Study On Corrosion Kinetics Of Microalloyed Marine Steel Under Simulated Marine Atmosphere Environment

As a typical engineering equipment material,high-strength marine steel has been widely used in various industries such as metallurgy,military,and marine engineering.A deep understanding of its corrosion behavior and corrosion resistance characteristics in service environments is of great guiding significance for the design,optimization,and quantitative production of new steel grades.This thesis selects AH36 steel for marine equipment and EH36 steel with microalloying elements(Nb,V)added;FH36 steel with Nb,V,Cu,Ni added and VL4-4 steel with Nb,V,Cu,Ni,Cr added were used as research objects.Indoor cyclic immersion simulation accelerated experiments were conducted to simulate marine corrosion environments with Na Cl solutions with concentrations of 0.5wt%,2.0wt%,3.5wt% and 5 wt%,respectively.The cyclic immersion experiments were conducted for 48 hours,120 hours,216 hours,360 hours,and 720 hours,respectively.Using the corrosion weightlessness method,the corrosion rate,environmental corrosion morphology,and phase composition of four types of marine engineering steels in simulated marine environments were analyzed,and the corrosion kinetics of marine engineering steels in simulated marine environments were analyzed using a power index equation.The main results of the thesis are as follows:The periodic immersion accelerated experimental study shows that the corrosion rates of the four types of marine steel show a decreasing trend with the extension of experimental time.The simulated corrosion behavior of marine steel in the marine environment is divided into three stages.In the first stage(after alternating wet and dry for 48-120 hours),the thickness of corrosion products on the surface of marine steel increases over time,and the corrosion rate shows a significant downward trend during this stage;In the second stage(216-360 hours),corrosion bubbles and corrosion pits occur on the material surface after fracture,and the trend of corrosion rate change is relatively small.The corrosion surface appears as a dark copper green metastable state;Stage 3(360-720 hours): The corrosion surface as a whole changes from dark to bright,and the corrosion rate continues to slow down,reaching the maximum corrosion weight loss.By fitting the power function dynamic model and the potentiodynamic polarization curve,the corrosion kinetics analysis of four kinds of marine steel showed that the microalloying reduced the parameter A reflecting the initial corrosion severity in the reaction kinetics equation,and the n values representing the corrosion trend of marine steel were all less than 1;At the same time,the self corrosion current density of the three microalloyed marine steel is lower than that of the unalloyed AH36 steel,indicating that alloying elements improve the corrosion resistance of marine steel.The analysis of the corrosion surface and cross-sectional morphology of marine steel using scanning electron microscopy,combined with XRD analysis of the composition of corrosion products,shows that all four types of marine steel corrosion products are composed of well protected α-Fe OOH and Fe3O4,as well as a small amount of instability in the early stages of corrosion γ-Fe OOH and β-Fe OOH composition.The corrosion rust layer structure of AH36 is relatively rough,hollow,and has poor adhesion between evacuation and substrate;Large clumps of flocculent Fe3O4 interspersed with needle like structures in EH36 and FH36 steel corrosion products α-Fe OOH;The section thickness and density of VL4-4 steel are slightly weaker than the first two alloying specimens,but the flatness is better than that of AH36 steel.