Failure Behaviors Of Tower Materials Of Offshore Wind Turbines Under Marine Corrosion And Stress Environment

The towers of offshore wind turbines have long been exposed to the complicated marine environment of high salt,high humidity and frequent alternation of wetting and drying,which at the same time are also subjected to localized high tensile stress due to cyclic loading impacts such as wind,waves,currents and even high-and low-temperature thermal stress.Under the action of environmental interactions in these two cases,they’re prone to stress corrosion failure.In order to better provide theoretical support for the performance,corrosion mechanism,stress corrosion failure mechanism and the evolution rule of micromorphology of the tower materials of wind turbines in marine environment,the paper conducted a variety of corrosion experiments under different marine dry and wet environments and a detailed research on the effects of tensile rate,medium concentration and temperature on the corrosion behaviors of Q345 steel in marine corrosive environment by LYW-015 salt spray corrosion test chamber and WDML-5 stress corrosion slow-stretch test machine,through methods like mechanics performance test,microstructure analysis by scanning electron microscopes,electrochemical behavior analysis.The following aspects are included in this thesis:(1)The corrosion behaviors of the materials in the marine environment was studied by performing salt spray corrosion tests on Q345 steel with different corrosion time.The results showed that the corrosion weight loss rate continued to rise before 150h,and the upward trend slowed down between 150h to 250h,and then the upward trend increased slightly after 250h;the appearance of surface corrosion products in the early stage of corrosion were needle-like dendritic crystal which stacked in the middle stage of corrosion,and finally developed into columnar and spherical products in the later stage of corrosion,which can protect the steel substrate;and with the increase of corrosion time,the rust layer thickened and bound more closely with the substrate.(2)Corrosion tests were conducted in dry and wet environments with different seawater-salt spray proportion to compare the effects of different marine environments on the corrosion behaviors of Q345 steel.The results showed that the corrosion rate of materials in corrosive environments where seawater and salt spray corrosion alternates was greater than that in environments only with seawater corrosion or salt-fog corrosion.The evolution rule of corrosion behaviors over time bore some similarities with single environmental corrosion,but the occurrence of microcracks was greater in thicker corrosion layers during the wet-dry alternating process,leading to severe localized corrosion.(3)At the same time,electrochemical tests were performed on the corroded samples in dry and wet environments with different seawater-salt spray proportion to determine the effects of different marine environments on the electrochemical corrosion behaviors of the materials.The results showed that the corrosion product film formed on Q345 steel in a single corrosive environment had a higher film resistance than that formed in a seawater-salt spray alternating corrosion environment,with a shorter time to form stable corrosion product films.(4)Slow strain rate tensile tests were carried out at different tensile rates,medium concentrations,and temperatures to investigate the stress corrosion failure mechanism of Q345 steel under different operating conditions.The results showed that the strain rate of 4×10-7s-1 was suitable for the stress corrosion study of Q345 steel-NaCl solution system.Besides,NaCl solution can accelerate the corrosion cracking of Q345 steel,and the sensitivity index Iδ and Iψ represented by extensibility and reduction of area increased as the concentration increased,with the change of Iδ more obvious than that of Iψ.Moreover,the proportion of dimple and quasi-cleavage regions in the fracture micro appearance was closely related to the concentration of the solution.Furthermore,the sensitivity index increased significantly with the increase of temperature.