Research On The Corrosion Behaviors And Mechanism Of A517Gr.Q Steel In Simulated Seawater

With the rapid development of human society, land resources are draining away. Marine resources’ exploitation became into the hope of solv ing the shortage of human resources. As the symbolic building of the marine resources’ exploitation, offshore platform’s depth is increasing with the offshore engineering developing. The high strength, large thickness, well welding performance and corrosion resistance, at present, are not only the most important properties, but also the concern developments of the offshore steels. Researches on the offshore engineering are developed so late in our country that there is a big gap between China and some developed countries. As a new product of our country, A517Gr.Q steel has high strength, large thickness,and well welding performance. The yield strength of A517Gr.Q steel reaches 700MPa, and the tensile strength achieves 790MPa. But there are no reports about the corrosion behaviors or mechanism about A517Gr.Q steel at present. So, in order to offer a science reference for security use and corrosion protection, it’s of great importance to research the corrosion behaviors and mechanism of A517Gr.Q steel in marine environments.In this paper, the laboratory simulated experiments were used to study the corrosion behaviors of A517Gr.Q steel in simulated seawater splashing zone and immersion zone. The weight-loss measurement was used to test the corrosion rate of A517Gr.Q steel. The change of the macro morphologies were-recorded by a digital camena. The micro morphologies and the surface of the sample after removing the rust were observed by SEM. XRD and EDS were used to characterize the corrosion products and analyze the elements’ distributions. At the same time,the corrosion potential, polarization curve and the electrochemical impendence spectroscopy were investigated using electrochemical measurement techniques. At last, the corrosion behaviors and mechanism of A517Gr.Q steel in simulated seawater were synthetically analyzed.The studies’ results were shown as follows:(1) The stable corrosion rate of A517Gr.Q s teel in simulated seawater splashing zone was 0.185mm/a., which classified 6～7 grade. The corrosion resistance of A517Gr.Q steel was classified into 4～5 grade with a rate of 0.081mm/a.(2) A517Gr.Q steel in splashing zone and immersion zone leaded to pitting corrosion. The pitting corrosion occurred during the 0h～24h quick corrosion phase and 96h to the stably corrosion phase in splashing zone. Caused by the small droplets attracted to the surface of the sample, they extended shallowly and widely. But they placed unevenly in stably phase, caused by the flashing function of simulated seawater, extended inwardly. The pitting corrosion occurred during 24h～96h slowly corrosion phase in immersion zone, caused by the cracks in rust layer and extended shallowly and widely.(3) The rust layer of A517Gr.Q steel in simulated water divided into two layers. The outer layer in splashing zone was characterized by loose texture and porous,as well as lots of large orifices existed. The inner layer was dense, but there were still some cracks existed. The outer layer in immersion zone characterized by loose texture and porous, but the orifices were smaller than that in splashing zone. The inner layer was dense. The rust layer’s protectiveness was promised on its wholeness.(4) The inner layer forming a more dense contributed to the concentration of element Cr,which can obstruct the diffusing of Cl in simulated seawater through the rust layer. So Cr is beneficial to protect the substrate. An increase in substrate Cr content without an influence to its mechanical properties would increase the corrosion resistance of A517Gr.Q steel.(5) Corrosion products of the outer layer of A517Gr.Q steel in splashing zone mainly contains: γ-FeOOH,Fe3O4/Fe2O3, Fe(OH)3, α-FeOOH and some non-crystalline substance. The inner layer mainly contains: Fe3O4/Fe2O3, α-FeOOH, γ-FeOOH, α-FeOOH and a few of FeOCl,Green Rust (I) , as well as some uncorroded metals. The outer layer in immersion zone mainly contains: Fe3O4/Fe2O3, Fe(OH)3, α-FeOOH and some insoluble calcium and magnesium salts.The inner layer mainly contains: Fe3O4 and α-FeOOH.(6) Corrosion tendency of A517Gr.Q steel in simulated seawater diminished at first then tended towards stability. The primary cathodic reaction process was for the reduction of the rust layer. Due to the iron oxide generated continuously, the dense inner layer formed, the corrosion rate slowed down. The corrosion reaction, at last, was controlled by the charge transfer and the diffusion of ion together.