mechanical properties gradually increases with the corrosion rate,and the reduction ratio of nominal yield strength to ultimate strength is about twice the corrosion rate.Compared with tensile specimens 2 and 3,tensile specimen 1 has the largest actual corrosion rate under the same corrosion time,and its mechanical properties also decrease largest under same corrosion time.